AudioCircle
Audio/Video Gear and Systems => The Vintage Circle => Topic started by: Bill Thomas on 12 Aug 2009, 05:42 pm
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While we are in the final stages on our FM-3 reconstruction, (I *should* have a final construction "update" posted in the next day or so, with an "Alignment Update" following shortly thereafter), it's time to become acquainted with our next "reconstruction project!" Here's a slightly "tilted" picture of our next "victim."
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SoDQwDKfgOI/AAAAAAAABI0/16W7OTh0M3Q/s720/Scott%20121-C%20-%20Front%20View.JPG)
This is an H. H. Scott 121-C Monaural Preamplifier from 1957. That's right - it's 52 years old! (Almost as old as *I* am!) The owner of this unit is an aficionado of early 78 RPM phonograph records and wants a Phono Preamplifier that will allow him to adjust the various equalization curves and turnover frequencies for more satisfying playback response from WILDLY varying sources. THIS little "gem" should do MUCH more than just "suffice."
If Dynaco equipment is a design study in "minimalist architecture", the H. H. Scott philosophy is almost the exact *opposite*. Dr. Hermon Hosmer Scott (1909 - 1975) was a true "Audio Pioneer." He and Avery Fisher were at the forefront of the "Hi-Fi Revolution" of the 1950's. Dr. Scott came from a SOLID background in engineering. His prior "work experiences" include an early "stint" with Bell Labs from 1929 through 1931; developing high quality sound for motion pictures and broadcasting, followed by a 15 year "hitch" with General Radio Company. General Radio Company was one of the "biggies" in the high-quality, lab-grade radio equipment field. Dr. Scott held more than 100 patents. He invented the R/C oscillator and pioneered a TON of research having to do with various R/C circuits. But he is perhaps BEST known for a development that made it possible for broadcasters to GREATLY improve the reproduction of existing (78 RPM) phonograph records. He called this invention the "Dynaural Noise Suppressor." While he successfully licensed this technology to other manufacturers, (and the principles of this design are utilized to this very day!) it became one of the "signature features" of H. H. Scott preamplifiers during the early days of the home Hi-Fi "boom." The subject of our rebuild also contains a version of the Dynaural Noise Suppressor.
Where Dynaco would distill their designs down to the absolute LOWEST "parts count," H. H. Scott designs generally did just the opposite. While Dynaco was using those "new-fangled" Printed Circuit boards back in 1957, H. H. Scott equipment was ALL "point-to-point" with nary a PC board in sight. Take a look at the underside of this preamplifier:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SoDQwqJ2vvI/AAAAAAAABJA/uBZbq2zMP0g/s640/Scott%20121-C%20-%20Bottom%20View.JPG)
Keep in mind that this is a MONAURAL preamplifier. Dynaco's PAM-1 preamplifier used two 12AX7's. THIS baby uses SEVEN tubes and HUNDREDS of parts. But there is no question that the 121-C is a much more "flexible" unit. Take a look at the rear panel and you'll get an idea of what I'm talking about:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SoDQwTxYnbI/AAAAAAAABI4/yrqnDyl48wY/s720/Scott%20121-C%20-%20Rear%20View.JPG)
Notice that each high-level Input has an individual "trim control" in order to allow the customer to "match audio levels" between sources. Not only that, there is an additional "preamp level control" to allow adjustment of the audio level coming from the Phono or Tape "equalizing amplifier" section as well. This all allows the end-user the ability to "tweak" nearly every parameter of operation. Of course, there IS a "down-side" to all this: We're going to be dealing with HUNDREDS of components before we're through.
H. H. Scott units were NOT built to achieve a "price point." They were built to achieve a MAXIMUM amount of flexibility for the Hi-Fi "enthusiasts" of the day. They were built like Battleships! Here's a picture from the top side of the unit:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SoDQwfZ-wrI/AAAAAAAABI8/qYE7zNl3Pz4/s640/Scott%20121-C%20-%20Top%20View.JPG)
Notice that EVERYTHING is completely shielded! Those two black "cables" are actually "shielding tubes" that carry audio back and forth between the Back and the Front of the preamplifier. Nice!
While many later H. H. Scott units were issued in kit form as "Scottkits", this unit was factory-assembled. The overall physical condition of this unit is quite good. While it will require some cleanup, everything is where it should be and the unit is complete.
Electrically, things are about what you would expect for a 52 year-old unit. You'll notice that this preamp is absolutely LOADED with "bumblebee" capacitors. They may have been the "cats pajamas" back then, but we now know they have an EXTREMELY high "failure-rate" and must all be replaced for reliable operation. Allen-Bradley carbon-composition resistors were used throughout the unit. There was absolutely NO "skimping" on "parts quality" in this unit.
Restoring this unit will be quite different from restoring a Dynaco product due to the "point-to-point" construction used. In many ways, this will be more like restoring an old television (and *just* about as complicated due to the high parts count.) But like most restorations, the task can be divided into "sections" that should allow us to keep things heading in a forward direction.
Electrically, we will begin by restoring the Power Supply first. Without good, CLEAN power, NOTHING will work correctly. As you might expect, after 52 years, EVERY electrolytic capacitor will need to be replaced. We'll also be replacing the "solid-state" rectifier. (No, that's NOT a mis-print, although the preamplifier's ONE solid-state rectifier was *probably* called a "chemical rectifier" back then.) We'll replace that old full-wave selenium rectifier with a more modern silicon diode unit that'll probably live longer than ALL of us.
The three "can capacitors" are the biggest "problem area" to be dealt with. As you might guess, these parts are now made of "unobtanium" and we'll have to devise a method to deal with this "issue" before we will be able to make any REAL progress in our restoration.
Great care will have to be used in dis-assembling the unit. Since this was built at the factory, rivets were used to fasten the various sockets and terminal strips to the chassis. In order to keep things looking "factory", I hope to avoid having to drill out any of the rivets. It's not a "deal-breaker", but it *will* make things a bit more complicated.
One area that *might* have been a "deal-breaker" has to do with the potentiometers used in the preamplifier. There are several "unusual" variants to "standard" potentiometers. One has a switch that operates at the END of the potentiometer shaft's rotation. Several of the potentiometers have one (or more) "taps" at various locations along the resistance element. These controls are *also* made from "unobtanium", but fortunately, preliminary testing shows that the original potentiometers will *probably* function ok. (I'll explain the method used to test the potentiometers in out next update.)
That's it for now. But we'll be getting "down to business" during the next few days (once the final photos of the Dynaco FM-3 tuner are posted). In our next update, we'll discuss the various "sections" we'll be restoring. There's LOTS of fun ahead so get ready to delve into one of the more "intricate" preamplifiers of the "Golden Age of Hi-Fi." Hopefully, we'll ALL learn something!
Sincerely,
Bill Thomas
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Hey Bill,
I want to add something to your bit of history. Mr Scott and another man started building microphones before the Depression and was $5000 in the hole during the Depression and almost lost it all. By the end of the of the Depression, the other man had left and Mr Scott, he had expanded his business and was growing. This is from a website I found by searching for information on this receiver that I bought over the weekend. Its a HH Scott. Pretty nice.
Steve
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It's AMAZING how many folks teetered on the "brink" of disaster, yet managed to persevere and "save their bacon" at the very last second. There's a HUGE "back story" to Dr. Scott and his Chief Research Engineer, Daniel von Recklinghausen. To read more about these folks, or to locate information regarding your H. H. Scott product, you NEED to visit www.hhscott.com (http://www.hhscott.com). Don't just use the tabs on top of the page. Instead, take a look at the site map to uncover a "Treasure Trove" of information. You will be AMAZED!
Thanks for the information, Steve! I appreciate it!
Sincerely,
Bill Thomas
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Very informative!
I've long been an admirer of Scott equipment. I'd love to hear one of (Mapleshade's) rebuilt Scott integrateds.
Don
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Bill, lots of luck with that unit! I've been slowly working on a 222c that has similar point to point wiring and rivited in tube sockets. Unfortunately, some of the solder tags on the sockets are just too fragile and break easily when removing old parts - I've had to replace two sockets already. I sure agree this beast is more complicated than your average Dynaco!
An interesting preamp that is "in between" is the Heath WA-P2. It has two switches to set phono playback equilization and 3 tubes (2 x 12AX7, 1x 12AU7). The up side is that it uses standard pots in the tone circuit and pretty easy to get filter cap. Of course it requires an external raw B+ and heater, but that's not necessarily a bad thing. I snagged an UNBUILT kit off ebay a few months ago and had a blast putting it together. Along side one I already had (from my late grandfather) it made a stereo pair. Sounded great, but I got a bit frustrated with trying to balance two volume controls. Like the Scott unit, the back of the Heath features level controls for each input, and lots of space between jacks, nice for modern size RCA connectors. Anyway, best of luck on your project!
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August 13th, 2009 - Update: "Formulating a 'Plan of Attack'!"
Rebuilding a complicated piece of electronics like the H. H. Scott 121-C preamplifier requires a LOT of preparation before the *FIRST* component is replaced. But before deciding to tackle a project like this, a few "tests" need to be performed. Here's the problem: As mentioned in our first "update", there are quite a few potentiometers and switches that are now made of "unobtanium." It would do us little good to completely replace the passive components in the preamplifier if the Volume Control is no good. There is NO suitable replacement available ANYWHERE. The same holds true for MANY of the potentiometers used in the 121-C. It would sure be nice if we had some way of "qualifying" the potentiometers. Oh sure, we can *try* to use an Ohm meter to see if they are continuous, but that will tell us VERY little about how *noisy* they are. So, what do we do? Try THIS:
Get a 1.5 Volt battery and a 1K resistor. Wire the resistor in series with the positive terminal of the battery and apply the other end of the resistor to the end of each potentiometer. Connect the negative side of the battery to the other end of the potentiometer under test. Now, put a capacitor in series with the Input to any general-purpose audio amplifier. Connect the other end of the capacitor to the wiper of the control to be tested. (Ground the amplifier to the negative side of the battery. Now you can rotate the control and listen to hear how "scratchy" the potentiometer is. If you run into a scratchy one, give it a shot of deoxIT and try it again. If it is too noisy to use, you *might* want to look for another "candidate" to rebuild. It's a VERY simple method, but it provides a very useful "go, no-go" result. As it turned out, a few of the potentiometers were a *little* scratchy prior to cleaning, but settled down *quite* nicely after treatment with deoxIT. That means, we're off to the races! With THAT little detail out of the way, it's time to formulate a "battle plan."
Documentation for the 121-C preamplifier is "sketchy", at best. The H. H. Scott website doesn't have a copy of the manual OR the schematic. I WAS able to find a copy of a Howard W. Sams schematic, but that's ALL I was able to come up with. No parts list. None of the usual photos in a Sams Photofact. Just the schematic. That means we will have a LOT more work to do. It all starts with LOTS of pictures! It'll also test my abilities as a "sketch artist" because we will need an accurate "pictorial" of the various parts of the preamplifier. This will all take a good bit of time. But in order to do ANY of this, we need to get a better look at the "innards" of this beauty.
The first step in dis-assembly is to remove the knobs. Here's a picture of the rear panel after losing all its knobs:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SoTFgh23ysI/AAAAAAAABJM/0X9Z8Af-8yo/s720/Scott%20121-C%20-%20Rear%20View%20-%20knobless.JPG)
ALL of the knobs on the preamplifier are secured by slotted set screws. The front knobs are anodized aluminum, while the rear knobs are plastic. (They "appear" to be made from bakelite, but they could easily be made from some other plastic. It doesn't really matter. We'll clean the rear knobs using dish-washing detergent. Since they appear to be in good condition, I'd prefer to avoid using anything that might scour or melt them, so let's just rely on Dawn liquid dish-washing soap and water. (I'll try to get a few Before and After shots of the results tomorrow - weather permitting.)
The anodized aluminum knobs on the front panel will require a bit more care. It is all too easy to scratch or damage the soft aluminum, so we'll start with, (you guessed it) Dawn dish-washing liquid and water! If THAT doesn't get them looking better, we'll get a bit more "assertive" in our cleaning efforts.
Once the knobs have been removed from the controls, remove the eight "sheet metal" nuts from the controls on the Front Panel. This will allow the anodized aluminum face plate to be removed. Next, remove both the front and the rear shields from the top of the chassis. Here's a picture of the result:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SoTFhCp3thI/AAAAAAAABJQ/Wr_z-nCJmU4/s640/Scott%20121-C%20-%20Top%20View-shields%20removed.JPG)
Now we can get a MUCH better look at the wiring and components behind the front panel. Here's a picture:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SoTFgGYBVqI/AAAAAAAABJE/TUUul_Q9GQQ/s720/Scott%20121-C%20-%20Front%20Panel%20-%20Rear%20View.JPG)
No doubt about it, we have our work cut out for us! One thing I noticed after removing the rear shield was a portion of the wire loom that covered the wires to the Power Switch had been "gnawed." A few "remnants" of the loom were found elsewhere in the "guts" of the preamplifier. Well, at least the mice had taste! Here's a shot of the damage:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SoTFgCFfW0I/AAAAAAAABJI/f7aKkAQPCEY/s640/Scott%20121-C%20-%20Gnawed%20Tubing.JPG)
There are only two heavy wires running inside this wire loom. Both of them go directly from the Power Switch to the back panel of the preamplifier. If that mouse had been just a *teensy* bit more "dedicated" to the task, we would have found a "mouse carcass" instead of just the damage to the loom.
Our next step is to take HUNDREDS of photos of the wiring and components. Our goal is to "map" every square inch of the unit so we can properly replace each component and orient it in its original location. These photos MUST be VERY well focused and a good macro capability is ESSENTIAL in order to achieve success. Here's an example of what I am talking about:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SoTMVpuqlcI/AAAAAAAABJY/EsqRiLk4xFw/s640/Scott%20121-C%20-%20Wiring%20-%20close%20up%20behind%20Volume%20Control%20%28sample%29.JPG)
We will not remove one single component until EVERY connection and EVERY component has been photographed. (I spent about four hours and wound up with over 800 photos!) It is often necessary to take multiple photos from different angles in order to know EXACTLY how a part is terminated, or how a wire is routed. DO NOT SKIMP ON THIS!!! There is no such thing as too many photos!
Once you have your photos safely transferred to your computer, it's time to get an old-fashioned pad and pencil. It doesn't have to be fancy, but you will NEED to make a diagram of how EVERY part and EVERY wire is connected. Don't rely on the hundreds of photos you take and NEVER rely on being able to just use the schematic to rebuild the unit. EVERY component MUST be detailed in your "home-brew pictorial" of the preamplifier. Make the diagrams BIG. You don't want to struggle to see how a part connects to a tube socket or terminal strip.
I think this is enough to "chew on" for now. Just remember: if you rely on your memory alone to rebuild your unit, you are almost CERTAINLY doomed to failure, or to a unit that will have a "mysterious 'hum'" that you simply can't track down. These tube preamplifiers are VERY high impedance devices. It takes VERY little in the way of mis-routing of components to cause problems. It would be a real SHAME if we wound up with something that was a poorer performer than the one we started out with.
In our next installment, we'll actually remove THREE parts! But those three parts are the foundation of everything else this unit is designed to accomplish. It shouldn't be TOO hard to guess which parts we will be removing. Meanwhile, it's time to go through the schematic and build up a complete list of resistors and capacitors. But don't be surprised if a few component values have been "juggled" between the schematic and the actual unit. If that turns out to be the case, use the value of the actual component used in the preamplifier, rather than the value indicated on the schematic. It's a pretty safe bet that this unit was once a "working" unit. But it is not unheard of for there to be mistakes in a Sams Photofact. It's not common, but it can happen.
Whew! that's a LOT of stuff to digest, but proper planning NOW will save you a TRUCKLOAD of problems later. Until our next update, keep those platters spinning!
Sincerely,
Bill Thomas
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Bill,
You might try toothpaste and a soft bristle toothbrush on those aluminum knobs thats what I use on my vintage stuff. HTH
Steve
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Hi Bill,
Looking forward to your ongoing updates. Over the past few years I've been slowly restoring my HH Scott 299A. Just so happens last week I pulled it out and its on my workbench again as I want to finish the restoration before the end of next month for our annual party we host for the Gateway Audio Society.
Its good advice you are giving so far. I do almost the exact same thing with tons of pics and the occasional hand sketch. I usually do one part at a time so a sketch isn't always required unless its removing layers of passives to get at a part below.
Rebuilds on gear like this are tedious. Its amazing how many parts are in an old Scott and just how many need to be replaced to make it reliable again.
Right now I'm going back and forth about removing the manual bias pots and replacing them with fixed resistors. On one hand I'll not have to worry about biasing (which is a pain on the 299) or the pot going/being bad but on the other hand the integrated looses some of its charm and originality. I'm leaning toward fixed bias as I'll probably never get rid of the amp. I picked it up for a song and its cosmetically perfect including the lettering. Even the gold on the knobs is still all there and not worn by age.
This weekend I'll get into removing the 'doghouses' and replacing the caps in the tone controls and phonostage. I might get a start on the power supply but I need to order in the first dropping resistor after the rectifier. I need to see what the new voltage is before I get that one on order. I'm saving the worst for last, the can caps and doing the fixed bias.
I need to immerse myself back into the schematic and the physical layout as its been a couple of years since I worked on it.
These are definitely fun projects for us hobbyists to take on. Word of warning to those reading, these rebuilds are not for the novice...at least with a Scott. If you've even been under the hood of one of these, it is quite menacing. You need to be able to read a schematic and trace circuitry fairly well before tackling a project like this otherwise you are better off sending it to someone like Bill to rebuild who obviously knows what hes doing.
Oh, back to the mono front...I've got an old Stromberg Carlson AR-425 that I use to play my 78s. I use a Dual 1229 turntable and an Altec 824 Iconic speaker. The Stromberg has the variable phono and tape EQing. People are amazed at just how good 78s actually sound. I haven't restored the Stromberg but shes still playing strong being well over a half century young (when I got her I brought her up really slow with my variac to reform the caps, none were leaking). I may just break that system out for our big party for those who have never heard mono and 78s.
Hey, what are your thoughts about going with a fixed bias (knowing I'm not worried about keeping things 'original')?
Anyway, I'll be reading your updates :thumb: Great article :thumb:
PS. You should get a hold of Steve Rochlin (my editor at EnjoyTheMusic.com). I'd bet he'd be thrilled to publish this restoration article you are writing.
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Thanks, guys! I appreciate the kind words. It kinda "spurs me on" to the next phase of the rebuild process, since I have already stuck my neck out and am patiently waiting for the "grim reaper" to stop by for a visit.
The toothpaste tip is a good one! My *favorite* use for "toothpaste therapy" is to use it to bring back fogged plastic "lenses" on automotive instrument clusters, or to *gently* polish a fogged "implosion shield" over a late 50's television's CRT. If the knobs don't clean up well using the *mild soap* approach, it's always good to have another "gentle" treatment available before breaking out the "heavy artillery!"
Scott, my first (otherwise known as "gut") reaction would be to keep the bias arrangement in "stock" form. With the cost of good Output tubes rising almost *hourly*, it makes sense to me to be able to reset the bias for longer life. But that's just *my* humble opinion. Sorry to hear about the weak tube socket lugs. I'm about to find out just HOW weak they really are. (Say a little prayer on my behalf!)
We probably won't have a big "update" today. I'm not *quite* ready to snap the next round of photos and the skies are clouding up for our usual afternoon "Monsoon" rain that we've imported from South Florida this year. Rest assured that progress is being made. I am just about to remove the "three parts" I mentioned in the last update. (Actually, I will be removing SIX parts before I'm finished today, but the first three are the ones that will require a LOT of attention. Two of the others will be set aside for restoration, while the last one will be added to the "dead components pile."
Once those parts are removed, I will be making preparations to remove the front panel. THAT will take a LOT of preparation due to the number of wires and parts that connect with the main chassis. Once that is accomplished, I'll be ready to post another "update." We'll be ready to start "replacing stuff" in the next few days (I *hope*!) Meanwhile, I'll be breaking out my arsenal of cleaning supplies in order to make an attempt at improving the "aesthetics" of the unit. Preliminary cleaning has given me some hope that I will be able to avoid having to use the "BIG guns" to do the cleaning. Let's HOPE so anyway.
More to come soon!
Sincerely,
Bill Thomas
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Sunday, August 16th, 2009 - Update: "It's a DIRTY Job!!!" - Part One
As promised, we have finally gotten around to removing *something* - THREE somethings. And THESE babies are gonna take a LOT of "sweat equity" to straighten out. Here's where they USED to reside:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SoiYdtncPxI/AAAAAAAABJc/dMnidHnLEcA/s640/Main%20Chassis%20-%20Quad%20caps%20removed.JPG)
When it comes to rebuilding ANY "Vintage" electronic device, one of the BIGGEST problems is finding replacement capacitors for the original "twist-loc" can capacitors. A small number of can capacitors are being manufactured in the US and there are also new ones being manufactured in Germany, but the three can capacitors inside the Scott 121-C have LONG been unavailable. The Power Supply is the absolute *foundation* upon which the rest of the device is constructed. If we expect to get another lifetime of service from this preamplifier, these capacitors MUST be replaced. Besides, we have evidence that this unit was stored for a LONG time in a hot environment - probably an attic. (More on *that* later.)
This gives us only three choices: We can try to add some extra terminal strips and *squeeze* discrete replacements under the chassis, but there really isn't a lot of room under there and it runs the risk of increasing hum due to inevitable wire re-routing. OR, We could design a Printed Circuit Board that would fit the top of the chassis, but that would run even MORE risk of additional hum, it would put some pretty high Voltage on TOP of the chassis and it can be argued that a Printed Circuit board would be "out of context" in a "point-to-point" wired preamplifier. It would certainly be a much "busier" solution. What's left? Re-stuffing!
If someone told me I would be "re-stuffing" quad capacitors, I would have ASSURED them that it's time to take off the tin-foil hats! This is uncharted territory for me. I have NEVER had a strong desire to slice open a can capacitor. That simply means, I have never re-stuffed a can capacitor! But apparently, a lot of other folks have, so let's give it a try and see what we wind up with.
The first task is to slice open the case of the can capacitor. There are LOTS of ways to actually do this. Some people use a pipe cutter. Other use a Dremel tool with a cutoff wheel. If you're EXTREMELY careful, I suppose you could even use a hack saw. I used... my Brother (and a small "hobby lathe") to "do the dirty work" of cutting the can capacitor open. My Brother just happens to LOVE doing lathe work. He graciously offered to help me "cut 'em open." (This step alone *probably* saved me several hours of frustration. While I've certainly done some lathe work myself, there comes a time when you defer to those possessing a higher "skill level" than YOU do. This was one of THOSE times!
Well, "He done good!" Here's a picture:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SoiYdi1DxLI/AAAAAAAABJg/Cc3_9uLvGF4/s512/Quad%20Cap%20-%20separating.JPG)
Looks simple, doesn't it? Well, forget THAT! The cutting was the EASY part. Things are about to take a real UGLY turn! The capacitor shown above simply slid out of the can, but the other two had NO such intentions! They were FIRMLY and SOLIDLY anchored to the can. The solution is controlled heat. (Never let it be said that I suggested you use the household oven to heat these things, but if you DO, set it to for around 325 degrees.) Your goal is to heat the can until the tar-like "goo" in the can releases the "core" of the part. You'll still have to yank and wiggle and twist the thing, but eventually, it WILL come out.
So what does the "core" of a can capacitor look like? Like this:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SoiYd6vH5dI/AAAAAAAABJk/zqJkBJ3sp5g/s512/Quad%20Cap%20-%20guts.JPG)
You can get a better idea how one is constructed by looking at the end of the "core", see?:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SoiYd3sTMDI/AAAAAAAABJo/syieweFJdZg/s512/Quad%20Cap%20-%20guts%20-%20top%20view.JPG)
This is not the only type of construction used, but it was the most common. Of course, it doesn't really matter because this "core" is headed for the "capacitor graveyard." We're going to re-use the base (containing the terminals), but we're going to stuff four nice, new individual electrolytic capacitors inside the can where this old "core" used to reside. But before we can do that, we have a rather onerous job ahead of us. We have to clean out all three cans before we can "stuff" them with new parts. What do we have to clean out? This:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SoiYeOaUOQI/AAAAAAAABJs/34snSwbarKA/s512/Quad%20Cap%20-%20internal%20tar.JPG)
THIS is some truly NASTY stuff. It looks like tar, it SMELLS like tar and it *acts* like tar! This is the stuff that glued the "core" inside the can. It all has to go!
There are HORROR stories floating around the web about attempts to remove this stuff. But I'm about to save you a TON of unnecessary scraping and digging. The secret is lacquer thinner! It takes time and it takes work, but lacquer thinner will *eventually* allow you to get the inside of the can acceptably clean. It will take several applications of fresh lacquer thinner and quite a bit of "digging" to get the gooey stuff to release. It will also take a good deal of time. It's a messy, nasty job, it really STINKS and the used lacquer thinner should be disposed of properly. Before you simply dump it down the drain, remember that lacquer thinner will melt PVC and other plastics (and I doubt your local waste treatment facility will appreciate having to deal with it as well.) The GOOD news is you won't have to deal with any PCB's. PCB's were used in the manufacture of *some* capacitors and transformers, but usually MUCH higher-powered stuff. These small capacitors are made with *relatively* benign materials and chemicals. Expect to spend MANY hours soaking, digging and cleaning these cans, but *eventually* you will be rewarded with a clean can. (OK, no snide remarks about my "clean can.") Here's what I'm, *really* talking about:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SoiY8d1nQUI/AAAAAAAABJ0/SK7bq7mVWjE/s512/Quad%20Cap%20-%20internal%20tar%20removed.JPG)
Since the ONLY way to effectively join the two pieces of the can capacitor back together will be to use a STRONG bonding agent, we want the can as clean as possible. When you get the "chunks" of tar out of the can, you'll have a slimy goo left. More lacquer thinner and steel wool will remove that and leave the surface fairly clean, but for better results, finish the cleanup with SOS pads and water. Dry thoroughly and set the cans aside for later reassembly. But before we put this one aside, here's a picture showing our "test fit" of the new capacitors:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SoiY8dI2q2I/AAAAAAAABJ4/qBjep17c_Fg/s512/Quad%20Cap%20-%20new%20capacitor%20fit.JPG)
Fortunately for us, modern electrolytic capacitors tend to be MUCH smaller than the parts we could buy 50 years ago, so we'll have no problem fitting individual new capacitors inside the original can.
I had hoped to get these three capacitors rebuilt over the weekend, but perfecting the "tar cleanout process" ate up over 18 hours, and we STILL have to clean the original base "buttons", install the new capacitors, encapsulate them inside the can and epoxy the can halves back together. But the end result will be worth it (I *hope*!) By re-installing the rebuilt capacitors, parts and wire positioning inside the preamplifier can remain "stock" which will minimize any increase in additional hum.
In the second part of this "capacitor update", we'll take a look at the "reconstruction phase" of our capacitor rebuild. (I feel the excitement level building as we speak!) More details to follow!
Sincerely,
Bill Thomas
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August 21st, 2009 - Update: "It's a Dirty Job - Part Two"
Our "Update Schedule" is *somewhat* up to the mercy of the "elements" since I depend upon sunlight for the photos you see in these updates. I have tried to use indoor lighting to shoot photos, but NOTHING beats a nice sunny day. But, when God says "it's time for rain", the quantity of sunlight becomes somewhat scarce. That means our updates *occasionally* have to wait for a sunnier day. Well, we *finally* got enough light to "press on regardless."
With that in mind, the next step in our "capacitor re-stuffing operation" has to do with the removal of the old capacitor "guts." In a way, it's not unlike unrolling a roll of paper towels. The four sections of the original quad capacitor are manufactured concentrically; similar to the construction of a transformer. To remove the "guts", all we have to do is UN-roll the capacitor. The capacitor "roll" is held together with Cellophane Tape. Remove the tape and un-roll the capacitor "element" from the terminals. When you get done (and after you clean the base) this is what you'll see:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/So7tg7HiX4I/AAAAAAAABKU/PCXoMf_LgY8/s512/Quad%20Cap%20-%20guts%20removed.JPG)
The four "electrodes" you see in the center of the photo are the connections to the four different sections of the capacitor. They are exposed as you unroll the entire capacitor from the base. The fifth "electrode" is the common ground electrode. It snakes its way through the capacitor "roll" and connects to the common edge of all four individual elements. These "electrodes" are made of VERY soft metal that is easily broken. Good thing we won't really need them.
By the way, as you unroll the capacitor elements, you can get a good idea of the condition of the capacitor. In the case of our three quad can capacitors, the electrolyte "paste" was heavily crystallized and in two of the capacitors, a significant quantity of the actual foil had decomposed and was GONE! If you have ever seen the base of a failed electrolytic capacitor, you'll *usually* see a light beige-colored crystallized deposit oozing from the terminal area. There is NO question that all three of these can capacitors were, at BEST, on their last legs.
Since we're not using those frail "electrodes" to make the connections to our discrete capacitors, we need to drill some small holes in the base of the quad cap to allow the individual wires to attach to each of the four terminals, like this:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/So7--tt2NRI/AAAAAAAABLI/l3eYTWjmBOc/s640/Quad%20Cap%20-%20Base%20with%20holes%20drilled%20for%20capacitor%20wires.JPG)
It's a *tad* blurry, but you can see the holes drilled next to each of the four terminals. There is another hole you really can't see in this picture. It is located adjacent to one of the grounding lugs and will provide our "common groun" for all four capacitors. (Ignore the thing that *looks* like a hole that is centered between the four terminals. It is simply a remnant of the capacitor being firmly chucked into the lathe in order to remove the can.
Now that the holes are drilled, we can do another "test fitting" of the individual capacitors. Here's a picture:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/So7zOoBEz5I/AAAAAAAABKg/euIk83-7iQk/s512/Quad%20Cap%20-%20initial%20test%20fit%20prior%20to%20soldering.JPG)
This picture of the base shows how the wires from the individual capacitors will protrude through the small holes we drilled in the base:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/So7zPJfWfmI/AAAAAAAABKo/haKkLHNbblM/s512/Quad%20Cap%20-%20Base%20with%20capacitor%20wires.JPG)
In this picture, you can see the fifth hole that is located next to one of the mounting lugs.
Now that we know everything will fit, let's "wire 'em up!" Here's a picture:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/So7zPcbCxLI/AAAAAAAABKs/-VhK8pqv_E8/s512/Quad%20Cap%20-%20Completed%20Internals.JPG)
Now, let's slide the can back on and check the final "fit" once again:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/So7zPjwB4DI/AAAAAAAABKw/H7TTkHNzWc0/s512/Quad%20Cap%20-%20Completed%20-%20partially%20covered.JPG)
Before anyone "takes me to task" about the capacitor values, you should know that I have increased the values of the 25 microfarad capacitors to 33 microfarads. Why? Because they don't MAKE 25 microfarad capacitors anymore. I *could* have subbed 22 microfarad capacitors, but that *might* have hurt the performance of the preamplifier. The 25 microfarad capacitors are used as Cathode Bypass capacitors. Increasing them to 33 microfarads will do no harm and *might* actually prove beneficial. Likewise, there is no such thing as a 300 microfarad capacitor, so I increased the value to 470 microfarads. The 300 microfarad capacitors are used in the filament supply for the "sensitive" stages in the preamplifier. Adding more capacitance can only improve the filtering a bit further.
This picture shows how the can will sit once we have glued the two halves together again:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/So75Zv48BII/AAAAAAAABK8/miHESHdpLFE/s512/Quad%20Cap%20-%20Completed%20-%20cover%20installed%20but%20not%20glued%20yet.JPG)
All three can capacitors have been constructed and will now be "glued" back together. (I'll show the final results in our next update.)
Before I close this "update", I wanted to share something funny with you. First, here's a picture of the "inspection stamp" that the factory applied to the chassis, indicating that the unit had been thoroughly inspected and approved:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/So75Z-WcxwI/AAAAAAAABLA/aCq1ZiDyMZQ/s720/Final%20Inspection%20Stamp.JPG)
The only problem is, they MISSED soldering one connection! Here's a picture:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/So75Z4PJMpI/AAAAAAAABLE/CiVLmWAPEuo/s576/Unsoldered%20wire%20on%20Selenium%20Rectifier.JPG)
That's right! One of the wires from the Power Transformer to the selenium rectifier was never soldered at the factory! Now, I'm SURE this unit probably operated PERFECTLY for many years, but it's also possible that the preamp suffered from a mysterious "noise" of undetermined origin. If it did, this would *certainly* explain the source of that noise!
Well, this seems like a *reasonable* stopping point for this update. In our next "update", we'll separate the front panel from the main chassis and also remove all the potentiometers from the rear panel. This will allow us to clean the main chassis without fear of destroying any parts that would be IMPOSSIBLE to source!
There's one thing I wanted to mention again: before attempting to dis-assemble ANYTHING, take LOTS of pictures and draw LOTS of pictorial diagrams! You don't need to be an artist, but make SURE you draw the positioning of the wires and parts. You can be certain that while you might see ten pictures in one of these updates, there are probably a HUNDRED more that you DON'T see. Every possible connection has been photographed. Every part and every wire has also been photographed from MANY different angles. When the time comes to re-assemble the unit, I'll be armed with HUNDREDS of photos and DOZENS of hand-written diagrams that *should* show how EVERY part and EVERY wire should be routed. (At least, let's HOPE so!)
Our next update will follow shortly. Until then, get those 78's ready for service! We'll be playing them in NO time!
Sincerely,
Bill Thomas
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Very interesting as always Bill. :thumb:
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Sunday, August 23rd, 2009 - Update: "Separation Anxiety!"
In EVERY total rebuild, there is a moment (or MANY moments) when you experience a feeling of: "What have I DONE??? I'll NEVER be able to put all this back together again!" Don't worry. It will pass. (As long as you have taken enough pictures and drawn enough pictorial diagrams!) The separation of the Front Panel from the Main Chassis is the FIRST of those moments on this project. Here's a picture of the Main Chassis with the Front Panel removed:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SpFe8tWFMvI/AAAAAAAABLc/fxwASObvx4M/s720/Main%20Chassis%20-%20Top%20-%20Front%20Panel%20removed.JPG)
And here's a picture of the separated Front Panel:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SpFe7wNqH7I/AAAAAAAABLQ/kyvLU9jDYlU/s576/Front%20Panel%20-%20removed%20-%20Top%20Shot.JPG)
Why should the project be so intimidating at this stage? Take a look at the underside of the Main Chassis:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SpFe8F98_JI/AAAAAAAABLU/MoXOUxA78w8/s720/Main%20Chassis%20-%20Bottom%20-%20Front%20Panel%20removed.JPG)
There are an AWFUL lot of wires and components that have been un-soldered or simply cut. NOW do you see why it is SO important to take many, MANY pictures of the original unit? If not, you WILL when we start re-assembling the preamp.
We have out work cut out for us. But the Main Chassis has been stripped as far as necessary now. All potentiometers have been removed. Any other components that might be damaged in the cleaning process have also been removed. The parts and wires that remain will all be replaced, so there is no need for concern over *their* survival. The only parts that will be re-used are the tube sockets, the terminal strips and the RCA jacks. Here's a picture of the Main Chassis showing the removal of the potentiometers from the rear panel:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SpFe8eidP5I/AAAAAAAABLY/9G-vXRWfYCU/s640/Main%20Chassis%20-%20Top%20-%20Front%20Panel%20removed%20-%20three-quarter%20shot.JPG)
The next step will be to thoroughly clean the topside of the Main Chassis. In the process, the bottom side of the chassis will also become cleaner, but it is not our primary concern. When we finish the cleaning process, the tube sockets will all have nice, shiny contacts and the chassis itself should be clean, shiny and bright. (Of course, we'll ALL see how THAT turns out in a later "update.")
It will not be possible to clean the Front Panel as thoroughly. There are simply too many switches that are riveted to the panel and they would NEVER survive intact. Instead, we will do a "cursory" cleaning of the front side of the Front Panel and *g-r-a-d-u-a-l-l-y* replace the passive components around each potentiometer and each switch. Of course, we will remove, clean and lubricate each potentiometer as we go along. We will also remove the two selector switches, dis-assemble them, thoroughly clean them and lubricate them, and then return them to the panel as we rebuild it. Simple, right? Well, take a look at the rear side of the Front Panel and YOU be the judge:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SpFe7mk3RRI/AAAAAAAABLM/PQCq16fsZ7s/s720/Front%20Panel%20-%20removed%20-%20Inside%20Shot.JPG)
Remember, ALL of the slide switches will remain attached to the panel. Everything else will be removed, cleaned and lubricated, and then returned to the panel as we begin to restore it with all new passive components.
There now. That wasn't so bad, was it? There's really no need to feel "anxious" as long as we take it slowly, work on a step-by-step basis and again, take LOTS of pictures and make LOTS of diagrams!
I had hoped to show our completed can capacitors, but they are currently "curing" and I would rather not disturb them until the curing time has completely elapsed. Pictures WILL follow as soon as possible.
That's it for this little "update." In our *next* chapter, we'll take a look at the results of our "chassis cleaning party." Meanwhile, I'll be preparing to rebuild the Front Panel. (I wonder if I can buy deoxIT by the gallon?) Just kidding!
Until our next "update", remember: "Always keep your powder dry!" (No, that has NOTHING to do with our project, but they can't all be "gems.") Seriously, we'll be posting another update within the next few days. I have high hopes that we'll have a nice, shiny chassis to show you.
Sincerely,
Bill Thomas
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You're a braver man than I am, Gunga Din! :thumb:
Don't suppose you have a link to a schematic for this beast? Fun to follow along in the journey.
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September 2nd, 2009 - Update: "Our 'Distinguished' Panel!"
We'll get to the rebuilding of the Front Panel in a moment, but first I wanted to share a "discovery" with you. When the Front Panel was separated from the Main Chassis, a NEW problem surfaced. Here's a picture:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Sp8u_nwbTTI/AAAAAAAABL4/sIrNQ7pb9rU/s640/Main%20Chassis%20-%20Bent.JPG)
With the Front Panel attached, it was hard to see how "tweaked" the Main Chassis had become. Once it was removed, the damage was painfully obvious. Somehow something heavy had slammed into the Power Transformer at some point in this preamplifier's life. The "hit" was SO hard that the corner of the Power Transformer wound up bending the Main Chassis. Unlike most other manufacturers, H. H. Scott used aluminum for the chassis parts. It was EASILY distorted by the "hit" the Power Transformer took. This will be straightened before the unit is reassembled, but it just goes to show you, you never know WHAT you'll find when you completely rebuild a unit like this. Now, on to the Front Panel!
Here's a picture of the right side of the Front Panel in "as-found" condition.
(http://lh6.ggpht.com/_Xp_4nF2BnZE/Sp8u_k9gj2I/AAAAAAAABL8/aE8VdhBxYMQ/s640/Front%20Panel%20-%20Right%20Side%20-%20As%20found.JPG)
Looks pretty "daunting" doesn't it? But we'll take it bit-by-bit and *gradually* replace all those resistors, capacitors and wires with all new parts. First, we need to remove a few things.
That Selector Switch contributes a HUGE amount of the "clutter" that makes this look so intimidating. Our first task is to isolate and remove it. In order to do that, we removed the two potentiometers to the left of that switch. Here's how that went:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Sp8u_x9mFbI/AAAAAAAABMA/D_s6YOmYcMw/s720/Front%20Panel%20-%20Middle%20-%202%20pots%20removed.JPG)
We removed these potentiometers because they wound up connecting to the switch we're removing, or they had only a couple of wires connecting them. First we made a diagram of their connections and then we yanked 'em!
Next, we disconnected the other wires leading to that Selector Switch and removed the Switch in order to isolate the right side of the panel so we can concentrate on rebuilding it. Here's how that looks:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Sp8vAMKJmtI/AAAAAAAABME/vv_OWF7AmsY/s720/Front%20Panel%20-%20Right%20Side%20-%20Switch%20Removed.JPG)
In reality, there were only about six wires connecting the Selector Switch to the other components on the Front Panel. Removing the Switch sure got rid of some pretty intimidating "clutter." Here's why:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/Sp8vAd59s-I/AAAAAAAABMI/cDpu1Hfr-64/s512/Selector%20Switch%20-%20as%20removed.JPG)
We'll address this "clutter" when we clean, lubricate and rewire the Switch itself prior to re-installing it on the Panel.
Now it's time to "get down to business" and rebuild the right side of the Front Panel. We start the same way we've started EVERYTHING involved with the preamplifier: More diagrams! SEVERAL more diagrams. By breaking the task down into several "sections", (and making several diagrams of each "section"), our "intimidating project" gets cut down to size! Here's a picture with about 75% of the parts removed:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Sp8vxu4Dh2I/AAAAAAAABMQ/eo3k0voWfBU/s720/Front%20Panel%20-%20Right%20Side%20-%2075%20percent%20stripped.JPG)
Without LOTS of diagrams, things would NEVER be replaced in the proper positions. But WITH lots of diagrams and lots of pictures, it becomes MUCH simpler. Here's a picture with about half of the rebuilding completed:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sp8vxyMd0UI/AAAAAAAABMU/DQVUE_4B8fM/s640/Front%20Panel%20-%20Right%20Side%20-%20Rebuilt%20-%2033%20percent%20completed.JPG)
And here's a picture with this portion of the Front Panel completed:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/Sp8vyEZ3s6I/AAAAAAAABMY/AHmc3Cd1Zlg/s720/Front%20Panel%20-%20Right%20Side%20-%20Rebuilt%20-%20completed.JPG)
You'll notice that all the parts are *basically* in the same locations as the original parts. Some of the new capacitors are quite a bit smaller than the originals, but that just allows us to make the whole thing "neater." You'll also notice that I prefer a bit more insulation on my component leads. It takes VERY little time to add a bit of heat-shrink tubing to the resistor and capacitor leads. The extra insulation certainly can't hurt and it might just help. It also lends a bit of "uniformity" to the visual "presentation" that also adds to the "neatness factor." This is a high impedance circuit so "wire dress" becomes quite important in order to minimize hum. Signal carrying wires are routed against the metal panel wherever possible. Again, this is basically the way the original wiring was "dressed."
Take a moment to compare the "before" and the "after" pictures. Here's the way things looked before the rebuild:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Sp8vAMKJmtI/AAAAAAAABME/vv_OWF7AmsY/s720/Front%20Panel%20-%20Right%20Side%20-%20Switch%20Removed.JPG)
And this is how things look AFTER the rebuild:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/Sp8vyEZ3s6I/AAAAAAAABMY/AHmc3Cd1Zlg/s720/Front%20Panel%20-%20Right%20Side%20-%20Rebuilt%20-%20completed.JPG)
I don't know about you, but to MY eyes, things look a LOT neater now. This should go a long way to make SURE the preamplifier will deliver another lifetime of faithful service.
I should mention that all of the potentiometers have been cleaned, de-oxidized, cleaned again and then treated with contact "conditioner." There were several slightly "noisy" controls when we started out, but all of them are now smooth and quiet - except ONE! Suffice it to say, one of our potentiometers had a problem that *almost* "sank" this entire project. We'll cover that in our next update, coming up almost immediately!
Restoring a vintage unit is a LOT like restoring a vintage car. You take LOTS of pictures, search for as much documentation you can find, you discover new "areas of concern" as things are dis-assembled and you clean, clean and CLEAN everything as much as possible. Along the way, you'll almost certainly discover a potential "roadblock" that may *seem* insurmountable, but with enough determination and a LOT of luck, you can find a way to work around the problem. I'll show you what I'm talking about in our next update. It's coming right up over most of this same station. Don't touch that dial just yet!
Sincerely,
Bill Thomas
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September 3rd, 2009 - Update: "Trouble! With a Capitol 'T' and That Rhymes With 'P' and That Stands for POT!"
The H. H. Scott 121-C uses quite a few potentiometers that are "single-purpose specific." In other words, they were manufactured exclusively for this particular unit. One of them is a 1 Meg potentiometer that has a switch. I know it doesn't sound THAT "exotic", but the switch activates at full CLOCKWISE rotation, rather than the usual counter-clockwise rotation. This particular potentiometer is used to set the "Rolloff" point of the Phono Equalization networks. It was manufactured by Clarostat and *probably* cost H. H. Scott less than a Dollar!
Imagine my surprise when little black "shavings" and "curls" of the resistance element began falling out of the control. A check with an Ohm meter revealed that the control was open! The switch was still fine. Needless to say, the odds of finding another control like this one were *slightly* poorer than hitting the Powerball Jackpot without buying a lottery ticket! Since this preamplifier is going to be used to play 78's, this control is ESSENTIAL! OK, let's investigate further. Here's a picture of the dis-assembled control:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Sp9K7tfzf7I/AAAAAAAABMc/Wk2lKm7oVao/s720/Potentiometer%20-%20exploded%20-%20original.JPG)
If you'll look at the phenolic wafer in the center of the picture, you'll notice that there is no resistance element at all. You can see a "tab" under the left terminal which was part of the substrate the resistance element was applied to. (You'll also notice that the middle terminal is missing, but that was MY doing.) This "exploded" shot shows how the control is assembled. The shaft has a clip (similar to a "C-clip") that keeps the shaft assembly in place. You literally have to "bend" that clip to allow the shaft to be removed from the front bearing and frame assembly. Once it is removed, you'll see the contact assembly that actually "wipes" the resistance element:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/Sp9K7-OEUOI/AAAAAAAABMg/-8taDxtZGqs/s512/Potentiometer%20-%20rotor%20and%20shaft%20-%20original.JPG)
The two tiny contacts on the left side of the picture are the wiper contacts. The "ring" inside these contacts is used to connect to the middle terminal by way of a "slip ring" assembly that is riveted to the center terminal. (This is the part I "clipped out" of the original control. You'll see why in a moment.)
What we NEED at this point is a new resistance element. But finding one that will be an exact match to the original one is pretty unlikely. But Radio Shack has a 1 Meg control available. Could we somehow use the resistance element in that control? Let's take a look!
Here's a Radio Shack potentiometer, manufactured by Alpha:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/Sp9LaVLi46I/AAAAAAAABM8/re3GlU788YI/s720/Radio%20Shack%20Potentiometer%20-%20Alpha.JPG)
While there are *some* simi8larities, it is pretty obvious that the construction details differ quite a bit. For one thing, the shaft doesn't have a "clip" to hold the assembly together. OK, let's tear into it to see what we DO have.
Here's a picture of the shaft assembly with the rotor assembly removed:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sp9K8OHclfI/AAAAAAAABMo/mULdd2TiM9o/s512/Radio%20Shack%20Potentiometer%20-%20Alpha%20-%20gutted.JPG)
It is the rotor assembly itself that is "staked" onto the shaft that holds the whole thing together. There are other differences too. Take a look at the contact assembly used in this Alpha potentiometer:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sp9K7yS93SI/AAAAAAAABMk/aOMNF2rKoS4/s512/Potentiometer%20-%20rotor%20-%20Alpha.JPG)
Notice that this contact assembly has three wiper contacts, but it also has two OTHER contacts on the right side. These make contact with a conductive "ring" on the resistance element phenolic wafer. This is VERY different from the original Clarostat arrangement. Still, the 1 Meg element is correct. If we could simply mount the slip-ring assembly to the wafer, we *might* be able to use the Clarostat shaft with the Alpha resistance element and wafer. But how in the WORLD will we find some way to mount the slip ring assembly?
The answer is as close as your local Hobby Shop that specializes in H. O. Trains! It turns out that a 0-90 screw and two nuts were all we needed. Ever heard of a 0-90 screw? Neither had I. But it was a tight fit inside the center eyelet of the Alpha contact assembly. This is a GOOD thing. It will keep the slip ring assembly located properly. Here's a picture:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sp9LaHbYnQI/AAAAAAAABM4/bLvRCGXuAew/s512/Radio%20Shack%20Potentiometer%20-%20modified%20element%20with%20slip-ring%20added.JPG)
Since the only screw I was able to find was a flat head screw, it required two nuts to keep the slip ring assembly nice and tight - one on either side of the phenolic wafer. A "test fit" of the original Clarostat rotor and the Alpha resistance element shows that this just *might* work:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Sp9Ladu0xXI/AAAAAAAABNA/gJxt-UI0-Rw/s512/Potentiometer%20-%20rotor%20and%20shaft%20-%20original%20plus%20new%20element.JPG)
But there were a few other "modifications" that had to be made in order for things to "fit." The front bushing was MUCH larger than the bushing area on the Alpha control, so we had to "chamfer" the hole to allow things to sit flat:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/Sp9K8MJnzRI/AAAAAAAABMs/H2TPLf19iX0/s640/Radio%20Shack%20Potentiometer%20-%20Alpha%20-%20modified%20element.JPG)
We also had to make some new "notches" for the tabs on the original control case to fit properly. While this isn't what I would call an "elegant" fix, it WILL work. When assembled, the control operates properly and the switch still operates at the maximum clockwise rotation. The resulting combination duplicates the function of the original Clarostat control. It only cost us $2.95 for the Radio Shack part (but it DID cost us about a week in lost time on the project.)
It's a good thing the tapped potentiometers all survived with "flying colors". There is ZERO likelihood that we would have been able to find a way to "make" a replacement for one of those.
Well, that's it for this installment. It's back to the Front Panel rebuilding for the next few days. I'll post an update when there is something *meaningful* to report. Until then, stay with us!
Sincerely,
Bill Thomas
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Beautiful work Bill. I can only imagine the patience it takes to complete such a restoration.
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Dear Bill,
Thank you for your VERY kind words. It'll take me all day to wipe this smile off my face. Coming from you, I find this to be an EXTREME compliment and deeply appreciated. YOUR work and your products are absolutely beyond ANY sort of reproach!
Although, *I* am not really the patient one. The owner of this unit is the one who has incredible patience. It's just *my* humble job to make sure his patience will be rewarded. I'm doing my absolute BEST to make sure this is the case.
MOST Sincerely,
Bill Thomas
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September 11th, 2009 - Update: "Our Latest 'Panel Discussion'!"
Yes, I have something "meaningful" to report! Our Front Panel is *almost* finished! It *would* have been totally finished, but we have hit another "glitch" along the way that will require a *bit* more work. But let's take a look at the latest progress.
In rebuilding a complex unit (with VERY little documentation), I mentioned that it is best to divide the restoration into "sections." The Front Panel can be conveniently divided into three sections and five tasks:
1. Rebuilding the Right-hand section (The Volume and Tone Control areas)
2. Rebuilding the Left-hand section (The Phono EQ/Dynaural section)
3. Rebuilding the Center section (containing the pilot and switch wiring)
4. Restoring the Main Selector Switch and pre-wiring it for installation
5. Restoring the Dynaural Range Switch and pre-wiring it as well.
Of course, then we have to install those switches and wire them into the panel. Piece of cake, right? Well...
Here's a picture of the Left side of the Front Panel in "as-found" condition:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SqoOyt6GcoI/AAAAAAAABNc/BaaiLlu7Re4/s720/Front%20Panel%20-%20Left%20Side%20-%20As%20found.JPG)
There's a LOT of work to be done here, but by making enough pictorial diagrams of the sub-assemblies, it's fairly straightforward - just VERY time-consuming! Here's a picture of this section after almost all the parts and wiring have been removed:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SqoPmhbE1HI/AAAAAAAABOM/6P6mGCDL6Ic/s720/Front%20Panel%20-%20Center%20area%20-%20Stripped.JPG)
Looks pretty naked, doesn't it? You'll notice that the Main Selector Switch has been dis-assembled. The ONLY way to do a proper job in restoring the two sections of this switch is to dis-assemble it and thoroughly clean and de-oxidize the contact surfaces. There are several parts that are "sandwiched" between the two sections, so it also makes sense to pre-wire the two wafers first, and THEN re-assemble the switch.
Let's start by working on the Rear Wafer of the Main Selector Switch. This section does the actual switching between the various sources. The Input Jacks on the rear panel are connected to this switch by wires that are routed through a shielded casing. To make things a bit easier, the wires are color coded and I have duplicated the color scheme of the original wiring. Here's a picture of the rear wafer after cleaning, with the new Input Wires connected:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SqoPQhtdspI/AAAAAAAABN8/4hHpYObdCH0/s512/Selector%20Switch%20-%20Rear%20Section%20-%20cleaned%20rotor.JPG)
The actual contact surfaces are now clean and bright. They are silver-plated, so they won't *stay* that way for long, but it's always good to start out with nice, clean contacts. As the silver surface is exposed to the atmosphere, it will develop an oxidized surface that appears black. Fortunately, silver-oxide is conductive and will not harm the function of the switch, or the electrical integrity of the connections. Here's a picture of the shielded wiring from a different angle:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SqoPQh23qhI/AAAAAAAABOA/QJjExUgsdAo/s512/Selector%20Switch%20-%20Rear%20Section%20-%20with%20Shielded%20wiring%20-%20alt%20view.JPG)
The black and the green wires are original wires that will also be replaced before we're finished. I'm just keeping them installed at *this* point in order to use them as "place-holders" to help me remember where everything goes.
The Front Section of the Main Selector Switch is used for the low-level switching functions; Phono, Tape Head, etc. The connections to the rear-panel are also routed through another shileded "tube". The Front Switch Section also has a few components on it. These parts and wires would be nearly impossible to install if the switch were re-assembled, so we're doing it now. Here's a picture:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SqoPQHnn89I/AAAAAAAABN0/gyZ1adx2naw/s720/Selector%20Switch%20-%20Front%20Section%20-%20Initial%20rebuild%20stage.JPG)
The 220K resistor is new, The black and blue wires that snake into the shielded tube are also new. The rest is still (mostly) original wiring. (There's also a jumper on the other side of the switch that is also new.) By taking each switch position one-at-a-time, we keep errors down to a minimum (I *hope!*) The 2.2 Meg resistor on the right side of the picture is an original part that will also be replaced.
Care must be exercised to make SURE the 220K resistor doesn't interfere with the operation of the rotor. Here's a picture from the side to give you an idea of what I'm talking about:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SqoPQTcyXtI/AAAAAAAABN4/YU9xvXmCRR8/s720/Selector%20Switch%20-%20Front%20Section%20-%20resistor%20detail.JPG)
The rest of the re-wiring is pretty straightforward, so let's just show you the final results of this "third" of the Front Panel. Here's the wiring around the potentiometer:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SqoOzLJQJrI/AAAAAAAABNk/pJJ9Xw5OoIA/s640/Front%20Panel%20-%20Left%20Side%20-%20Potentiometer%20wiring%20-%20Completed.JPG)
And here's the wiring around the Main Selector Switch:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SqoOzbJO9-I/AAAAAAAABNo/mG20FCGnbKg/s640/Front%20Panel%20-%20Left%20Side%20-%20Selector%20Switch%20wiring%20-%20Completed.JPG)
It's pretty crowded around this side, but by breaking things down into sub-sections, it all goes back together again. Here's what we started with:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SqoOyt6GcoI/AAAAAAAABNc/BaaiLlu7Re4/s720/Front%20Panel%20-%20Left%20Side%20-%20As%20found.JPG)
And here's what we wound up with:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SqoOy24RfmI/AAAAAAAABNg/hExQqC1YDlY/s720/Front%20Panel%20-%20Left%20Side%20-%20Completed.JPG)
Quite a bit "cleaner", don't you think? Wiring is routed to keep crosstalk to a minimum. Low level signal-carrying wires are routed against the front panel for minimum hum pickup. While there are still a few "finishing touches" needed, things have come a LONG way from where we started!
We still have another wafer switch to cover and we'll do that in our next "update" coming up a bit later. Meanwhile, the Main Chassis is being thoroughly cleaned in order to remove as much corrosion and "grunge" as possible. This is a long, slow process since the actual surface of the chassis was "etched" by the corrosion. But NOW is the time to clean it, rather than making a feeble attempt after the chassis has been rebuilt.
All things considered, we're actually making pretty good progress; just a LOT slower progress than I had hoped. But there HAVE been a few little "bumps" in the road that simply HAD to be addressed before we could proceed. More to come VERY soon! Honest!
Sincerely,
Bill Thomas
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Friday, September 11th, 2009 - Update: "Are We Ready to Get Deranged?"
The Front Panel of our H. H. Scott 121-C preamplifier is coming together quite nicely now. The final area we have yet to tackle is the Dynaural Range Switch. Here's a picture of the removed switch in "as-found" condition:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SqoPmxCcvjI/AAAAAAAABOQ/3nGJ5ktCJ1g/s576/Range%20Switch%20-%20As%20found.JPG)
Now THAT is one "busy" little switch! LOTS of components to deal with. One of the good things about more modern parts is that they are *usually* a good deal smaller than the "old-school" components. But, this switch will still wind up being pretty "busy". Take a look at the finished "pre-wired" switch:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SqoPmy-q2lI/AAAAAAAABOU/uyastgiGJDs/s576/Range%20Switch%20-%20Rebuilt%20and%20ready.JPG)
Here's what it looks like when mounted to the Front Panel:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SqoPmeeUnMI/AAAAAAAABOI/W_WaakBGqoE/s640/Front%20Panel%20-%20Center%20area%20-%20Range%20Switch%20mounted.JPG)
With the switch wired-in to the Front Panel, we come up with this:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/Sqq_NZWjlZI/AAAAAAAABOs/jAsftGWBvMM/s720/Range%20Switch%20-%20Wired.JPG)
We do have one *teensy* little problem due to the smaller size of our current-production components. The leads of the .01 uF capacitor that run from the Range Switch to the Slide Switch are just a *wee* bit too short. A little bare wire and some heat-shrink tubing takes care of that problem. Here's a picture:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/Sqq_Mr3q8wI/AAAAAAAABOc/V6jcoQH0UYQ/s720/Front%20Panel%20-%20Center%20Section%20-%20Completed.JPG)
One final area to deal with is the Pilot Lamp. A new NE-2 bulb and the associated dropping resistor make short work of that problem. Since the NE-2 is fed with DC, only one of the electrodes will glow. We have positioned that electrode closest to the "window" in the Front Panel. Take a look:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sqq_MwvG3bI/AAAAAAAABOk/PbAJJBoSbIE/s512/Pilot%20Lamp%20-%20replaced.JPG)
Put it all together and here's the result:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Sqq_M3D_1LI/AAAAAAAABOg/n9EToVEM9a8/s720/Front%20Panel%20-%20Completed.JPG)
Compare our final result with the "before" picture:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SpFe7mk3RRI/AAAAAAAABLM/PQCq16fsZ7s/s720/Front%20Panel%20-%20removed%20-%20Inside%20Shot.JPG)
I think you'll agree that our completed Front Panel is quite a bit neater and tidier than the way we found it. Ahh, but there is still one last "problem" we need to deal with - the Power Switch!
(http://lh6.ggpht.com/_Xp_4nF2BnZE/Sqq_NbQahOI/AAAAAAAABOo/t-A8io5D55I/s720/Power%20Switch%20-%20problem.JPG)
I had *hoped* to be able to save the original Power Switch, but sadly, that was not to be. Notice that one of the wires to the switch is missing? Well, the remaining wire is also hanging by a tiny thread as well. This switch is a high-quality part manufactured by Carling. It is actually a toggle switch with a slide switch actuator for the front of the switch. Unfortunately, Carling hasn't made this part in MANY decades and we will have to replace it with a standard Slide Switch. In reality, the switch we will be using to replace the original Carling Switch will actually have a heavier rating than the original part. If there were any way to replace the wires on the Carling Switch, I would have retained it, but the wires are crimped to the terminals on the inside of the switch, so it will HAVE to be replaced.
You'll notice that I have left a LOT of excess wire trailing off the bottom edge of the Front Panel. Some of the wires will have to snake their way around the Main Chassis and I wanted to make SURE there is enough wire available to reach ANYWHERE on the Main Chassis. We'll "trim-to-fit" when we re-attach the Front Panel. That will be awhile, but for now, it's time to get back to the cleaning and polishing of the Main Chassis. Then, we'll divide the Main Chassis into sections in order to rebuild it; just like the method we used to rebuild the Front Panel.
To this point, we've replaced dozens of capacitors and resistors. Most of the "bumblebee" capacitors had cracked cases. The modern epoxy-dipped capacitors we are replacing them with will provide DECADES of faithful service without a *hint* of a crack. When I get some time, I'll check the original bumblebees to see exactly how bad they really are. If they are like other bumblebees from the 50's, chances are good that ALL of them are leaking badly, open or shorted!
That's it for this update! In our next update, we'll take a look at the cleaning and straightening of the Main Chassis in preparation for the rebuilding process. I also hope to have the Power Switch on the Front Panel replaced. Don't stray too far, there's LOTS more fun in store!
Sincerely,
Bill Thomas
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This was a fun and informative read....!!
I am so glad for you guys that can fix our vintage gear in this day and age..and I hope there is a next generation brewing...for that reason I had all my Pilot gear redone.
basically, thank goodness for you guys who know shit!!!
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The best cleaners I have used on vintage audio equipment is "MeanGreen" it disolves all kinds of grime including nicotine from white plastic !! and dosent seem to harm the original finish, great stuff and it's cheep!
I buy basket cases from E-*** they are usualy filthy but clean up rather nice with it.
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"Mean Green" is a great product and does work EXTREMELY well. Unfortunately, in the case of our Scott preamp, the aluminum chassis suffers from a good deal of "pitting" or "etching." In fact, the ONLY way we are going to wind up with a nice, shiny chassis is to drill out all the rivets and do an extensive sanding, polishing and buffing job. This changes our "methodology" a bit because we'll need to draw ALL of our chassis pictorial diagrams and remove all of the tube sockets prior to sanding and polishing.
As you might guess, this is the reason for the delay in our next "update." I DO hope to have some news to report within the next couple of days. There WILL be pictures as well.
Sincerely,
Bill Thomas
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Just a quick note: I have been "internet poor" for over two weeks since my cable company changed hands and they had problems during the transition. (They disabled ALL of the DVR boxes on the system for over a week and also lost the ability to provide internet access, system-wide. Apparently, they have most of their "issues" worked out and my internet access has returned. Look for an update sometime in the next 24 hours. Sorry for the delay in posting updates. Progress IS being made (as you shall see).
Sincerely,
Bill Thomas
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October 5th, 2009 - Update: "Beginning To See the Light!"
You would think that each restoration project follows the same path. You would *think* that, but you'd be dead wrong! Oh sure, the basic steps are similar, but each project tends to take on a life of its own, dictating "modifications" in the manner we approach things. In the case of our H. H. Scott 121-C, our *original* idea was to attempt to rebuild it without removing any of the riveted components. Unfortunately, the condition of the aluminum chassis precluded us from following our original approach. As a result, the Main Chassis had to be completely stripped of all parts in order to clean up the corrosion and pitting so we could "make it prettier." We'll hold off on showing you pictures of the results until we get closer to the end of our project. Meanwhile, let's take a look at the initial stages of repopulating the Main Chassis with the parts we removed and are re-using.
Here's where the re-assembly begins: The Phono Preamplifier section. Here's a picture showing the initial mechanical assembly:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/Sso-jcSzsAI/AAAAAAAABQw/bB_Q8fmMXSk/s720/Main%20Chassis%20-%20Initial%20Mechanical%20assembly%20-%20Picture%20One.JPG)
All of the tube socket pins and the terminal strip terminals have been cleaned up. The sockets have also been pressure-cleaned and de-oxidized; cleaned again, and then treated with a contact conditioner. It takes several hours to complete this process, but the results are WELL worth the time invested. Since there are so many multiple connections to most of the tube socket pins, complete solder removal is ESSENTIAL to insure a minimum number of problems during reconstruction.
While I would *normally* install all of the sockets and terminal strips before any wiring is done, the terminal strip on the left side of the chassis is *hidden* a bit by the "lip" of the chassis, so it made more sense to install the components on the terminal strip before most of the other sockets or terminal strips were installed. Here's a picture:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Sso-iyY3rNI/AAAAAAAABQk/HFa26LFqbVk/s640/Main%20Chassis%20-%20Initial%20Electronic%20assembly%20-%20Picture%20One.JPG)
With that out of the way, we can add more sockets and terminal strips. Along the way, we'll add a few of the MANY wires that run to and from the terminal strips. In MOST cases, the wires are added first and THEN the individual resistors and capacitors are added "to the mix." It just makes for a neater, "cleaner" result when the project is completed. Here's a picture of the next "phase" of the mechanical assembly:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sso_GOw1iII/AAAAAAAABRA/pQi2A6pkvjk/s720/Main%20Chassis%20-%20Initial%20Mechanical%20assembly%20-%20Picture%20Two.JPG)
Let's add more "stuff" in this picture:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sso-jl8D4CI/AAAAAAAABQ0/8QoS5CY1PdY/s720/Main%20Chassis%20-%20Initial%20Mechanical%20assembly%20-%20Picture%20Three.JPG)
This picture shows all of the terminal strips installed:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sso-jWUg8cI/AAAAAAAABQs/5VOjE5ZtYxE/s720/Main%20Chassis%20-%20Initial%20Mechanical%20assembly%20-%20Picture%20Four.JPG)
And here we have a picture with the rest of the tube sockets installed:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/Sso-jDwfNMI/AAAAAAAABQo/laQ-1Fe9hMo/s720/Main%20Chassis%20-%20Initial%20Mechanical%20assembly%20-%20Picture%20Five.JPG)
H. H. Scott supplies the filaments of the two low-level tubes and the Output tube with DC for the filaments in order to keep hum to a minimum. As a result, these three tubes are wired in series. The orange wires connect the three tube filaments together. The rest of the tubes are supplied with AC from the Power Transformer, but there is a "hum adjust" potentiometer thrown into the mix. We'll show you how that all connects in our next "update."
"Back in the day", so-called "bumblebee" capacitors were considered to be high-quality parts. Of course, we have since learned otherwise. They may have worked well initially, but they didn't remain that way for long. Here's a picture showing *most* of the removed "bumblebees":
(http://lh6.ggpht.com/_Xp_4nF2BnZE/Sso-2-24LwI/AAAAAAAABQ8/gLNCBT0GHe8/s512/Removed%20Bumblebees.JPG)
You'll notice that a couple of them are physically broken in two. Most of the rest have cracked cases. Needless to say, the chance of ANY of them working correctly is close to ZERO! Our modern replacements will go a LONG way to insure that our preamplifier will have another long lifetime ahead of it.
That's it for this update. But, we'll be posting MANY more updates in the days to come. We'll finish up the filament wiring and begin the long process of repopulating the unit with all new passive components in our next update. We'll also install our completed front panel to the chassis and begin the task of sorting out "which wire connects where" so don't give up hope just yet. There's a LOT more to cover before we're "making music" again. Stay tuned!
Sincerely,
Bill Thomas
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October 8th, 2009 - Update: "The Plot Thickens!"
With "point-to-point" wired units, the FIRST wires to be installed are the filament wires. These wires should be as close to the chassis as possible; ESPECIALLY the wires carrying AC. We have added one extra little step: We have twisted the AC wiring that feeds the various tube sockets. This little step accomplishes two things. First, we reduce the AC radiated by the filament wires and Second, we also reduce the susceptibility of noise pickup by the filament wires. Granted, this is a SMALL contribution, but when you are dealing with high-gain devices, ANY reduction in hum and noise pickup is a potentially GOOD thing. Let's follow the trail of the AC filament wiring.
We start off with the AC filament wiring at the 6X4 rectifier:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/Ss5a9Vo6EyI/AAAAAAAABSA/RmbN5uC2zUg/s640/Main%20Chassis%20-%20Initial%20Electronic%20assembly%20-%20Picture%20Five%20-%20AC%20Filament%20Wiring%20-%206X4.JPG)
Pins 3 and 4 are the filaments. These pins will be fed directly from the Power Transformer (which will be installed a bit later) and the brown and gray wires from these pins then feed the hum adjust potentiometer here:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/Ss5a9d4mBrI/AAAAAAAABSE/aZcc_3aWUmU/s512/Main%20Chassis%20-%20Initial%20Electronic%20assembly%20-%20Picture%20Six-%20AC%20Filament%20Wiring%20-%20Hum%20Adjust%20Pot.JPG)
The wires from the 6X4 are on the right. The wires on the left then feed pins 3 and 4 of the 6AV6 bias rectifier and amplifier for the "Dynaural" noise reduction circuitry. Here's a picture:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Ss5bzP9S2pI/AAAAAAAABSM/V84Sq7kg0p8/s512/Main%20Chassis%20-%20Initial%20Electronic%20assembly%20-%20Picture%20Seven-%206AV6%20AC%20Filament%20Wiring.JPG)
The wires on the bottom of the picture come from the hum adjust potentiometer. The wires on the top of the picture feed the two 12AU7's for the high-frequency and low-frequency Dynaural circuitry AFTER the brown wire goes to a 2.7 Ohm filament dropping resistor here:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/Ss5bzI3_XEI/AAAAAAAABSQ/A_EOyOTI34U/s512/Main%20Chassis%20-%20Initial%20Electronic%20assembly%20-%20Picture%20Eight%20-%20V3%20AC%20Filament%20Wiring.JPG)
This 2.7 Ohm resistor reduces the filament Voltage to the two 12AU7's by a VERY small amount. I'm not quite sure why this was done, but it was certainly not uncommon to see something like this in all SORTS of applications during the 1950's. Needless to say, we're not going to second-guess the design engineer!
Anyway, after the resistor, the filament Voltage is fed to the two 12AU7's, first V3 as shown in the previous picture, and then to V5 as shown here:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Ss5bzTIESRI/AAAAAAAABSU/RHkUr04OeKU/s576/Main%20Chassis%20-%20Initial%20Electronic%20assembly%20-%20Picture%20Nine%20-%20V5%20AC%20Filament%20Wiring.JPG)
And that's the end of the line for the AC filament wiring!
With THAT little chore out of the way, it's time to mount the Front Panel to the Main Chassis and begin to wire up the whole "shootin' match." The accepted practice for "point-to-point" wiring is to connect the actual wires to the terminal strips first, THEN we add the passive components. (Of course, there are always a *few* exceptions, but in GENERAL, this is how it's done.) That means that we had to connect the wires from the left side of the Front Panel first. Here's a picture showing that wiring:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Ss5a9NNmLDI/AAAAAAAABR8/Etgmb28lSUM/s640/Main%20Chassis%20-%20Initial%20Electronic%20assembly%20-%20Picture%20Four%20-%20Front%20Panel%20-%20Left%20Side%20Wiring.JPG)
In this "bundle" of wires, there are three green wires, three black wires and a blue wire. Needless to say, it is CRUCIAL that these wires all connect to the proper locations. Sure, you can trace where they go on the schematic (if you have one), but it's a LOT easier to use the photos and the pictorial diagrams we drew up when we took it all apart. Still, before you cut ANY of the wires, double and triple-check the wiring locations! (Measure twice - cut ONCE, right?) You should know that even with ALL of the photos and ALL of the pictorials I drew up, there was *still* a bit of ambiguity involving two of the black wires. They are both ground wires, but one connects to the center of tube socket V1 and the other connects to lug #4 of the top terminal strip, along with the third black wire. Moral: you can NEVER take too many pictures or draw up too many pictorials!!!
NOW we can begin to repopulate the Main Chassis with new components. We start things off by installing the passive parts around V1; the 12AU7 used for the equalized preamplifier section. Here's a close up of the completed wiring at the first tube:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/Ss5a83RFEvI/AAAAAAAABR4/CjGO6VbWH-Y/s640/Main%20Chassis%20-%20Initial%20Electronic%20assembly%20-%20Picture%20Three%20-%20V1%20complete.JPG)
You'll notice that we are using a "mix" of Allen-Bradley 5% carbon-composition resistors and metal-film or metal-oxide resistors. The "rule of thumb" is to use metal resistors for Cathode and Plate resistors, while retaining carbon-composition resistors for grid resistors and other locations where audio passes through them or is "shaped" by them. This way, we retain the "warmth" attributed to the Allen-Bradleys, while reducing the potential for noise in the Cathode and Plate circuitry associated with the tubes, since appreciable current is drawn through these circuits. Of course ALL of the resistors were checked to make SURE they were well within 1% of nominal values. (Most were EXACTLY as marked!)
You'll notice that we have a LOT more room than the original parts allowed. Modern capacitors are a LOT better in quality and a LOT smaller than those old tubular "bumblebee" capacitors. Having a little extra room allows us to "dress" the leads for a neater "look" while still keeping the lead length short.
Here's a picture of our Main Chassis at this stage of completion:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/Ss5a8vp0zZI/AAAAAAAABR0/xxRGqs5LkHw/s720/Main%20Chassis%20-%20Initial%20Electronic%20assembly%20-%20Picture%20Two.JPG)
What's done looks pretty nice. Now we just have to "fill in" the rest. We'll cover the following stage in our next update, coming up in a day or so. Don't tarry at the waterin' hole TOO long. You'll want to have a clear head to make sense of the next "bunch of parts" that will be installed in our next update - coming up VERY soon! Stay with us!
Sincerely,
Bill Thomas
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Bill, I suggest use Dale Vishay RN60C metal film resistors for your plate resistor applications. RN60D types or similar tend to be very noisy with high voltage across them.
Regards,
Frank Van Alstine
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I love watching Bill's work. :thumb:
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Bill,
Anyone who ever picked up a soldering iron has great admiration for your work! The intricate attention to detail, both function and "looks" in your work is amazing. That and your ability to preserve the essential best of a classic really top it off. :thumb:
I'm only sorry the art and science of preserving and updating classic electronics is not more well known and practiced.
Mark
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As usual, amazing work, Bill. Any chance for a peak at the top of the chassis now that you've cleaned it up?
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Thanks for the comments, guys. It helps when you're knee-deep in "now WHERE did that wire go?" stuff!
Frank, I agree with you regarding Vishay/Dale resistors, but in this particular circuit location, the Voltage seen by any of the resistors is not much more than 100 Volts. Most of them see MUCH less than 50 Volts. For example, the 150K resistor in the Plate circuit has approximately 103 Volts across it. The 82K resistor sees about 57 Volts. Of course, the current is less than one mA. These resistors *should* work just fine here and they should certainly be quieter than carbon-comps. I generally don't see a noise problem until the Voltage goes past 250 Volts and the currents are MUCH higher. Of course, the proof will come when we "fire up the beast." Still, there WILL be some Vishay/Dales coming up further in the rebuild.
As for pictures of the top of the chassis, they are "in the works" and WILL be posted a bit later. I *will* tell you that the pitting was VERY deep in the aluminum, so not ALL of it was completely eliminated, but it looks much better than the corroded wreck we started with. The final result, while not a perfect "mirror finish you could shave with", is still quite acceptable and will be in keeping with the "Vintage" look of the unit when all is said and done. I just didn't want to "spoil the surprise" just yet.
It's funny how these restorations go. My original time estimate from start to finish was for the "project" to take about two weeks. Hah! Not even CLOSE to what was eventually involved. But this little preamp simply DESERVES the extra time involved in "making it right." Besides, I couldn't LIVE with myself if I really "cut corners" to meet a self-imposed time estimate. The TRUTH is, it takes as long as it takes to do the job RIGHT! As I have ALWAYS stated, my goal is to wind up with a unit that will serve the end-user faithfully for ANOTHER lifetime of use. Cutting corners is simply NOT an option!
My "better half" always complains about how long it takes for me to do the dishes, but I GUARANTEE that you will NEVER see a SPECK of ANYTHING on ANY dish *I* wash!
In the midst of all this, I'm STILL having some "issues" with that prototype FM-3 tuner with new circuit boards. I'll be taking a small break from the preamp in the hope that a fresh perspective will FINALLY get to the bottom of the alignment problem on the IF board. I'll be posting another update on the preamp during the weekend and I *hope* to have more information on the FM-3 tuner "issues" before the weekend is done as well. MUCH more to come.
Sincerely,
Bill Thomas
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Bill, the noise issue reared its ugly head with our original Super Pas rebuild for the Dyna Pas-3 preamp. Even with a 100V drop or less, we had to go to RN60C plate resistors throughout to be sure of long term quiet operation.
One of these days I hope you can get to that rebuild kit too.
Regards,
Frank Van Alstine
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Friday, October 16th, 2009 - Update: "Loose Ends and a Few Observations."
While things are progressing a good deal slower than I originally planned, progress IS being made. But before we look at more parts and wiring, I wanted to conclude our look at the can capacitors. First, a little "Visual Detective Work" will show exactly WHY we had to rebuild the can capacitors. Here's a picture of the Bottom Cover for the preamplifier:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/StkYfHWuyvI/AAAAAAAABS4/gbC8xXw-8tM/s640/Bottom%20Cover%20-%20before%20cleanup.JPG)
This is a "before" picture (before cleanup, that is.) Notice the two "spots" or "stains" on the bottom cover? These "spots" are directly UNDER two of the three can capacitors. These "spots" are due to electrolyte leakage from the can capacitors above these locations. If you see this sort of "staining" on ANY piece of electronic gear, REPLACE THOSE CAPACITORS. Don't even THINK about trying to reform them. An electrolytic capacitor that shows ANY sign of physical leakage simply MUST be replaced. There is NO other choice. This leakage is proof positive that a capacitor is (as Jerry Clower used to say), "Graveyard Dead!" If it is working at all, it will fail completely VERY soon. Why take a chance on trying to use a component that is BOUND to fail? It makes NO sense at all! Take a closer look at these "stains" and YOU decide. Here's one of them:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/StkYfGMBN5I/AAAAAAAABS8/ZbtiidyTQCY/s720/Capacitor%20Leakage%20Stain%20-%20Number%20One.JPG)
Notice that you can even tell where the electrolyte leaked out from the two terminals directly above the "staining." Now here's a close-up of the other "spot":
(http://lh5.ggpht.com/_Xp_4nF2BnZE/StkYfXQeEpI/AAAAAAAABTA/vNXdlfUGHa0/s576/Capacitor%20Leakage%20Stain%20-%20Number%20Two.JPG)
Again, you can barely see an indication that the electrolyte leaked out from two of the terminals on the capacitor. Not good at ALL!
Now, here is a picture of our three "re-stuffed" can capacitors after complete assembly:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/StkYe6g55aI/AAAAAAAABS0/NBGHgckdSt0/s720/Can%20Capacitors%20-%20rebuilt.JPG)
Yes, there is a visible "parting line" where the cans were cut open, but it's really not *that* visible. We used good old J-B Weld to permanently "glue" the can back onto the base. In order to make SURE the epoxy will hold, a bead of J-B Weld was applied to the base and another bead of J-B Weld was applied to the inside of the can top. The can was then replaced onto the base and the assembly was held together with large rubber bands to maintain alignment. The bead of J-B-Weld inside the can will gradually descend onto the bead we applied to the base, fusing the two parts together. By the way, we used the original J-B Weld formula which takes about 12 hours to cure. This gave us plenty of time to assemble all three capacitors before the epoxy "took a set."
I should mention that while J-B Weld labels one of the two tubes of their epoxy "Steel", in reality there is NO steel or metal of any kind in regular J-B Weld and it IS an insulator. (It says so right on the package!)
By using a small hobby lathe to cut the cans open cleanly, the size of the "parting line" is kept to a minimum. When using the J-B Weld to "glue" the cans back together, you *might* have some "oozing" from the parting line. A Q-Tip dampened with lacquer thinner will clean things up *IF* you use it before the epoxy begins to harden and "cure."
Well, that's brings us up to date on the can capacitors! We'll take a look at the latest wiring on the main chassis in our next update, which will be posted VERY soon! We'll show you the completed installation of the Front Panel with ALL of its associated wires and parts connected to the Main Chassis. (That's over TWENTY wires and five leads from parts that connect to the terminal strips on the Main Chassis!)
There's still a lot of "fun" left to enjoy, so don't touch that dial *just* yet!
Sincerely,
Bill Thomas
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Bill,
That one spot on the bottom cover where the caps may have been leaking looks more like a arc burn. I say this because thats what Aluminum looks like when its not clean and you try to Tig weld it. Black, sooty and porosity. Just my opinion. Keep up the outstanding work!
Steve
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Bill,
Beautiful work as always. Almost as pretty as the one of kind preamp you did for me. Good luck.
Howard :wink:
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Thanks for the kind comments, guys. Steve, you're right that the damage DOES look like an arc, but I can assure you it is chemical in nature. There is simply too much distance between the capacitor terminal and the bottom cover for there to be an arc. If the cover had been pushed against that terminal, it would have been SERIOUSLY bent! There is ZERO trace of any bending or warping of the bottom cover.
Howard, I am SO glad you are still enjoying your unit, both visually and audibly. I only wish I had more time to devote to these "projects" so they would be completed a bit faster. But in a way, they are a bit like a fine wine. They are ready when they are ready and rushing them would spoil the end result.
A lot of folks wonder why these projects take so long. The answer is simple: "Because they do!" In the case of this Scott 121-C, the lack of documentation means that for every connection that is made, I have to refer to a pictorial that I have drawn, a picture or ten that I have snapped of the original unit, and the flawed photofact schematic that I was able to locate. It takes a LOT longer to install EVERY component or wire up the connections throughout the unit. I should also add that point-to-point wiring is MUCH more time-consuming than "stuffing" parts onto a printed circuit board. But, I truly *hope* the end result will be worth it!
And with THAT out of the way, let's proceed with our next "update!"
Sincerely,
Bill Thomas
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Thursday, October 22nd, 2009 - Update: "More, More, More!" ("How Do You Like It?")
While I truly doubt there are many Disco records on 78, it's time to add more, more, more parts to our preamplifier and complete the wiring of the second tube in the chain. Our previous wiring update showed the completed first tube stage - a 12AU7 that is used as the equalized preamplifier stage, now let's complete the second gain stage that utilizes a 12AX7.
The 12A?7 family of tubes are actually two tubes in one envelope. The 12AU7 has a gain of 30. The 12AT7 (not used in this amplifier) has a gain of 70 and the 12AX7 has a gain of 100. It's a bit unusual for a 12AU7 to be used in the equalized preamplifier stages. In most other preamplifiers, a 12AX7 would be used for more gain. Dynaco did things that way, as did MANY other manufacturers. But some people find the "sound" of a 12AX7 to be a bit "strident" when used at maximum gain. Not much chance of that in THIS preamplifier. The second stage tube (the 12AX7) is run WELL below maximum gain! The first half of the 12AX7 is used to amplify the output of the equalized phono preamp stage, since the 12AU7 used there doesn't output the necessary audio level to match the line-level sources. The first half of the 12AX7 also acts as a "buffer" to isolate the load on the phono preamp stage. If there weren't a "buffer", changing the setting of the "preamp gain" control *might* upset the equalization circuitry. We CERTAINLY don't want THAT result! Neither did the folks at H. H. Scott!
The second half of the 12AX7 provides *some* gain, but it also acts as a buffer, or driver for the tapped inductor that feeds the Dynaural noise reduction circuitry. Here's a picture showing the completed first and second tube stages:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/StkZmmwFZzI/AAAAAAAABTI/46oavKwfdos/s640/Main%20Chassis%20-%20Front-left%20quadrant.JPG)
Notice that the top terminal strip is now fully wired and all terminals have been soldered. The vacant terminal is not used in this application. The terminal strip on the left side of the picture is *almost* completely wired. Three of the four terminals have been soldered. The fourth terminal (located at the bottom of the strip) has one more small electrolytic capacitor to be added, so *that* terminal has yet to be soldered. We'll add that part once the can capacitors have been re-mounted to the chassis.
In the picture above, you can see the inductor I was talking about earlier. It is mounted to the Front Panel and is used somewhat like a crossover inductor to assist in splitting the audio into a low-frequency and a high frequency "band" which is then processed by the Dynaural noise reduction stages.
Now, let's jump ahead to the Output Stage tube. It is also a 12AX7. One half of the tube is used to amplify the Output of the Tone Control circuitry, while the other is used as an Impedance reducing circuit called a "Cathode Follower." Here's a picture showing the initial wiring of that 12AX7:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/StkZm8hfi3I/AAAAAAAABTM/ON8BhYiZ6vI/s640/Main%20Chassis%20-%20Front-right%20quadrant.JPG)
The reason we have "skipped ahead" has to do with the completion of the wiring at the right-side front terminal strip. If you look closely, you'll see that all four terminals have been soldered. This means that ALL of the terminal strips adjacent to the Front Panel have been wired and soldered! Also notice that the terminal strip to the right of the Output Tube has three of its four terminals soldered as well. The fourth terminal will have a few resistors added once the can capacitors are installed.
Do you notice a pattern here? In order to make CERTAIN that all connections are soldered, our "course of action" was dictated by the terminal strips, NOT the tube sockets. By systematically going "down the line" of terminals, our preamplifier will be properly completed. We WON'T have any connections that AREN'T soldered. We'll continue in the same manner throughout the rest of the preamplifier.
So, once we complete the wiring and componentry around the Output Tube, what's left? The most interesting circuitry in the entire preamplifier: The three tubes involved in the Dynaural noise reduction section. Here's a picture of the tube socket used for the High Frequency "Gate" (It's the tube socket on the Left side of the picture):
(http://lh4.ggpht.com/_Xp_4nF2BnZE/StkZm1PbD0I/AAAAAAAABTQ/e-xc0lMHs04/s640/Output%20Stage%20wiring%20-%20%28partial%29.JPG)
We also have the Low Frequency "Gate" and the Bias amplifier left to wire. Here's a picture of those two tube sockets:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/StkZnDHm2TI/AAAAAAAABTU/LMQ8Nd0aKWY/s720/Main%20Chassis%20-%20area%20left%20to%20wire.JPG)
Finally, here's an "overall" shot that shows what has been completed, and what remains to be done:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/StkZnVsBsOI/AAAAAAAABTY/HIqfQZfGjmM/s640/Main%20Chassis%20-%20Front%20Panel%20wired%20in.JPG)
Yes, things are *definitely* shaping up, but there's still a good deal of "fun" left. We'll cover much of that in our next "update", coming soon! We'll also install the three can capacitors and begin the "clutter reduction" of many of those wires that are currently "waiting in the wings" for their new home!
Don't miss it! It's going to go a LONG way toward the completion of our preamplifier "project." Stay with us!
Sincerely,
Bill Thomas
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Sunday, October 25th, 2009 - Update: "It's Getting Better!"
No, we're not ready to play that unopened vinyl original pressing of "Sergeant Pepper's Lonely Heart's Club Band" *quite* yet, but we ARE getting closer "all the time!"
Let's continue with our restoration of the Output Stage. Here's a picture of the Output 12AX7's tube socket:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SuFecNlfZAI/AAAAAAAABT8/yC-PLIJo97U/s576/Output%20Stage%20-%2080%20percent%20completed.JPG)
While it isn't *quite* complete, this shows the bulk of the construction for the Cathode Follower section of the Output Tube. We have made a *slight* change in the parts used. In the "stock" unit, the Input Network for the Cathode Follower used dipped silver-mica capacitors. We have replaced them with the same values in polystyrene, rather than mica. Both types of capacitors are OUTSTANDING capacitor, but some people feel that polystyrene capacitors are a bit "kinder" to audio. Since ALL of the audio flows through these capacitors, (and since we're not in a high heat environment), polystyrene capacitors will perform admirably!
Now, *finally*, as promised, let's add our "re-stuffed" can capacitors to "the mix" and begin connecting the wiring to the respective units. Here's a picture:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SuFeccgA66I/AAAAAAAABUA/q5hWdlaI9wY/s720/Main%20Chassis%20-%20Can%20capacitors%20added.JPG)
With the can capacitors added, we can begin to reduce the "clutter" by connecting some of those loose wires to their respective terminals. Here's how things start on the Right Side of the preamplifier:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SuWM7ohCppI/AAAAAAAABUM/lSalkUMuG8E/s640/Initial%20Can%20Capacitor%20wiring%20-%20Right%20Side.JPG)
Here's the other can capacitor, located on the Left Side of the chassis:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SuWM74V9YWI/AAAAAAAABUQ/3yFL9rB4SB4/s640/Can%20Capacitor%20-%20Left%20Side%20-%20un-wired.JPG)
Several of the loose wires will connect to this capacitor, but first, let's finish off the Output Stage. Completed, it looks like this:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SuWM7n8CvuI/AAAAAAAABUI/rn2fcW6xiV8/s640/Output%20Stage%20-%20Completed.JPG)
Pretty "busy" around the Output Tube, isn't it? This is, for the most part, unavoidable when point-to-point wiring is used.
It's interesting to note that with point-to-point wiring, components are added in a different order than when restoring Printed Circuit Boards. With Printed Circuit Boards, you (generally) start with the lowest profile components like resistors and diodes. Then you add the next larger component until the largest parts are mounted. But with point-to-point wiring, the larger components (mainly the "tubular capacitors" are generally mounted first, followed by smaller and smaller parts. This technique assures us that there won't be any parts hitting the bottom panel of the preamplifier when we are finished.
Now that our Output Section is complete, let's "fill in the middle" by wiring up the first of three tubes associated with the Dynaural Noise Reduction section. Here's a picture of the completed High Frequency "gate":
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SuWM8Lm4V6I/AAAAAAAABUY/c1lRgxr4yjg/s640/1st%20Dynaural%20Stage.JPG)
Before we leave this area, let's take a closer look at that little 1 Meg resistor. This gives you a good example of how the component leads are formed and how we measure the length of the heat-shrink tubing that covers the component leads:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SuWM8Ft2zNI/AAAAAAAABUU/tGhDHt-hx3g/s720/Component%20Lead%20Forming%20detail.JPG)
This 1 Meg resistor will wind up connecting to the Low Frequency "gate" and the Bias Amplifier tube circuitry (as soon as we can get to *those* sections!)
Let's bring this update to a close by taking an overall look at the Main Chassis wiring to this point. Here it is:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SuWN15igkhI/AAAAAAAABUg/3uZ15arENRo/s720/Main%20Chassis%20-%201st%20Dynaural%20Stage%20added.JPG)
For all practical purposes, the top half of the preamplifier is now completely wired! In our next "update", we'll begin wiring the bottom half of the preamp, starting with the Can Capacitor located on the Left Side of the Main Chassis, followed by the wiring of the Bias Amplifier/Bias Rectifier tube (the 6AV6) located just to the Right of the Can Capacitor.
Well, we're getting closer and closer to a completed preamplifier, but there's still *quite* a bit remaining. Don't "bug-out" on us now! This is just *beginning* to get really GOOD! More to come VERY soon! Stay tuned!
Sincerely,
Bill Thomas
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This is a total work of art. I love it, thanks for taking the time to show how it should be done.
I cant wait to see it finished and hear your impressions on how it sounds...... :drool:
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Thursday, October 29th, 2009 - Update: "For the Benefit of Mr. Scott"
(Sorry, I couldn't resist with all the Beatles "Hoopla" going on lately.) But with THAT out of the way, let's get down to business!
Our H. H. Scott 121-C is getting closer and closer to the "smoke test." As I mentioned in the previous "update", we're wiring the bias amplifier/rectifier for the Dynaural circuitry in this installment. You would *think* that wiring up five more tube socket lugs would be "no big deal." But, with the number of small components that had to be added here, along with the crowded conditions at several of the tube socket pins, it isn't exactly a "walk in the park." But "slow and steady wins the race." Just take things in order and work your way up from the connections that are closest to the chassis, then the "mid-level" connections, and finally, the "top-level" parts. What initially appears to be an IMPOSSIBLE task, winds up being a series of *teensy* little steps that are quire possible. But don't let me keep you in suspense here. Let's take a look at the finished wiring around the 6AV6 tube socket, the associated Quad Capacitor to the left of that socket, and the connections to the various terminal strips "in the neighborhood":
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SuohFzRICwI/AAAAAAAABUo/P-RTUuuEabQ/s640/6AV6%20and%20quad%20cap%20-%20Complete.JPG)
As you can see, things are pretty crowded around those seven pins! If it weren't for the MANY photos and SEVERAL hand-drawn pictorial diagrams, I would have NEVER gotten through it. In case you're wondering, this was about three night's worth of work! (Yeah, I'm REALLY slow, aren't I?) But each connection requires CONSTANT checking against the pictorials, the photos AND the photofact schematic diagram to make CERTAIN everything goes where it should. It's a LOT easier to make SURE things are correct when you are BUILDING it, than to try to troubleshoot it and "make it right" AFTER the thing is complete. ("Measure twice, cut once", right?)
You may wonder why I used polystyrene capacitors. As it turned out, I either forgot to order .0033 film capacitors, or I "left them someplace." (Hey, we're ALL human!) Seriously, I looked and looked, but they weren't to be found ANYWHERE. But I DID have these lovely polystyrenes available, so in they went! (A .0033 microfarad capacitor is the same value as a 3300 picofarad capacitor; it's just expressed differently.) Polystyrenes are actually more stable than typical film capacitors, so let's just call this an "upgrade" (and conveniently ignore my sloppy "parts organization", ok?) Seriously, polystyrene capacitors are considered IDEAL for use in audio circuitry. They have extremely low leakage, are usually "spot-on" in value and their only "downside" is that they don't like to play in hot environments. No problem with that here.
You'll also notice that we've completed the wiring of our nearby quad capacitor. Two sections of that capacitor are used as Cathode Bypass capacitors and the other two sections provide part of the B+ filtering for the preamp.
So, what's left? Well, we *still* have one more tube used in the Dynaural noise reduction circuitry. Here's a picture of the "work to be done" around that tube socket:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SuohF8tuOrI/AAAAAAAABUs/cpUwK1SRgfo/s720/Final%2012AU7%20wiring%20remaining.JPG)
Since five connections are already completed at that socket, there are really only four tube socket lugs that need to be wired. Piece of cake, right? Well, we'll see about that in our next update.
There's one more area that needs to be completed: The rest of the Power Supply and the associated quad capacitors. Take a look:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SuohGI8tp3I/AAAAAAAABU0/Txq1ijTll98/s640/Final%20Power%20Supply%20area%20-%20Yet%20to%20go.JPG)
Since selenium rectifiers are so "last century", we're using a more modern silicon bridge rectifier to replace that old selenium rectifier in the filament supply, see?:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SuohGBRiUeI/AAAAAAAABUw/TlIFKWl63q8/s512/Filament%20Supply%20-%20Bridge%20Rectifier.JPG)
Selenium rectifiers may have been OK "way back when", but they have several drawbacks. They start deteriorating as soon as they are manufactured, (If I could even FIND a NOS replacement today, it would most likely be bad after sitting on a shelf for 50 years!) they have a fairly low current capacity, (the original was rated in milliamps), they really *STINK* when they fail; generating fumes that can actually be poisonous, and simply weren't that good at rectifying AC to begin with. Modern silicon rectifiers are DEFINITELY the way to go here!
This silicon bridge rectifier is rated at several AMPERES! We'll have NO problem powering the three tubes that are fed from our DC filament supply. But there IS one "gotcha!" Since silicon rectifiers have a lower forward Voltage drop than selenium rectifiers, we will *probably* be delivering higher Voltage to the tube filaments than they are rated for. Since the tubes are in series, we need to deliver no more than 18.9 Volts to the filaments. With the silicon rectifier in place of the selenium rectifier, we will *probably* be delivering closer to 22 Volts to the filaments! No good can come from THAT, so we'll "juggle" the resistor values in the filament supply in order to bring the Voltage back down to something closer to 18 Volts. That's right, I said 18.0 Volts! By running the filaments slightly BELOW their typical ratings, we will extend the life of those tubes by a significant amount. That tiny reduction in filament Voltage will cause NO problems "in-circuit", but *could* help our valuable tubes live a few YEARS longer! (Honest!)
As I usually do, here's a picture of our overall progress to this point:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SuohGU6v0fI/AAAAAAAABU4/_AlBdgxgXOA/s720/Main%20Chassis%20-%2085%20percent%20complete.JPG)
I'm calling it about 85% complete. But we STILL have some "fun stuff" ahead. We'll be "improving" the cosmetics of the Power Transformer and wiring it in, getting the rear panel ready to re-mount to the main chassis and THEN - the ever-popular "smoke test!" (I can feel the anticipation building!)
My sincere thanks go out to everyone who has posted their comments or e-mailed me regarding this "project." A *special* boatload of thanks go out to the owner of this preamplifier who has shown INCREDIBLE patience during this VERY slow rebuild. We'll soon see if that patience will be rewarded!
There's still a bit of "fun" coming, so don't stray too far from Audiocircle! (Hey, you've stuck with me THIS far, do you REALLY want to miss the "exciting conclusion?") Stay tuned!
Sincerely,
Bill Thomas
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Friday, October 30th, 2009 - Update: "And In the End..."
Well, we're not *quite* to the end of our journey, but we certainly ARE getting close! All of the "signal" tube sockets have now been wired. Here's a picture of the final tube socket used in the Dynaural noise reduction circuitry:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SusSw5DrhMI/AAAAAAAABVE/Cox5Yz5FnLY/s512/Final%2012AU7%20-%20Completed%20wiring.JPG)
Unlike the 6AV6 socket, most of the components for this part of the circuit are mounted on the terminal strip immediately below the socket. Still, there were a *few* parts surrounding the tube socket. A couple of hours of checking, re-checking, re-re-checking and THEN soldering, and we're gettin' close to the final wiring on the Main Chassis.
The High Voltage Power Supply sections are also now complete. Here's a picture of the wiring around the two final quad capacitors:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SusSw9rOsPI/AAAAAAAABVI/6rbrStbkqj4/s640/HV%20Power%20Supply%20wiring%20-%20Completed.JPG)
You'll notice that two sections of the right-hand quad cap are still yet to be wired. They are part of the DC filament supply; which is the final stage of wiring on the Main Chassis, along with the installation of the Power Transformer. Here's a picture of the area we'll be focusing on next:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SusSwz-Q6jI/AAAAAAAABVM/kSBN_sHP8s0/s640/Filament%20Supply%20%20-%20remaining%20work%20to%20be%20done.JPG)
While the 6X4 rectifier socket looks pretty vacant, all but two of the socket lugs are fed directly from the Power Transformer. The other two pins are used as "tie points" for the filament supply components. Since they aren't internally connected inside the 6X4, using them this way is just FINE!
By the way, that two-lug terminal strip will also be wired when the Power Transformer is installed. Those terminals are used to terminate the two wires from the Power Switch on the Front Panel.
While there IS more to come, take a look at our *nearly* completed Main Chassis:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SusSxNwsmUI/AAAAAAAABVQ/29SK-iQyjaw/s720/Main%20Chassis%20-%2090%20percent%20complete.JPG)
Other than the filament supply, the Power Transformer and the wires that lead to the back chassis, we're REALLY close to the completion of our "project." (I can feel the anticipation building!) Don't go away! We'll be installing our "beautified" Power Transformer next! Stay with us!
Sincerely,
Bill Thomas
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Tuesday, November 3rd, 2009 - Update: "Closer To (its NEW) Home"
With apologies to Grand Funk Railroad, here are a couple of pictures of our Power Transformer in "ready to install" state. It was pretty breezy and dusty when this was shot, so my apologies for the "particles" on the cover. It actually looks a good deal prettier than the pictures show. First, here's a picture from the side. This shows the copper eddy-current strap reflecting the grain of the wood the transformer is sitting on:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SvBm3aSzTVI/AAAAAAAABVw/V_sqthB95DY/s512/Power%20Transformer%20-%20completed%20-%20side%20shot.JPG)
And here's a picture from the front of the transformer. The red highlights are actually reflections from the shirt I was wearing:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SvBm3tWSCII/AAAAAAAABV0/dAVmJg5XFYk/s576/Power%20Transformer%20-%20completed%20-%20front%20shot.JPG)
While I mentioned we would be installing the transformer next, I have decided to install the rear panel first. Otherwise our nice, new paint job might get damaged. With the rear panel installed and the chassis inverted, the Power Transformer won't rest on the surface of the bench. While the new paint on the transformer cover is dry, it won't achieve maximum hardness for about a month. I'd just rather not tempt fate by resting the unit on the Transformer cover at this stage.
The Rear Panel will require a good deal of work before it's "ready to go." We still have seven potentiometers to clean and lubricate, as well as whole BOATLOAD of RCA jacks to clean and de-oxidize. Since my "French Maid" isn't skilled in the ways of H. H. Scott, (Don't ask me what she IS skilled at! I'm still trying to figure THAT one out!), I'll have to tackle the chores myself!
Just to show you what's next, here are a couple of pictures of the Rear Panel in "as removed" condition. First, here's a picture of the side everyone will see from the back of the preamplifier:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SvBuREks7jI/AAAAAAAABV8/aKgByonvqZI/s720/Rear%20Panel%20-%20As%20removed%20-%20Rear%20Shot.JPG)
Notice that there are a couple of "stickers" on the back. The foil "H. H. Scott" sticker will *probably* survive just fine! But the paper "patent notices" sticker will be a bit more problematic. All I can say is, I'll do my best to save it!
Now, here is a picture of the side most folks WON'T see, the INSIDE of the Rear Panel:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SvBuRA7MuhI/AAAAAAAABWA/zp4zQybWlhc/s720/Rear%20Panel%20-%20As%20removed%20-%20Inside%20Shot.JPG)
LOTS of dust and dirt to remove, as well as a few "stragglers"; a few small parts that remain to be replaced. The "stragglers" are the least of our worries. They'll all be replaced with new parts.
Sorry about the poor lighting of the pictures. Even at High Noon, the sun is now VERY low in the sky so there are LOTS of shadows to deal with. But this will at least give you an idea of what is yet to come.
We'll cover the panel cleanup and installation, the Power Transformer installation, the remaining installation of the DC Filament Supply and the incoming Power wiring in our next update. At THAT point, we'll be *just* about ready to "fire her up!" We'll STILL have another "Update" after that. In that final construction update, we'll show you the installation of the two cleaned up cover/shields for the front and back panels, the (cleaned up) Front Panel, the nice, clean knobs on the controls and we'll take care of one more *teensy* little detail - a cosmetic part that was missing when I received the unit. Details, details. But as they say, "The Devil is in the DETAILS." Let me tell you, they KNEW what they were talking about!
Our next update will follow VERY soon. We really ARE "Getting Closer to Home" on this one now. I can almost SMELL the anticipation building! (No wait, I *think* that was the CAT!) Anyway, until our NEXT update, stay warm and stay tuned!
Sincerely,
Bill Thomas
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November 29th, 2009 - Update: - "I'm Headin' For That Last Roundup!"
And "hell-bent for leather" to get this thing completed by TOMORROW! As things stand, the chassis is now completely electrically assembled. Let's take a look.
Last time, we showed you the inside of the rear panel in "as-found" condition. Now, take a look at the pre-wired rear panel with the new parts installed:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SxL6_9_LGhI/AAAAAAAABXo/Rgw3Mj9v5vY/s640/Rear%20Panel%20-%20Inside%20Shot%20-%20Ready%20for%20assembly.JPG)
I had planned to re-use the original "Level Trim" potentiometers, but one of them had a fractured element that worked *most* of the time. Sorry, but that just won't do. We're after a RELIABLE preamplifier, so all six of the "Level Trim" potentiometers were replaced. The 100K "Cartridge Loading" potentiometer was in FINE shape so it stays. Here's a picture of the pre-wiring around the new potentiometers:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SxL7Aekc0mI/AAAAAAAABXs/H-daCyYSbZg/s640/Rear%20Panel%20-%20Inside%20Shot%20-%20Trim%20Controls%20-%20Ready%20for%20assembly.JPG)
Scott used bare wires from the potentiometers to the Input Jacks, but to make things a bit neater, I used the same color-coded wires that are used in the shielded multi-conductor "tube" that runs from the Front Panel area. No chance of an inadvertent short here!
Now, here's a picture of the pre-wiring at the low-level Input Jacks:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SxL6_vTpzfI/AAAAAAAABXk/BMKJmT1jsdY/s512/Rear%20Panel%20-%20Inside%20Shot%20-%20Low-Level%20Input%20Jacks%20-%20Ready%20for%20assembly.JPG)
And finally, here's a shot of the pre-wiring around the AC supply area:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SxL6kNU8QOI/AAAAAAAABXY/ZokhAuTDX4A/s512/Rear%20Panel%20-%20Inside%20Shot%20-%20AC%20Wiring%20-%20Ready%20for%20assembly.JPG)
New fuse holder, new line cord but we DID retain the original "convenience receptacles." They are in FINE condition so there was no reason to change them.
We still had a bit of work to do on the main chassis before we could "button it up" and call it done. It all hinges on the installation of the cleaned-up Power Transformer. Well, here's the result:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SxL6jVKzIOI/AAAAAAAABXM/pRKK2UnVJh0/s640/Completed%20Chassis%20-%20Top%20View%20-%20Power%20Transformer%20Area.JPG)
Looks pretty nice, doesn't it? It certainly beats the original appearance of the Power Transformer by a LONG shot! Remember what THAT looked like? No? Well, take a look:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SxMEb5lWAkI/AAAAAAAABX4/d8sbTy7m638/s576/Power%20Transformer%20-%20as%20found.JPG)
With the Power Transformer mounted, it was time to complete the wiring of the filament circuitry and the wiring around the High Voltage rectifier, the 6X4. Here's a picture:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SxL5tLxfaKI/AAAAAAAABXE/XtXm4vIoM0E/s640/Completed%20Chassis%20-%20Bottom%20View%20-%20Power%20Transformer%20Area.JPG)
In the final analysis, we only changed the values of two resistors in the DC filament supply. Originally, the value of the two current-inrush limiting resistors was a total of 5.4 Ohms. Since we increased the size of the capacitors in the filament supply, I thought it *might* be a good idea to increase those resistors a bit. Now, instead of 5.4 Ohms, they are a total of 7.6 Ohms. As it turns out, this little change was all that was necessary. The 33 Ohm dropping resistor remained the same as the original value. With the three tubes installed, the Voltage delivered to the series string filaments is 17.6 Volts. This is within 1/10th of a Volt of the original Voltage value of 17.5 Volts.
You might wonder why H. H. Scott ran these tubes below their normal "rated" Voltage. According to the tube specifications, these tubes SHOULD be running at a total of 18.9 Volts, so what gives? Simple! By running the tubes at a *slightly* lower filament Voltage, tube life increases DRAMATICALLY! This also explains the 2.7 Ohm resistor in series with the tubes fed with AC from the Power Transformer. Since there is gain to spare, running the tubes a *teensy* bit below their nominal Voltage rating extends the life of the tubes.
With THAT out of the way, let's wire in the rear panel! Here's a picture of the wiring to the "Level Trim" potentiometers:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SxL6kNLE6pI/AAAAAAAABXc/syRA5DbUGi8/s640/Rear%20Panel%20-%20Inside%20Shot%20-%20High-Level%20Input%20Wiring.JPG)
It's pretty hard to show the rest of the wiring to the rear panel, so let's wind things up with an "underside" shot of the completed preamplifier:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SxL5tbXkzvI/AAAAAAAABXI/0ojID-9j23Y/s640/Completed%20Chassis%20-%20Bottom%20View.JPG)
And finally, here's a picture of the top side of the completed wired preamplifier:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SxL6jjLqiNI/AAAAAAAABXQ/ZNO-Qw1hPeQ/s640/Completed%20Chassis%20-%20Top%20View.JPG)
Oh! I *almost* forgot! I mentioned there was one little "detail" we had yet to cover. Well, here it is - the "jewel" for the pilot lamp:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SxL5s-PjAII/AAAAAAAABW4/ht92jH8RhjA/s640/Cleaned%20Face%20Plate%20-%20Head-on.JPG)
No, this isn't an original jewel, but it captures the "flavor" of the LONG extinct original jewel pretty well. It's installed the way the original one was mounted - glue!
While there are still a few "Final Assembly" details yet to go, they are of a "cosmetic" nature. In the final "Update", we'll show you the completed unit with all shields in place. We'll also mount the face plate to the front panel, add all the knobs and then we'll "go for the gusto" with the "smoke test." Be still my beating heart!!!
As I mentioned, I'm shooting for a completion date of TOMORROW, so don't make any plans for tomorrow night! We're going to hear this baby in operation! Stay with us!
Sincerely,
Bill Thomas
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I felt this all needed a comment, but the only thing I could think of was, Wow! :D
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Absolutely stunning work Bill.....as always :thumb:
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Monday November 30th, 2009 - Update: "Problems, Problems!"
Thanks for the kind comments, guys. Right about now, I need all the "positive waves" I can find. We have a problem and it's NOT going to be an easy fix!
I always tell folks there are TWO ways I do things; LIKE an idiot, or AS an idiot. It looks like I have BOTH bases covered with THIS little *error.* But first, let's look at the GOOD news.
We've had out initial "smoke test" and nothing exploded. The missing 6AV6 has arrived so all tubes are present and accounted for. Upon initial application of power, there was a TRUCKLOAD of noise. It was traced to a bad (and microphonic) 12AX7. A NOS RCA gray plate solved THAT "issue" with a minimum of "fuss." I also replaced the other 12AX7 with another NOS RCA gray plate "just to be sure." For testing purposes, I use a little Sony portable CD player; not because it has FABULOUS sound quality, but because it is battery powered and has a Line-level Output. This eliminates any possibility of Power Line-induced hum. Of course, I initially forgot all about the little back panel "Level Trim" controls, so I DID waste about ten minutes trying to figure out why I wasn't hearing anything.
Advancing the Level Trim control for the "Spare" Input, (and plugging the CD player INTO the "Spare Input"), I was rewarded with the UNMISTAKABLE sound of Tom and Ray Magliozzi, laughing their heads off. (OBVIOUSLY, I grabbed the wrong CD!) Nevertheless, our little Scott preamplifier was "doing the job" quite nicely. There's a *little* bit of hum, but since it changes as I reposition the unit on the bench, I'm chalking it up to the missing shields and the missing bottom cover. Bringing my hand near the 12AX7 Output Tube, the hum increases DRAMATICALLY! I guess there really IS a reason why Scott used a tube shield at this location. Needless to say, I will ALSO put a shield on the two tubes that require it when we get to the final testing phase.
OK, now that we have audio, I tried each of the Line-Level Inputs. Yep! They ALL worked flawlessly. The "Level Trim" controls are noise free! (Whew!) Checking the Volume and Tone Controls, they all function normally. So far, so good!
The "star" of the show in this preamplifier is the "Dynaural Noise Reduction" circuitry. It is designed to minimize "impulse noise" on old 78's (or trashed LP's). Since I was dealing with a (relatively) noise-free source, I could only verify that the Rumble Filter was working properly and that the different bandpass settings of the Dynaural Selector Switch were having an appropriate effect. Well, they did indeed! And the Dynaural Level Control also operated noiselessly. It certainly appears that the Line-Level circuitry, the Dynaural Noise Reduction circuitry and the Tone Control circuitry are all doing what they are supposed to do!
Felling pretty good at this point, I switched the unit to one of the Phono Inputs and used the "fingertip method" to check for a signal from the Mag 1 Input.
Nothing! Not ANYTHING! Not a TRACE of any hum or noise of ANY kind! I tried the other Magnetic Phono Input - Mag 2. Again, NOTHING! I switched the Main Selector Switch to the NARTB Tape Input and tried the "fingertip test" again. THIS time, I was rewarded with a LOUD hum when my fingertip touched the Input jack.
Digging a bit deeper, I turned the unit upside-down and applied my fingertip (VERY carefully) to the Input point of the Equalized Preamplifier tube (a 12AU7). With the Selector Switch set for the NARTB Tape position, I was again rewarded with a LOUD hum. But when I switched the Selector Switch to either of the Phono Inputs, I got NOTHING! This Input point occurs AFTER the Selector Switch, so I *should* get a rip-snortin' hum! OBVIOUSLY there is something that is shorting out this point whenever the Selector Switch is set to the Phono positions.
After spending several HOURS tracing ALL of the wiring to the Selector Switch, nothing revealed itself. All of the wiring to the switch was found to be correct.
But the GOOD news is, all "active" gain stages of the preamplifier are working, we just have a little *problem* lurking SOMEWHERE around the Main Selector Switch. But WHAT??? Could the switch be defective? Could a wire be touching somewhere around the Phono Section of the switch?
Nope! In the immortal words of "Pogo", "We have met the enemy and he is US!" Using an Ohm meter, I was able to ascertain that the wiper of the switch was making contact with the correct terminals. (This chewed up several MORE hours of troubleshooting.) Still, after all this, NOTHING was found to be remiss. All of the wires are connected to the appropriate terminal numbers of the Selector Switch.
We've all experienced the "Eureka Moment!" That's the point where the light bulb over our head lights up with the dawning of realization. In MOST cases, this is a VERY good thing! In THIS case, it confirmed my worst fear - I had goofed when re-assembling the Main Selector Switch! HOW I did this, I will NEVER know, but nevertheless, I DID do it!
In this case, I had the wipers of the two switch sections properly aligned, but one set of stationary contacts was rotated 180 degrees from where it SHOULD be. It's the only POSSIBLE explanation for this problem! This is the point where my own words have come back to haunt me: "It is all too easy to "goof" and re-assemble the switch with the wafers incorrectly oriented." The trouble is, this little "goof" will require unsoldering ALL of the wires to the Front Section of the Selector Switch, dis-assembling the switch AGAIN and rotating the front wafer 180 degrees and then re-soldering ALL of those wires!
And that's where things stand right now. I have begun the LONG and ARDUOUS process of attempting to get to the switch and FIX my stupid mistake! Since it is COMPLETELY buried, this is no EASY fix, but with care, patience and persistence, we'll EVENTUALLY get things where they SHOULD be!
Like an idiot? Or AS an idiot? YOU be the judge!
If anything can be learned from this, it is this: No matter how silly you think it is to check and RE-CHECK your work, the fact is, you can NEVER check it TOO many times. Check EVERYTHING and check it over and over until you are absolutely CERTAIN you have things "in order." Even then, don't be surprised if, after all this checking and re-checking, you find something wrong after assembly. When that happens, take a break. Have a drink! Go watch something mindless on the tube. Whatever! Just get AWAY from it for a bit. Then, when you come back to it, use your BRAIN to determine the cause of the problem and take some time to think about *exactly* what course of action you are going to use to FIX the problem. Be creative! But BE THOROUGH!
Well, we've taken a bit of a detour, but we *should* be back on the "straight and narrow" path within a couple of days. I'll keep you posted!
Sincerely,
Bill Thomas
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Bill, consider this a VERY positive wave! :wink: You do great, meticulous work. I've got 4 Scott integrated tube amps and 2 Scott tuners and would like for you to work your magic on a couple of them, but I don't dare get a quote from you, based on the number of hours I assume you put into your work. I've got one of the amps almost completely re-capped and have replaced critical resistors and will continue on with it.
Your caution to check and re-check is a good reminder for us all. I would add that the mental attitude to do your checking in should be that of TRYING to find something wrong rather than trying to confirm you did perfect work. With the latter 'tude, I've missed mistakes. With the former, I've not missed mistakes.
Keep on truckin'! :thumb:
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Thanks, Hank! I TOTALLY agree with you regarding the idea of looking for something WRONG, as opposed to confirming that something is RIGHT!
I'm "digging out" the Selector Switch as we speak. It *looks* like we're going to be doing quite a bit of "re-working and re-wiring" before all is said and done. I have un-bolted the Front Panel from the Main Chassis and "tilted" it slightly to allow access to the selector switch. Next, I have separated the rear wafer, the spacers and the shield, but access is EXTREMELY tight. It appears that I'm going to have to un-solder three connections on the rear wafer in order to swing it out of the way to allow access to the front wafer. Once I can do THAT, there are nine connections that must be un-soldered from the front wafer. This is a real BEAR of a job. Best to consider this a "learning experience." in order to keep me from kicking myself around the block for a while!
Once this is finished, I will finally finish off our second FM-3 tuner. There is a problem with THAT little beastie that has kept me scratching my head, off-and-on, for the past few months. I have found that once I get too intensely involved in tracing a problem, it becomes necessary to get away from it for a while in order to face it with a "fresh brain." The FM-3 plays, but it isn't possible to align it properly. The problem is on the new IF board *somewhere* and I'm comparing a working tuner to this one. It helps to have a working tuner in order to inject "known good signals" from one tuner into the other one.
After this, we'll do a nice, easy, simple Stereo 70 restoration. I think I'm gonna need a little rest before I tackle another H. H. Scott of ANY sort! (BOY did they like to use LOTS of parts!)
How many hours? That's a question for the ages. I'm not doing this as a "money-making" operation so I'm not keeping track of the total number of hours "invested" in these rebuilds, but if I did, I'll bet I would be working for a LOT less than minimum wage! (lol) But these are not "cheap and dirty" repair jobs. As always, my goal is to give these old and tired pieces a "second lifetime" of reliable operation. "Quick and dirty" has NEVER appealed to me. I think people are getting tired of the "quick and dirty" approach. The downside of this is that it takes a LOT of time to complete each unit. I wish I worked faster, but I would rather have it done RIGHT than done FAST!
Well, back to the salt mines!
Sincerely,
Bill Thomas
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Yes, Scott used lots of parts because Scott over-engineered their products, but that's why they sound great today :D Oh yeah, I also have my original Dynaco AF-6 which needs an alignment since I have no such experience nor proper equipment.
I agree with you on the number of hours thing. I've never kept track when building speakers for me or the occasional customer; otherwise I too would be working for less than minimum wage :wink:
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December 13, 2009 - Mini Update: "Right or Wrong?"
After carefully UN-building a significant portion of our preamplifier in order to get down to the selector switch, I discovered that I DIDN'T assemble the selector switch with a wafer 180 degrees off. In other words, my only real goof (so far) was dis-assembling this thing to get down to the selector switch.
What I DID discover (and the thing that has had me going around in circles for several days) is that my limited documentation has errors in their depiction of the selector switch! These errors show a switch that couldn't POSSIBLY work correctly which led me to believe that I had rotated one of the wafers. While we still DO have a problem, it is NOT in the selector switch area. It is apparently in the variable equalization circuitry that goes between the selector switch and the second stage of audio amplification. I'm guessing that our "homemade" repair on the 1 Meg potentiometer may not be working properly. While I work out the "issues" on THAT little area, I wanted to make SURE folks knew about the errors in the documentation that wasted two weeks of my time.
My limited documentation is a (poor) photocopy of a Howard W. Sams photofact schematic. The H. H. Scott 121-C is covered in Photofact number 380, folder 10. While the *orientation* of the selector switch is shown correctly, the actual switch contacts are NOT depicted properly. Several of the "short" and "long" contacts are out of place. I can only thank God (and my retentive NEED to take LOTS of pictures) for FINALLY confirming the documentation errors.
The GOOD news is the selector switch IS wired correctly. The BAD news is that it will take several days to get everything back where it was before I had to dig down DEEP to get to the selector switch! Such is life.
I hope to be able to report on the ACTUAL problem by tonight.
Sincerely,
Bill Thomas
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December 14th, 2009! - Update: "At Last!"
(The Etta James version, of course!) We FINALLY have a working Phono Section! In other words, we are DEFINITELY heading down the "home-stretch" of this most *interesting* project! The problem with the Phono section was FINALLY traced down to the "home-brew" repair of the potentiometer. Oh, the pot itself was working just FINE! But there was one TINY little detail that caused a problem. Here's a picture to help everyone see what finally caused the problem:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sp9LaHbYnQI/AAAAAAAABM4/bLvRCGXuAew/s512/Radio%20Shack%20Potentiometer%20-%20modified%20element%20with%20slip-ring%20added.JPG)
If you'll look CAREFULLY at the protruding screw threads from that TINY little screw holding the wiper contact surface in place, you'll see the "offender!" As it turned out, once the control was re-assembled, mounted and the nut holding the control to the panel was tightened, this little protruding thread made intermittent contact with the rear of the Front Panel. A few minutes with a file eliminated the excess threads AND the problem!
In troubleshooting ANY piece of electronic equipment, "intermittents" are the most time-consuming, (expletive-deleted), drive-you-crazy types of problems. As it turned out, simply "nudging" the preamp on the bench would make this "short" come-and-go. Of course, I *thought* I had checked the clearance, but there was *just* enough extra "give" in the front of the metal framework of the potentiometer for the screw thread to make *slight* contact.
I just thought you all would like to know what the FINAL cause of the problem was. I'll have a wrap-up report on the finished unit coming up as soon as my final testing is completed.
More to come (with pictures, no less.)
Sincerely,
Bill Thomas
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Friday, December 18th, 2009 - Final Update: "Goodbye Yellow Brick Road!"
As I type this, our H. H. Scott 121-C is about ten hours into the "burn-in" process. Our little trip down the "Yellow Brick Road" has FINALLY come to an end! It is making some pretty music and *appears* to be functioning properly and reliably! But BOY, has it taken a long time to get to this final stage!
ALL of these restoration projects take on a life of their own, but THIS one threw a LOT of "challenges" in our path! Having to dis-assemble a significant part of the unit just to find out that I *didn't* make a mistake was probably the biggest "buzz kill" of the whole project, but it simply HAD to be done due to the "documentation errors" in the Howard W. Sams Photofact schematic. We live and learn!
OK, let's get down to business by looking at some photos of the final assembly stages of the "project." First, here's a "View From the Top" of the completely wired unit prior to the addition of the chassis shields:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/SysxzJmLHII/AAAAAAAABZU/D_w3MHN0SZc/s640/Completed%20Chassis%20-%20Top%20View%20%28Initial%20Photo%29.JPG)
While I *usually* HATE having reflections of my arm holding the camera, in THIS case it helps to show how much nicer the chassis turned out than the corroded mess we started out with! Remember how things looked when we began? Here's a little "reminder":
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SoTFhCp3thI/AAAAAAAABJQ/Wr_z-nCJmU4/s640/Scott%20121-C%20-%20Top%20View-shields%20removed.JPG)
While nothing is ever *perfect*, I'd say we're in MUCH better shape now than when we started this journey! At this stage, the final metal parts hadn't been installed yet, but the bare front panel looked like this:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sysx0CIop1I/AAAAAAAABZg/C4nlA1rPX_g/s640/Completed%20Chassis%20-%20Front%20Panel%20-%20minus%20face%20plate%20and%20knobs.JPG)
You'll notice that we had to "elongate" the mounting holes for the replacement Power Switch a bit. Other than that, it all looks pretty "stock", but there's a LOT going on *behind* that stock-appearing Front Panel. A "Three-Quarters" shot from the rear of the preamplifier helps to "fill in the blanks" a bit:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sysxz4hJDcI/AAAAAAAABZc/qJZjZCw6JSU/s640/Completed%20Chassis%20-%20Rear%20Three-Fourths%20View%20%28Initial%20Photo%29.JPG)
A "Three-Quarters" shot from the Front of the preamplifier shows a bit more detail of the Line Level circuitry on the Rear Panel:
(http://lh3.ggpht.com/_Xp_4nF2BnZE/Sysxzr2FKSI/AAAAAAAABZY/ev5rxBRTIwE/s640/Completed%20Chassis%20-%20Three-Fourths%20View%20%28Initial%20Photo%29.JPG)
I must admit, having individual "Trim" controls for the Line-Level Inputs is a pretty nice feature. Coming from a background in Recording Studios, I REALLY appreciate the ability to properly match the levels of the various Line-Level sources! This comes in ESPECIALLY handy when dealing with a CD player that is putting out over 2 Volts of audio, compared with a "standard" Line-Level source that "only" supplies ONE Volt of audio! (By the way, there is an honest-to-goodness REASON why today's CD players supply Two Volts of audio!)
When the CD system was originally "envisioned" it was assumed that CD's would be issued with a 6 dB "headroom margin" and you will often find MANY early CD's that adhere to this "standard" arrangement. In *that* case, a CD with a 1 kHz tone recorded at -6 dB from absolute MAXIMUM level will output a 1 Volt audio signal (or thereabouts) from the Line-Level Output jacks! Of course, as CD's production progressed, engineers found that the 6 dB headroom was unnecessary so they "cranked up volume" to the max! (Personally, I think this was *probably* a step in the wrong direction, but it CAN help to improve the low-level "definition" of the recorded audio on a CD. It just doesn't allow for "overshoots" through the analog portion of the CD player.)
Anyway, let's get back to our little preamp! Here's a picture of the underside of what I *thought* was the completely wired preamplifier:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sysx0bVDFHI/AAAAAAAABZk/KnDM3nkLVUs/s640/Completed%20Chassis%20-%20Bottom%20View%20%28Initial%20Photo%29.JPG)
This picture shows the preamplifier in "totally stock" condition. As it turns out, "totally stock" wasn't completely acceptable. Here's why:
I mentioned that this unit is EXTREMELY sensitive and susceptible to hum pickup, but what I DIDN'T expect was an excessive amount of 120 Hz ripple from the Power Supply using "stock" capacitance values. While it *might* have been acceptable in 1957, I consider it "borderline" when compared with other preamplifiers of the day. Clearly, we needed to add more filtering! Thanks to the smaller size of modern electrolytics, we were able to add two more capacitors underneath the chassis. Here's a picture:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SysybC01dHI/AAAAAAAABZw/4xnhVQZ23kM/s640/Added%20Filter%20Capacitors.JPG)
There was *just* enough room available to "beef up" the Power Supply to bring the 120 Hz ripple to acceptable levels - well BELOW the original published specifications for this preamplifier. I didn't want too much capacitance across the Output of the 6X4 rectifier; THAT could shorten the life of the tube. Instead, I added 47 microfarads to the SECOND stage of the filtering and I added an additional 10 microfarads to the third stage that supplies the Output section of the preamplifier.
With THAT little problem resolved, it was time to "button her up" by adding the rest of the "cosmetic" parts and the shields.
The Bottom Cover is an ESSENTIAL part of the overall shielding of the preamplifier and it turned out a little nicer than I expected. Here's a picture:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/Sysybe4vJfI/AAAAAAAABZ0/ZX60l4v7HUY/s640/Completed%20Unit%20-%20Bottom%20Cover.JPG)
There were a couple of "dings" that I wasn't able to *completely* remove, but all things considered, it's not TOO shabby! (Besides, this is a part of the preamplifier that isn't *normally* "on display" anyway, right?)
Now, here's a picture of the Rear Panel with all of the newly cleaned knobs properly in place:
(http://lh4.ggpht.com/_Xp_4nF2BnZE/SysybsKO0QI/AAAAAAAABZ8/i5N_QM1Ru_g/s640/Completed%20Unit%20-%20Rear%20Panel.JPG)
I did my BEST to try to "save" the little paper "patent notice" label, but it DID suffer a bit of damage during the rear panel cleanup process. I guess you simply can't win them all, can you? Still, it was necessary in order to fully clean up the RCA jacks and remove some interesting "stains" that appeared to be from a spilled soft drink.
Now, here's the part EVERYONE will see the most - the Front Panel, with all of the knobs installed and properly indexed:
(http://lh6.ggpht.com/_Xp_4nF2BnZE/SysybQul5oI/AAAAAAAABZ4/ivi1oN2FKpQ/s640/Completed%20Unit%20-%20Front%20Panel.JPG)
While there are a couple of small scratches that have occurred somewhere during the past 52 years of this preamplifier's lifetime, I think things turned out pretty nicely, don't you?
And our FINAL shot shows the front and rear "cover shields" that serve to "button up" the exposed Front and Rear panels:
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SysybpknasI/AAAAAAAABaA/s4WL6eNIJgo/s640/Completed%20Unit%20-%20Three-Fourths%20View.JPG)
These shields are also an ESSENTIAL element needed to keep hum and noise pickup to a minimum. Unfortunately, I don't have the "proper" tube shields "in-stock" for this unit. Instead, I have added a couple of shields from my stock of Dynaco FM-3 tuner shields. They "do the job", but they are *obviously* not "correct" for this preamplifier. The shield on the left covers the 12AU7 used in the Low-Level Phono stage and the tube shield on the right covers the 12AX7 Output tube. BOTH shields are absolutely ESSENTIAL to keep hum and noise to a minimum!
That about "covers" it! (Sorry! I just couldn't resist the bad pun!) I'll discuss the operation of the unit next.
Sincerely,
Bill Thomas
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Friday, December 18th, 2009 - Update: Part Two!
Let's discuss a few additional points:
Since I had NO "audio reference point" when this project began (after all, ALL of those "bumblebee" capacitors and ALL of the electrolytic capacitors were totally toasted!), there was NO way I could make a "before and after" audio comparison. I CAN report that the completed preamplifier sounds VERY nice and there is NO question that the variable equalization circuitry adds an INCREDIBLE amount of flexibility necessary for the proper playback of early 78's. The "Dynaural Noise Reduction" circuitry goes a LONG way to help reduce surface noise from old shellac records too.
The variable "Turnover" and "Rolloff" potentiometers operate quite differently. The "Rolloff" control has a great deal of effect on the audio, while the "Turnover" control can best be described as "subtle." I also noticed that the audio level of the "Variable Equalization" setting of the Main Selector Switch is substantially lower than the "Fixed RIAA EQ" setting. The schematic diagram also shows this to be the case. I assume this is to keep the extreme settings of the EQ controls from overloading the Low-Level circuitry. Whatever the reason, it works! The Phono Preamplifier stage has the ability to impart a "full-bodied" quality to even acoustically recorded 78's! The Tone Controls also operate VERY well. The shelving frequencies were well-chosen and the amount of boost and cut is VERY helpful! The "Loudness" contours add a bit of boost to both the Low AND High frequencies and under the right low-level listening conditions, they are quite "pleasant" (Unlike many other "Loudness" contours I can name.)
The variable loading for the "Mag Phono 1" Input is a welcome addition and can help "tame" cartridges that have a particularly high Output Level. That's a GOOD thing since this little preamplifier is VERY sensitive! In the "Fixed RIAA Equalization" position, I found that a cartridge with a very high output CAN overload the Phono Preamp stage. (The cartridge I tested this with has an EXTREMELY high Output when compared with most other cartridges out there.) With a Shure V-15 type IV-G cartridge, there was no trace of any overload. (The Shure V-15 type IV-G cartridge was supplied by Neumann with their VMS-70 disc cutting lathes in the 1980's in order to verify proper frequency response from a cut lacquer. I just happened to have one left in my "parts stash.")
It should be mentioned that this is NOT the *quietest* preamplifier I have ever heard. But it is EASILY one of the finer preamplifiers of its time. The amount of flexibility available is simply AMAZING when compared with more "conventional" preamplifiers with "fixed" equalization curves.
Somewhere, I hope Hermon Hosmer Scott has a smile on his face, knowing that one of his "children" has been brought back from the dead!
(http://lh5.ggpht.com/_Xp_4nF2BnZE/SyvI_QYEwtI/AAAAAAAABaI/VSmbF3UvuUo/hhs_port_2%20.JPG)
(This image of H. H. Scott was "borrowed" from the EXTREMELY useful H. H. Scott Hi-Fi Stereo Archive. Visit http://www.hhscott.com/ (http://www.hhscott.com/) for MUCH more information.)
Well, that's about it! I hope this has been an interesting journey back to the early days of "Hi-Fi." It has certainly been *quite* an "education" for ME! We've covered some of the "pitfalls" of restoring a VERY difficult preamplifier to rebuild. *I* have learned a LOT through the "process" myself! I have CERTAINLY learned that with enough time, patience and persistence, almost ANYTHING can be accomplished - even a "home-brew" rebuild of a now extinct potentiometer!
This project has also increased my appreciation for the SIMPLICITY of Dynaco's offerings when compared with the UNBELIEVABLE complexity of this unit! Two VERY different paths to the same overall goal - TRUE audio excellence!
Sincerely,
Bill Thomas
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Kudos and admirations, Bill! :thumb: :D Outstanding work! She's a beauty now!
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Thanks, Hank! I'm just glad to FINALLY be able to call this one finished! I DO think it turned out quite "pretty" from a cosmetic standpoint as well. (It wasn't exactly a "dog" in the looks department when I started, but all the visible metalwork *did* seem to clean up pretty well. That aluminum chassis was ANOTHER story! It was VERY oxidized with a good deal of pitting and corrosion. Without stripping the chassis COMPLETELY, it would have been absolutely IMPOSSIBLE to get it even *this* nice.
And a sincere "Thank YOU" for your encouragement along the way (as well as everyone else here for theirs too!) When a project like this runs "into a wall" the encouragement helps to motivate me into seeking an "alternative", but "acceptable" solution! THIS thing hit more walls than Spiderman did during his first day of web-slinging!
Ahhh, but the GOOD thing is we are now nearly 24 hours into the "burn-in" phase and it sounds *quite* lovely. All controls are quiet, the Power Transformer is nice and "tepid" to the touch and the music still sounds lovely!
Whew!
Sincerely,
Bill
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Sunday, December 20th, 2009 - Update: "No News is GREAT News!"
"Burn-in" continues with absolutely ZERO problem! The music sounds great and the preamplifier is running cool and comfortable to the touch. No hints of any Voltage on the chassis. In other words, all is well!
I'll give it another full day of "burn-in" and then give it one final checkout of all inputs and controls. After that, it's going to its new home for the holidays!
Sincerely,
Bill Thomas
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Superb as always Bill. Thanks for all your excellent work. Happy Holidays :)
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Hi, I just came across this awesome thread! The pictures are not visible -- can they be turned back on please?
Thanks for posting!
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Unfortunately, Mr. Thomas has passed away and I don't have the capabilities to locate the missing pictures as I'd like to see them, too.
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Hi HiFi Geezer,Bill Thomas was the heart and soul of this Vintage catagory.His Dyna rebuild articles are suberb,ask Frank VanAlstine who knows!He died not to long ago,but when I had a printer with my computer,he sent me a message that encouraged me so much,it hangs on my wall,art! I don't know who the moderater here is,but not showing Bills photos are like supressing Ansel Adams pictures of the west...Bill told us all he brought his pictures,that I think are fanstasic,into real sunlight,outside....the Audio Circle should put those H.H. Scott pics on!.not throw into the fire a great library of know how....Mark Korda
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I'd put them up if I could but I see the same thing that you guys do.
It could be the way that the photos were loaded.
JohnR found where the photos were hosted so I'll do my best to try and get them reinstated in this thread.
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Until I can get to it, here's where you can find the photos:
https://www.google.com.au/search?q=GREAT+SCOTT!!!+-+It%27s+NOT+a+Dynaco%3F%3F%3F+-+H.+H.+Scott+121-C&sugexp=chrome,mod%3D12&um=1&ie=UTF-8&hl=en&tbm=isch&source=og&sa=N&tab=wi&ei=dk_IUMOvHMjdigeR8oHABA&biw=1206&bih=589&sei=e0_IUMLsKYTmmAWjp4GgBA#q=GREAT+SCOTT!!!+-+It%27s+NOT+a+Dynaco%3F%3F%3F+-+H.+H.+Scott+121-C&um=1&hl=en&tbo=d&tbs=imgo:1&tbm=isch&tbas=0&source=lnt&sa=X&ei=E1DIUN_vH-PMmgXLwIC4CA&ved=0CBwQpwUoAA&fp=1&bpcl=39942515&biw=1206&bih=589&bav=on.2,or.r_gc.r_pw.r_qf.&cad=b&sei=TwTJUIT5As670QG68YGQCA
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Steve: thank you for working on getting those photos accessible. :thumb:
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Greetings from Reno.
Been working on one of these--complex bitch, especially for a newbie like me.
The pix no longer seem to be available on Google images, or anywhere else. Being able to view them would be a great visual aid for me. I did take digital photos myself of my 121c and tried to make some diagrams, mostly successfully. But I can't, for the life of me, seem to get the Range switch reconnected properly. Dynaural is involved, as is the rumble switch, and I've become hopelessly buggered up. 'Bout ready to heave it in a dumpster. No, not true. But I am severely frustrated. Wish Bill were still around. Does anyone have one of these HH Scott 121c preamps or is familiar with them?
I've studied and studied the schematic but it doesn't make a lot sense (to me--probably similar to a situation of one who doesn't know how to read music attempting to). In addition, in the few photos I've been able to find of the undersides of these devices, they seem to vary, in the wiring, the capacitors used, from unit to unit (same model--121c).
I did alright with a Scott 99D, recapping, installing new resistors where needed, and it sounds great, but it was a cakewalk by comparison. This preamp is infinitely more complex, even convoluted. Maybe I should take it to a pro, now that I've totally screwed it up.
Any comments or advice are welcome. Copies of the photos of Bill's work would be very welcome.
Thank you.
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This post has not been updated in a long time now, but I've been using this since '09 as a reference while restoring my 121C. I happen to have saved every photo that has long disappeared. I have saved them to Flickr so they will not disappear again. I will upload them again for you all here, and thank you to the one and only Bill Thomas who made this possible.
Post #1
(https://farm2.staticflickr.com/1731/41835282614_aeab136feb_b.jpg)
(https://farm2.staticflickr.com/1721/42505771682_9b9fcb5f25_z.jpg)
(https://farm2.staticflickr.com/1733/40749333670_45bf343e92_b.jpg)
(https://farm2.staticflickr.com/1753/41835282484_bf6f9b4f5e_z.jpg)
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August 13th, 2009 - Update: "Formulating a 'Plan of Attack'!"
missing photo.
Top View Shields Removed:
(https://farm2.staticflickr.com/1741/40749587900_5194e1fda2_z.jpg)
Front Panel Rear View:
(https://farm2.staticflickr.com/1747/27687969627_0eaef3a67f_b.jpg)
Gnawed Tubing:
(https://farm2.staticflickr.com/1726/40749573830_55e93fff4e_z.jpg)
Wiring Control Sample:
(https://farm2.staticflickr.com/1749/41835622934_1962b410eb_z.jpg)
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Sunday, August 16th, 2009 - Update: "It's a DIRTY Job!!!" - Part One
Main Chassis Quad Caps Removed:
(https://farm2.staticflickr.com/1730/28684340168_60620a6900_z.jpg)
Separating Quad Cap:
(https://farm2.staticflickr.com/1759/42558258501_118ffcc6a3_z.jpg)
Quad Cap Guts:
(https://farm2.staticflickr.com/1748/42558258561_d80ce4ceed_z.jpg)
Quad Cap Internal Tar:
(https://farm2.staticflickr.com/1749/41835901664_6cb3b28fbc_z.jpg)
Internal Tar Removed:
(https://farm2.staticflickr.com/1745/42558258631_a470684700_z.jpg)
New Quad Capacitor:
(https://farm2.staticflickr.com/1747/42558258661_9764ec9a5c_z.jpg)
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August 21st, 2009 - Update: "It's a Dirty Job - Part Two"
Quad Cap Guts Removed:
(https://farm2.staticflickr.com/1737/28685451838_d78679b75b_z.jpg)
Quad Cap Base With Holes Drilled For Capacitor:
(https://farm2.staticflickr.com/1756/28685452058_68f7d41f2c_z.jpg)
Initial Test Fit Prior To Soldering:
(https://farm2.staticflickr.com/1731/42559386671_308906ee26_z.jpg)
missing photo.
Quad Cap Completed Internals:
(https://farm2.staticflickr.com/1752/28685452098_1683cb787a_z.jpg)
Completed Quad Cap Partially Covered:
(https://farm2.staticflickr.com/1733/28685451968_7cbe44d42f_z.jpg)
Quad Cap Cover Installed Not Glued Yet:
(https://farm2.staticflickr.com/1744/28685452198_e3b4b0d1d7_z.jpg)
Final Inspection Stamp:
(https://farm2.staticflickr.com/1745/28685452298_1aa01a377b_b.jpg)
Unsoldered Wire On Selenium Rectifier:
(https://farm2.staticflickr.com/1746/42559386731_ca625fdcdc_z.jpg)
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Sunday, August 23rd, 2009 - Update: "Separation Anxiety!"
Top Front Panel Of Main Chassis Removed:
(https://farm2.staticflickr.com/1741/42508476402_1b74965521_b.jpg)
Front Panel - Removed - Top Shot:
(https://farm2.staticflickr.com/1736/41838223184_8392bbd4bf_z.jpg)
Bottom Front Panel Of Maine Chassis Removed:
(https://farm2.staticflickr.com/1722/42560540741_58166ae1d2_b.jpg)
Main Chassis - Top - Front Panel removed - Three-Quarter Shot:
(https://farm2.staticflickr.com/1746/42560540921_93eb5cd046_z.jpg)
Front Panel - Removed - Inside Shot:
(https://farm2.staticflickr.com/1748/41838223124_ff8a2f235e_b.jpg)
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September 2nd, 2009 - Update: "Our 'Distinguished' Panel!"
Bent Main Chassis:
(https://farm2.staticflickr.com/1741/40752766530_c9eecc4fb9_z.jpg)
Front Panel - Right Side - As found:
(https://farm2.staticflickr.com/1745/28687448168_c54863af4d_z.jpg)
Front Panel - Middle - 2 Pots Removed:
(https://farm2.staticflickr.com/1759/42561043761_1d88216624_b.jpg)
Front Panel - Right Side - Switch Removed:
(https://farm2.staticflickr.com/1743/40752767160_c55773673d_b.jpg)
missing photo.
Front Panel - Right Side - 75 Percent Stripped:
(https://farm2.staticflickr.com/1743/40752766740_ffa311348d_b.jpg)
Front Panel - Right Side - Rebuilt - 33 Percent Completed:
(https://farm2.staticflickr.com/1721/42561044081_3dff5aebd6_z.jpg)
Front Panel - Right Side - Rebuilt - Completed
(https://farm2.staticflickr.com/1726/28687448058_dd01c5a117_b.jpg)
repeat photo.
repeat photo.
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September 3rd, 2009 - Update: "Trouble! With a Capitol 'T' and That Rhymes With 'P' and That Stands for POT!"
Original Potentiometer Exploded:
(https://farm2.staticflickr.com/1730/42561412491_89cf7c7f67_b.jpg)
Original Potentiometer Rotor And Shaft:
(https://farm2.staticflickr.com/1752/42561412781_630dc896bb_z.jpg)
Radio Shack Potentiometer:
(https://farm2.staticflickr.com/1757/42509461022_9f65d71041_b.jpg)
Radio Shack Potentiometer Gutted:
(https://farm2.staticflickr.com/1741/41839219854_23a7419a0b_z.jpg)
Potentiometer Rotor:
(https://farm2.staticflickr.com/1760/42561412951_c7fa1473db_z.jpg)
Radio Shack Potentiometer - Modified Element - With Slip-Ring Added:
(https://farm2.staticflickr.com/1733/42561412641_65b78e9eb0_z.jpg)
Potentiometer - Rotor And Shaft - Original Plus New Element"
(https://farm2.staticflickr.com/1725/42561413061_11a6737df7_z.jpg)
missing photo.
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September 11th, 2009 - Update: "Our Latest 'Panel Discussion'!"
Front Panel - Left Side - As Found:
(https://farm2.staticflickr.com/1736/41840003464_ef71fc6b71_b.jpg)
Front Panel - Center Area - Stripped:
(https://farm2.staticflickr.com/1729/41840003644_3d0996da77_b.jpg)
missing photo.
Selector Switch - Rear Section - With Shielded Wiring:
(https://farm2.staticflickr.com/1757/42510097572_07324ac283_z.jpg)
Selector Switch - Front Section - Initial Rebuild Stage:
(https://farm2.staticflickr.com/1722/42510097672_1612f4ece8_b.jpg)
Selector Switch - Front Section - Resistor Detail:
(https://farm2.staticflickr.com/1730/42510097712_21b835144a_b.jpg)
Front Panel - Left Side - Potentiometer Wiring:
(https://farm2.staticflickr.com/1758/42510097862_fc93d04078_z.jpg)
Front Panel - Left Side - Selector Switch Wiring - Completed:
(https://farm2.staticflickr.com/1743/41840003604_87a4274d14_z.jpg)
repeat photo.
Front Panel - Left Side - Completed:
(https://farm2.staticflickr.com/1729/42510097622_000a8a8f0a_b.jpg)
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This post has not been updated in a long time now, but I've been using this since '09 as a reference while restoring my 121C. I happen to have saved every photo that has long disappeared. I have saved them to Flickr so they will not disappear again. I will upload them again for you all here, and thank you to the one and only Bill Thomas who made this possible....
That is AWESOME, Shakedown. Thank you for taking the time to restore Bill's photos to the thread - you Rock. :thumb:
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Yes, thank you for doing that! It’s fantastic that Bill’s work can be preserved so that others can benefit from it. Bill Thomas was a true gentleman and a kind soul who willingly shared his knowledge and enthusiasm with the audio community. But, more than that, his humility and civility always impressed me greatly. I could never imagine him posting an unkind word or a snarky remark. Would that there were many more like him on the internet and in the real world.
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I think Mr. Thomas passed not long after I joined here, so I didn't really get to know him, but I remember he used to post these vintage restoration projects all the time, complete with very good instructions and photos; that he answered many questions, and that he had this idiosyncrasy of "putting stuff in quotes" and ending practically every sentence with an exclamation point!
Excellent addition to the vintage library here! Amazing! It would be "Super mega amazing" if "someone with mod privileges" could put them back into the original posts! Oh jeez! Now I'm doing it!
Thanks Shakedown AND Mr Thomas. +1on joeriz' comment.
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So thrilled when I found them on an old hard drive I was about to toss. I had no idea he had passed until recently, very glad I kept them around.
More photos to come throughout the week once I get them online, I'll keep you guys posted. - Ben K.
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Thanks for offering these Ben. It's really great to see some of these photos made available again. All of the Bill Thomas restoration series were a real gift to those of us who pursue vintage audio restoration. It's a shame some of many photos are no longer linked to the threads. I hope you still have plans for sharing any remaining photos for this series.
Dennis C.
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Thank you for taking the time to do this.
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Friday, September 11th, 2009 - Update: "Are We Ready to Get Deranged?"
Range switch in "as-found" condition:
(https://farm8.staticflickr.com/7836/45896044045_12d3273e73_z.jpg)
Range Switch - Rebuilt and ready:
(https://farm5.staticflickr.com/4824/39845763223_e42f9f081e_z.jpg)
Front Panel - Center area - Range Switch mounted:
(https://farm5.staticflickr.com/4830/46811174691_5b55ec22fe_z.jpg)
Range Switch - Wired to Front Panel:
(https://farm8.staticflickr.com/7828/46811186081_1f6473a030_b.jpg)
Front Panel - Center Section - Completed:
(https://farm5.staticflickr.com/4891/46086107804_c023dfaaa5_b.jpg)
Pilot Lamp - replaced:
(https://farm5.staticflickr.com/4873/46758747672_aa6f015b61_z.jpg)
Front Panel - Completed:
(https://farm5.staticflickr.com/4881/46086133844_1162e194dd_b.jpg)
Front Panel - removed - Inside Shot:
(https://farm5.staticflickr.com/4870/46811254641_4988393270_b.jpg)
Power Switch Problem:
(https://farm8.staticflickr.com/7873/45896139715_a6605c6223_b.jpg)
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October 8th, 2009 - Update: "The Plot Thickens!"
Main Chassis - Initial Electronic assembly - Picture Eight - V3 AC Filament Wiring:
(https://farm8.staticflickr.com/7839/32935455998_2fd3e48918_z.jpg)
Main Chassis - Initial Electronic assembly - Picture Three - V1 complete-1:
(https://farm8.staticflickr.com/7923/45896209775_0648070f26_z.jpg)
December 14th, 2009! - Update: "At Last!"
Radio Shack Potentiometer - modified element with slip-ring added:
(https://farm8.staticflickr.com/7890/45896277005_bc9bc9bb3a_z.jpg)
Friday, December 18th, 2009 - Final Update: "Goodbye Yellow Brick Road!"
Scott 121-C - Top View-shields removed:
(https://farm8.staticflickr.com/7872/45896288545_f3f887e05d_z.jpg)
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I made a promise to come back and wouldn't forget. Sorry folks it took so long between transitioning between computers.
Unfortunately all other photos are missing, including the finished restoration but so glad I could save a lot of them.
Thank you Bill for all of this, you were a true legend. - Ben K.