GREAT SCOTT!!! - It's NOT a Dynaco??? - H. H. Scott 121-C Preamplifier - DONE!

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Bill Thomas

     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."

       

     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:

             

     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:

     

     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:

               

     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
« Last Edit: 18 Dec 2009, 08:20 pm by Bill Thomas »

acwd1950

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

Bill Thomas

     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.  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

doorman

Very informative!
I've long been an admirer of Scott equipment. I'd love to hear one of (Mapleshade's) rebuilt Scott integrateds.
Don

smbrown

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!

Bill Thomas

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:

     

     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:

               

     Now we can get a MUCH better look at the wiring and components behind the front panel.  Here's a picture:

       

     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:

               

     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:

               

     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
« Last Edit: 14 Aug 2009, 05:51 pm by Bill Thomas »

acwd1950

Bill,
You might try toothpaste and a soft bristle toothbrush on those aluminum knobs thats what I use on my vintage stuff. HTH

Steve

Scott F.

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.

Bill Thomas

     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

Bill Thomas

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:

               

     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:

                                                 

     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:

                                           

     You can get a better idea how one is constructed by looking at the end of the "core", see?:

                   

     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:

                   

     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:

                       

     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:

                   

     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

Bill Thomas

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:

                                                 

     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:

               

     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:

                                       

     This picture of the base shows how the wires from the individual capacitors will protrude through the small holes we drilled in the base:

                             

     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:

                                             

     Now, let's slide the can back on and check the final "fit" once again:

                                                     

     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:

                                                 

     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:

     

     The only problem is, they MISSED soldering one connection!  Here's a picture:

                   

     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

Listens2tubes

Very interesting as always Bill. :thumb:

Bill Thomas

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:

   

     And here's a picture of the separated Front Panel:

                   

     Why should the project be so intimidating at this stage?  Take a look at the underside of the Main Chassis:

   

     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:

           

     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:

   

     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

smbrown

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.

Bill Thomas

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:

             

     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.

             

     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:

     

     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:

       

     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:

                         

     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:

     

     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:

               

     And here's a picture with this portion of the Front Panel completed:

     

     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:

         

     And this is how things look AFTER the rebuild:

         

     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

Bill Thomas

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:

     

     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:

                           

     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:

     

     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:

                             

     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:

                               

     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:

                                       

     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:

                           

     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:

                 

     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

     


Bill Baker

Beautiful work Bill. I can only imagine the patience it takes to complete such a restoration.

Bill Thomas

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

Bill Thomas

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:

     

     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:

     

     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:

                         

     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:

                         

     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:

     

     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:

     

     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:

               

     And here's the wiring around the Main Selector Switch:

               

     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:

     

     And here's what we wound up with:

     

     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

     

Bill Thomas

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:

                 

     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:

                 

     Here's what it looks like when mounted to the Front Panel:

         

     With the switch wired-in to the Front Panel, we come up with this:

     

     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:

     

     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:

                         

     Put it all together and here's the result:

     

     Compare our final result with the "before" picture:

     

     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!

     

     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