One final (mostly) NEW Dynaco SCA-35 Integrated Amp - "E Pluribus Unum" FINISHED

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Bill Thomas

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October 16, 2008 - Update:  "Well, What ELSE Are You Going To Do With Bauxite?"

     In 1963, Dynaco left the "Brass Era" of audio equipment styling with a totally NEW Integrated Amplifier.  Dynaco called it a "Stereo Control Amplifier" and since it offered approximately 35 Watts RMS of Output Power, the SCA-35 was born.  This was the FIRST Dynaco product to showcase the sleek, modern, "New Generation Styling" using a "Champagne Gold" anodized aluminum faceplate, along with gold-tone die-cast knobs.  But, it wouldn't be the last.  The "New Generation Styling" quickly spread to almost every other unit in the Dynaco product line.  The brass faceplate PAS-2 became the PAS-3 (and later, the PAS-3X).  The brass faceplate FM-1 became the FM-3.  Variations on the "New Generation Styling" continued all the way through the AF-6 Tuner and the Stereo 400 MONSTER Amplifier; in other words, almost until the end of the original Dynaco era.  (Let's not debate whether Dynaco actually continues to exist.  The NAME is still around, but it has ZERO connection with the company David Hafler created.)

     The SCA-35 was Dynaco's FIRST Integrated Amplifier.  It basically combined a simplified version of the PAS Phono Preamplifier stage with a *slightly* revised version of the Stereo 35 Power Amplifier (more gain was needed to allow the addition of bass and treble tone controls).  All this, for less than $100.00 (in kit form)!  For a 14 year-old kid with his first job (that would be ME!), this was a true Godsend!  The SCA-35 became my FIRST Dynakit!  I had the GREAT fortune of having a local electronics store that was owned by a truly WONDERFUL gentleman named John Brookshire, Jr.  John is no longer with us, but his store, "Brookshire Radio Supply" is STILL in operation in Anderson, South Carolina. Sadly, the days of being able to walk in and buy a Dynakit are LONG gone.  But in 1964, John made it possible for this 14 year-old kid to experience the joys of high-quality sound reproduction (once I actually BUILT the thing)!  My SCA-35 served me almost continuously until it was destroyed in a Storage Warehouse fire in the early 1980's.  It NEVER needed a repair and was only on its second set of tubes when it was destroyed.

     When I was amassing my collection of Dynaco gear, I was always on the lookout for another SCA-35.  I eventually wound up with several, but not a single one was in operating condition.  A couple of them had been "hacked-up" internally and a few were assembled so badly, I didn't DARE try to power them up! But, that's all about to change!

     Allow me to present our next restoration/rebuild candidate.  I'm calling this one: "E Pluribus Unum!"  The literal translation is: "Out of many - ONE!"  Usually, this "motto" refers to the founding of our great Nation, but it also seems perfectly appropriate for this particular build chronicle because it's going to take metal parts from SEVERAL amplifiers in order to make one BEAUTIFUL one.  Electrically, this will be as close to a new unit as possible.  While inventorying my stock of NOS Dynaco parts, I came across a complete set of NOS potentiometers, a set of four NOS PEC modules and even a NOS Selector Switch for a complete SCA-35.  We'll add all new Slide Switches and new component parts in order to make this as close to a NEW SCA-35 as humanly possible.  THIS one will be a real treat!  (And, it will allow me to revisit my misspent youth!)

     Let's start our story with a picture of the front of an SCA-35 in "as found" condition:

                             

     This is ONE of our "Donor Units."  This particular SCA-35 was a factory-wired unit.  Normally, that would be a GOOD thing.  But since we are looking for a good set of metal parts upon which to build the equivalent of a NEW SCA-35, this one isn't a great choice.  For one thing, the chassis isn't in the best condition.  Here's a "view from the top":

                             

     There's an AWFUL lot of rust and corrosion.  And since everything is riveted to the chassis (even the printed circuit boards themselves), trying to do any major chassis restoration is nearly impossible.  But, there are MORE problems.  Take a look at the back of the amplifier:

                             

     Besides being VERY dirty and discolored, the back panel has been damaged and bent.  Once again, poor packaging prior to shipment allowed the fuse holder to get "punched" into the back panel.  Take a look at the extent of the damage:

                             

     At least the transformers are still good and THAT is a VERY good thing because our other "donor chassis" has a shorted Power Transformer!  It's too bad the face plate also suffered damage or we could have had it re-screened to restore the lettering.  But that is not to be because of this:

                             

     At least we have our Transformers.  Now all we need is... another DONOR!  We'll look at some of the "interesting" details of our OTHER donor in the next update - coming soon!

Sincerely,

Bill Thomas
« Last Edit: 18 Nov 2008, 10:57 pm by Bill Thomas »

Bill Thomas

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Hi, Otis!

     We're in a "holding pattern" on the "Ultimate Stereo 70" while the chassis is off gettin' "gussied up!"  I was too busy just twiddling my thumbs while waiting for a response from the guy doing the "gussying", so I just decided to turn out a nice SCA-35 while I'm waiting for the chassis to return.  I ran into *just* the NOS parts I needed for one final (mostly) NEW SCA-35, so it jumped to the head of the line.  Once the chassis returns for the "Ultimate Stereo 70", I'll get right back on it - PROMISE!

Sincerely,

Bill Thomas

Bill Thomas

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October 17, 2008 - Update:  "Another Day - Another Donor!"

     OK, we have our transformers, but now we need to find something to mount them to!  Well, in order to come up with a complete set of metal parts, we need to call on our next donor - a kit-built donor that is ANYTHING but well built!  In fact, THIS unit was assembled SO poorly, it probably caused a short that took out the Power Transformer.  Take a look at a few little anomalies.  First up, here's a picture of the wiring around one of the Output stages:

                                       

     The other Output stage isn't a whole lot better.  Take a look:

                         

     More scorched insulation.  Not a good thing!  But even worse is the soldering of the wires to the eyelets on the Printed Circuit Boards.  Do you think they missed reading the manual when it explained how to solder to the eyelets?  They MUST have because the solder doesn't even come CLOSE to surrounding the wire, nor does it do ANYTHING to solidify the electrical integrity of the eyelet to circuit trace connection.  Of course, they might have just forgotten to do a proper soldering job.  We DO have some evidence of this possibility.  Take a look at the wiring to one of the AC receptacles:

                         

     Yep!  Not a *trace* of solder on one of the more "important" connections in the entire amplifier.  The funny thing is, the little black wire that connects to the side of the fuse holder ALSO missed getting soldered.  I'll bet this amplifier suffered from intermittent noise and arcing at these locations.  It suffered other problems as well because one of the Output Tube sockets was replaced with a "generic" plastic socket.  Not only that, one of the original sockets is missing one of its mounting screws, which allowed that particular Output Tube to "wiggle around" an AWFUL lot!  Here's the end result of all these errors:

                                       

     Notice the top 5 Watt wire-wound resistor?  It is the Cathode resistor for all the Output Tubes.  Notice the crack in the case?  Yep!  This is now an open circuit!  And take a look at the lower resistor.  It got so hot, the case literally cracked and fell off the resistor.  This is an "EX" resistor!  Now, it is simply "junk!"  No WONDER the Power Transformer wound up "toasted!"

     Obviously, this particular "donor unit" won't be allowed to remain in THIS condition!  Let's see if it can yield the metal parts we'll need (once we strip all the parts and poor wiring out of it)!  Here's a picture of the main chassis after stripping, but before cleaning:

                                                 

     We'll take a little time to thoroughly clean all the metal parts so we have a nice, clean foundation upon which we'll add our NOS Controls, NOS Selector Switch and NOS PEC modules.  Here's a picture:

                               

     In our next update, we'll show you the results of our cleanup operation on the metal parts, as well as the initial mechanical assembly with the NOS components.  Stay tuned!  This is *beginning* to get interesting!

Sincerely,

Bill Thomas

Bill Thomas

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October 17, 2008 - Update:  "...It's a Clean Machine!"

     Well, the FOUNDATION of our SCA-35 is a LOT cleaner.  Let's run through the results together.

     I'm a total SUCKER for a pretty face, so let's start out with a "face shot" of the cleaned up front panel:

                             

     I'd KISS the thing, but THAT would mess it up!  Not bad at all!  (I was surprised, considering the electrical condition of the amplifier!)  Now, here's a shot of the back of the Front Panel:

                               

     Not quite *perfect*, but certainly acceptable.  Those little dark spots are basically cosmetic blemishes.  No problems here!

     So, let's load it with NOS parts.  The front view looks like this:

                         

     And here's a picture of the Front Panel from the back - all loaded with NOS Parts:

                         

     Other than the bit of staining on the lip of the panel and the few "freckles", things look amazingly good for a 44 year-old unit!  The picture actually makes the staining look worse than it really is.  Here's a shot "from the top" to give you a better look:

                         

     I'm guessing that we're looking at the result of a spilled Coke that dripped its way down the back of the Front Panel.  The *teensy* little bit of Phosphoric Acid (and 40+ years) caused a reaction that stained the cadmium plating.  The plating is still good, just discolored.

     OK, let's turn our attention to the Back Panel.  It's a little harder to clean the Back Panel due to the silk-screen printing.  The printing on our "donor chassis" wasn't "pristine", but it's not bad either.  Here's a picture of ours with a few NOS parts installed:

                         

     When cleaning yours, know when to quit!  If you don't, you'll start rubbing all the lettering off the back.  Now, here's an "inside view" of the Back Panel:
               
                         
     
     We'll mount the fuse holder and the RCA jacks and then we'll start wiring the Front Panel.  But first, let's take a look at the Main Chassis.  Here's a picture of the underside of the Main Chassis after a thorough cleanup:

                                   

     No major problems here!  And now, here's a shot of the top of the Main Chassis:

                                   

     Once again, the camera makes the coloration a bit more pronounced.  Here's what it looks like if we put 'em all together in a "mock-up" arrangement:

                                   

     That wraps up this installment.  We now have a truly SOLID foundation for Dynaco's ONLY tube-type Integrated Amplifier.  In our next installment, we'll finish the mechanical assembly of the Rear Panel and the Main Chassis.  Then, we'll begin wiring the Front Panel.  I'll cover it all and a whole lot more, in our next "thrilling" chapter, coming SOON!  Don't miss it!

Sincerely,

Bill Thomas

jon_010101

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Holy cow, where did you get replacement tone control networks?  :o

Great work as usual!  :thumb:

Bill Thomas

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Hi, John!

     Thanks for the kind words.  I have several sets of NOS SCA-35 PEC modules in my "parts stash" that I acquired over the years.  When I mentioned that I had quite a few "rare" or "hard to find" parts, these parts are certainly on that list.  I'm STILL sorting through inventory to make it easier to just FIND things.  This will probably take the better part of a year to put in order.  (It took almost 15 years to acquire this stuff in the first place!)  I am *desperately* trying to clear out a 10 by 30 Storage unit that was nearly FILLED with Dynaco amps, preamps and tuners, along with a HUGE amount of either NOS Dynaco parts, or NOS parts like Allen-Bradley Carbon Composition resistors and other OEM parts.  Anyone need new 500 Ohm 5 Watt wire-wound resistors for an FM-3?  I probably have a thousand of them.  No, I don't have EVERY part to restore EVERY unit, but I do have an awful LOT of parts than might help get a LOT of units working again.

     My FIRST goal is to rebuild or restore as many of the complete amps, preamps and tuners as I can, in order to give as many folks as possible the BEST chance to hear what Dynaco was able to create "back in the day."  Yes, it's a labor of love.  While most teenagers were into Fords or Chevy's, *I* was a Dynaco "junkie."  I guess *this* is the result!

     Onward!

Sincerely,

Bill Thomas

dewaldv

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Hello Bill

Very interesting project - I am following with great interest!

Dewald V

Lydenburg, South Africa

Bill Thomas

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Hi, Dewald!

     Sorry the next "update" is a little delayed.  Much is going on "behind the scenes" and I will have a more complete update coming up in the next couple of days.  But, to "tide you over", I'm going to do a "mini-update" next.  I must say that this is turning into a really nice amplifier, but that would be getting a bit ahead of the story.

Sincerely,

Bill Thomas

Bill Thomas

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October 21, 2008 - "Mini" Update:  -  "A Little Detective Work"

     Our SCA-35 is coming along nicely, but a bit slowly.  Here are a few things I found "along the way" while preparing for the re-assembly.

     Our first picture shows the two Output Modules.  One is in "As Found" condition.  The other has been totally stripped (except for the 7199 socket which is in great condition):

                                                 

     If you'll take a look at the tube sockets, you'll notice the black socket that was on the right side of the circuit board.  You have to wonder why the previous owner had to replace a tube socket, but his choice of replacement socket was NOT a good one.  That socket was designed to mount to a chassis or circuit board from underneath the surface.  But, he decided to deform the mounting "ears" and mount it topside.  Yeah, it *worked*, but it was *beginning* to cause problems for the phenolic substrate.  That tan socket on the left is a NOS OEM Eby socket from my Dynaco "parts stash."  Here's another look at the two sockets:

                                     

     Needless to say, that black socket "wiggled" all over the place when it had a 6BQ5/EL84 plugged into it.  But more importantly, it was beginning to deform the circuit board underneath the socket.  Here's a better look at that deformation:

                               

     We'll replace the incorrect socket with the nice, new Eby socket and all will be right in Dynacoland once again.  Here's a look at the underside of the circuit board. 

                 

     It has been cleaned, but a little residue remains on the left side of the board.  That will also be cleaned off and we'll rebuild the module (along with its mate).

     As Monty Python might say: "And now for something completely different!"  Here's a picture of the necessary parts to rebuild the main Power Supply for the SCA-35:

                             

     The diodes are common Radio Shack items and are rated a LOT higher than the original diodes.  The filter capacitors are, of course, new production.  We're using our familiar 80-40-30-20 capacitors to replace BOTH of the original can capacitors on this SCA-35.  A little judicious wiring (along with one additional small radial electrolytic), and they'll serve our needs quite well.  That's a good thing because only one of the original value can capacitors has been available recently.  The original dual 50-50 uF unit isn't available anywhere!  When we're finished, the Phono Preamp stage will have some extra capacitance to play with.  Nothing wrong about that!

     We'll leave it here for now, but there is much more coming very soon.  We'll be covering the assembly and wiring of the Front Panel in our next installment - coming soon.  Don't miss it!

Sincerely,

Bill Thomas

     

dewaldv

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Hi Bill,

Thanx for the reply.

I wish I can tackle such a project but alas, I dont think here is any Dynaco products in Africa.

I am working on a SE amp that uses a single 6C33C tube per channel and huge Hammond Output Transfos.

D

Bill Thomas

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October 26, 2008 - Update:  "The Circuit Board Circus"

     As promised, here's a little update on our SCA-35 project.  Rather than updating you on the Front Panel wiring, we've been getting the Printed Circuit Boards into MUCH better shape.  A couple of new tube sockets, twenty-eight passive components, (fourteen per module), and a LOT of cleaning, have put us in MUCH better condition.  Here's a look at our two Output Modules:

                                       

     The top one is in "as found" condition.  The lower one has been cleaned and repopulated with new passive components.  Quite a difference, wouldn't you say?  Now, here's a look at both modules after they have both been rebuilt:

                                         

     These two modules make up the entire Power Amplifier circuitry of the SCA-35.  That's 35 Watts in an area not much bigger than a Post Card!  These two modules generate quite a bit of heat, so make SURE you solder all the components carefully and completely.  This is NO place for a cold solder joint to cause noise in the finished product.  It's a bit of a balancing act.  You need to use enough heat to allow the solder to make good connections, but not so much heat that the circuit board traces begin to lift from the phenolic substrate.

     Two of the Output Tube sockets have been replaced with NOS Eby sockets because the originals had "issues", while the other two are the original Eby sockets that have been cleaned and de-oxidized.  They're in fine shape.

     Two circuit boards done with one more to go.  We'll take a look at the Preamplifier Circuit Board in our next update.  We'll also take a look at the mechanical assembly of the Main Chassis as well.  With any kind of luck, we should be listening to some tunes later this week!  Our updates will be coming more frequently until then, so stay with us!  The BEST is yet to come!

Sincerely,

Bill Thomas

Bill Thomas

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October 29, 2008 - Update:  "Phono Board Phun!"

     I guess we start this by my apologizing for the delay in posting this update.  I have a solution for this.  I'll stop TELLING you when the next update is scheduled!  That way, I simply CAN'T be late, right?  Seriously, doing two projects at the same time is a lot like herding cats.  There's always SOMETHING that is trying to go out of control!  But we really HAVE made some good progress on our SCA-35 amplifier.

     As promised, this update covers the Phono Preamplifier Circuit Board.  Our first photo shows the first step in the rebuild process:

                                       

     The Phono Preamplifier Circuit Board consists of two identical channels, so the EASY rebuild method is to do one channel at a time.  We started by stripping the components on the left side of the board (except for the tube socket, but it was removed later), and then we clean, clean and clean again.  Here's a picture of the underside of our "half-cleaned" circuit board:

                               

     All the circuit traces are now nice and clean, but you MUST use EXTREME care when un-soldering components.  Use too much heat and the circuit pads and traces will lift right off the phenolic substrate!  We must be "living right" because not a single trace lifted, or even showed any signs of lifting.  However, all is not totally rosy on the "cleaned" side.  Take a look at the solder joints for the tube socket pins:

                                   

     While some of that "stuff" is leftover flux (which lead me to the conclusion that this socket has been previously replaced), the solder joints don't look very good at all.  I had originally toyed with the idea of keeping the sockets, but they really didn't grip the tube pins all that well, so they're goners!

     By leaving the Right side of the board alone, we have a guide that  really simplifies "stuffing and soldering" the new parts on the Left side.  Here's a picture of the result:

                                             

     The left side of the board has been completely rebuilt with all new parts.  Now, we can clean off the right side like this:

                                               

     This time, we'll use the LEFT side of the board to help us "stuff" the RIGHT side.  The end result gives us a finished product.  Take a look:

                                               

     Eagle-eyed viewers will notice that the Left side has 1/2 Watt grid resistors, while the Right side has 1/4 Watters.  Since there is no grid current, (There'd better NOT be any grid current!), the Wattage makes ZERO difference.  It just happened to work out that these were a closer match between channels.  All the resistors are matched to better than one-tenth of one percent.  You might also notice that we have "beefed-up" the coupling capacitors from the first amplification stage.  The original value was "22 nF" which works out to .022 uF.  We have increased the value up to .047 uF.  This will help to improve the bass response of the Phono Preamp section.  The SCA-35 is *occasionally* criticized as having insufficient bass in the Phono Preamp section.  Hopefully, this will improve that situation a bit.  We'll see when we fire it up later this week.

     We ARE getting closer to a complete amplifier.  Here's a picture of the Main Chassis with all of the circuit boards installed:

                               

     But before we get TOO excited, here's a look at the underside of the Main Chassis:

                     

     Yeah, we've still got a good bit of wiring left.  But this is a good stopping point for this update.  Coming up next, we'll take a look at the Front and Rear Panel wiring and MAYBE even the Main Chassis wiring.  Why, with any kind of luck, we'll be "making music" by the weekend!  I promise to keep you posted on our progress.  As always, feel free to ask questions, or post comments.  OR, you can ALWAYS e-mail me at: nostubesforyourdynaco@gmail.com.

     I've GOTTA tell you, this has REALLY been a "Blast From the Past" for me!  No, I don't feel fifteen again, but I sure do remember putting my FIRST one together back in 1965!  It's kind of amazing how building this one has triggered so many DEEPLY buried memories from so long ago.  We'll make MORE memories with our next update - coming VERY soon!  Stay tuned!

Sincerely,

Bill Thomas

salb203

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Bill,
Once again you simply amaze me with your workmanship. Just 3 days ago I picked up a SCA-35 in working nice condition, but it will need recapping and a general rebuilding as the EL84's in one channel started to glow red.

I am eager to see your final results.

Regards,
Sal

Bill Thomas

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Dear Sal,

     That is high praise coming from YOU!  The caliber of your work is beyond reproach!  Many humble thanks!  And with that...

Bill Thomas

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November 3, 2008 - Update:  "We Ain't Done Yet, But We Sure Are Closer!"

     In the SCA-35 Construction Manual, Bob Tucker wrote:  "The construction of the SCA-35 should take no more than a few hours."  I suppose that depends on how you define "a few!"  To be honest, I spent more than "a few hours" just cleaning the metal parts!  Construction of THIS SCA-35 will run probably run closer to 50 hours, when all is said and done - maybe more!  But, enough of my "venting", let's get down to BUSINESS!

     We've FINALLY gotten around to actually WIRING something!  Here's a picture of the initial wiring of the Front Panel:

                   

     The SCA-35 uses "PEC Modules" for the tone control circuitry.  PEC stands for "Packaged Electronic Circuit" and when you think about it, these resistor-capacitor networks were the forerunners of integrated circuits.  Centrelab made these for Dynaco, but they made MANY different little modules for radios, televisions and all *sorts* of electronic devices.  They were generally inexpensive and tended to be pretty reliable (in non-pulse applications).  Of course, TODAY they are nearly extinct!  finding a complete NOS set, along with a set of NOS potentiometers, made this project possible.  Let's look a little closer at the modules:

                   

     This shows why the SCA-35 is NOT a kit for the first-time kit builder.  There are 14 wires coming from those two little modules.  They must be routed VERY carefully to avoid a short.  This is ONE place where neatness REALLY counts!  Ahh, but there's MORE fun in store!  Here's a picture of the front panel with about two-thirds of the wiring completed:

                   

     The wiring of the Filter Switch is now complete.  But, take a closer look at the connections to the Balance Control:

                                       

     Two of the terminals on the Balance Control have FOUR wires attached to them!  These terminals are designed to accommodate one or two wires - three is a stretch!  But FOUR?  You gotta be kidding!  You REALLY have to be living right to get four wires through those holes, but it CAN be done!  Keeping it neat is the REAL trick!

     Compared with the Balance Control, the Volume Control wiring is a piece of cake!  Here's a picture with the Volume Control wiring about 75% done:

                                                   

     There are still three "Ground" wires remaining to be connected, which brings us to the topic of "Grounding", in general.

     Compared with the circuitry in a PAS preamplifier, the SCA-35 uses a VERY different grounding arrangement.  In the PAS preamplifier, "Ground" is shared between the two channels, but in the SCA-35, separate, independent "Grounds" are maintained for each channel.  They are only connected together at the main filter capacitor ground.  This reduces crosstalk between channels, but it increases the complexity of the Front Panel wiring.  And we've made it even harder by using shielded wiring for the bulk of the Front Panel audio path.  But the payoff should be worth it!

     Here's a picture of the completely wired Front Panel (with the exception of two more shielded wires that will connect to the Selector Switch):

                   

     Here's a closeup of the Loudness Switch and the Stereo-Mono Switch wiring (with the two shielded wires added):

                         

     So, with our Front Panel completely wired, it's time to move on to the Rear Panel.  And we WILL, in our next update coming up within the next 24 hours!  (Honest!)  Stay tuned!

Sincerely,

Bill Thomas

Bill Thomas

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November 4, 2008 - Update:  "Advance to the Rear!"

     Let's take a quick look at the pre-wiring of the Rear Panel of our SCA-35.  Here's an overall picture of the pre-wired panel:

                   

     Quite a jumble of wires at this point.  One of the "cute" elements of the original SCA-35 wiring was the use of two "ribbon cables" to connect the Line-Level jacks to the Selector Switch.  While it might have been "cute and colorful", it didn't do much to minimize hum and crosstalk between sources.  So, "cute and colorful" are out!  Instead, we are using shielded cables to connect the Line-Level jacks back to the Selector Switch.  Here's a closeup of the RCA Jack Field:

                   

     This will all bundle into a nice, neat arrangement once we get everything assembled.  Also, rather than rely on twisted wires for the Low-Level Inputs, we have used shielded wiring here too.  There is more of a chance to pick up a stray AC field in the SCA-35 Amplifier, than in a PAS preamplifier because the filaments of the preamp stage are fed with AC, rather than DC in the PAS preamplifier.  Shielding of the Low-Level Inputs will minimize the possibility of hum pickup.

     The other side of the Rear Panel deals with the Hum Adjust Potentiometers and the AC wiring:

                   

     We have made a slight change from the original construction in order to make certain the "Convenience Outlets" are wired with the same polarity.  In the original construction instructions, the two convenience outlets are wired opposite each other.  Of course, since there is no polarizing socket "in the mix", this isn't a BIG issues, but it just makes things more consistent between the two outlets.  Other than that, everything else is wired "by the book."

     Now, all we have to do is "fill in the middle" between the Front and Rear Panels!  We'll get to that in our next update, coming up VERY soon!  Stay Tuned!

Sincerely,

Bill Thomas
« Last Edit: 5 Nov 2008, 10:03 am by Bill Thomas »

Bill Thomas

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November 4, 2008 - Update TWO:  "Let's Talk About POWER!"

     An amplifier can only deliver as much Power as the Power Supply makes available.  Our first Update on the Main Chassis begins with the Power Supply.  The Power Supply in the SCA-35 is a conventional, full-wave, solid-state design.  There's nothing at all fancy about it.  The PA-774 Power Transformer has two 6.3 Volt filament windings, and a 630 Volt, center-tapped High Voltage winding for the B+ Supply.  The center-tap of the High Voltage winding connects to one of the first multi-section Filter Capacitor's can connections.  The other wires go to Terminal Strip TS-1, and two silicon diodes:

                   

     There's nothing special about the diodes.  They are common Radio Shack parts and are rated significantly higher than the original diodes.  While the construction manual doesn't suggest it, a little insulated heat-shrink tubing over the diode leads never hurts.  Notice that both Cathodes of the diodes are connected together.  The Anodes are connected to the transformer windings.  The red wire from TS-1 then connects to the first section of the first multi-section can capacitor.  There's a good bit going on around the can capacitors, so let's take a look at them now:

                   

     The First multi-section can capacitor is the one on the right side of the picture.  The red wire from TS-1 connects to the 80 uF section (20 uF more than the original capacitor.)  The 50 Ohm resistor connects between the first and second sections of the capacitor.  The two red, cloth-covered wires go to the Output Transformers.  The 2,200 Ohm resistor then connects between the second and third sections of the capacitor.  From there, another 2,200 Ohm resistor connects to the first section of the OTHER multi-section can capacitor.  The second multi-section can capacitor is quite different from the original two-section can capacitor, so let's talk about the differences.

     In a stock SCA-35, the second can capacitor is a two-section 50-50 uF unit that supplies the B+ Voltage to the equalized (phono) preamplifier circuit board PC-11 and the 7199 Driver Stage on the two PC-10 Output Modules.  Since that capacitor is not being produced ANYWHERE, we are substituting another 80-40-30-20 uF capacitor (the same capacitor as the FIRST can capacitor) for the original unit.  If you look closely at the wiring of the second can capacitor, you'll see that three sections are wired together!  That is NOT a mistake!  What we are doing is paralleling the 40, 30 and 20 uF sections together to give us a total of 90 uF.  So, instead of a 50-50 uF capacitor, we have the equivalent of an 80-90 uF capacitor.  The extra capacitance will actually HELP us by "stiffening up" the B+ source for the preamplifier and driver circuits.  (And we aren't taking chances on a 40 year-old capacitor that might just be ready to explode!)  But there is one more little "deviation" from stock that we have yet to cover.

     The fourth section of the first multi-section can capacitor was originally rated at 100 uF.  It is used as the Cathode resistor (that 95 Ohm resistor in the photo) bypass capacitor for both Output Modules and all four Output Tubes.  Our current replacement capacitor is only rated at 20 uF for that section.  To solve that problem, we have added a common Radio Shack 100 uF, 50 Volt capacitor (that *teensy* little blue electrolytic capacitor) in parallel with the 20 uF section of the can capacitor.  This brings the total capacitance up to 120 uF which will replace our original 100 uF capacitor just FINE!

     Whew!  I *told* you there's a lot going on around those can capacitors.  But the end result will be a Power Supply that will deliver more consistent DC than the original Power Supply could.  Here's a picture of the Main Chassis after completing the Power Supply:

                             

     This is a GREAT beginning, but we've got a lot ahead of us.  Here's the next area we have to tackle, the two Output Modules:

              

     I see four tube sockets that are pretty naked looking.  We'll fill them full of wires in our next update, as well as complete the filament wiring from the Power Transformer to the three Printed Circuit Boards.  PLUS, I'll pass on a REALLY great tip for cleaning up those old die-cast knobs.  You are simply going to SWOON over the results!  (I know *I* did!)  It's all coming up in our next fun-filled installment, coming VERY soon!  Don't MISS it!

Sincerely,

Bill Thomas

Bill Thomas

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November 6th, 2008 - Mini-Update:  "Ready to Swoon?"

     While I'm still knee-deep in the re-wiring of the Main Chassis, I simply HAD to pass on this tip regarding those old, ugly, oxidized die-cast original Dynaco knobs.  Here's a picture of one of the small ones that came from the Front Panel of our original Factory-wired SCA-35:

                                       

     Pretty grubby looking, isn't it?  Wouldn't it be GREAT if we had something that could bring this knob back from the "Ugly Farm?"  We do!  Here's a picture of the same knob after about 5 minutes of cleaning.  (Trust me, this picture doesn't really do justice to the amazing difference!):

                             

     Further cleaning would improve the appearance even more!  Here's the "magic elixir" that can literally bring these knobs back from the DEAD:

                                             

     It's available at most supermarkets.  It's cheap and it does an AMAZING job!  If you have a unit with grubby knobs, try it for yourself and report back here with YOUR results.  Just be prepared for the ammonia smell and if you have a cut, you will want to wear rubber or latex gloves when using it.  (Don't try it without removing the knobs from your unit.  You really NEED to wash the stuff off with water.)  I haven't tried using this on the later machined aluminum knobs, so make SURE you have the die-cast knobs before trying it.

     That's it for this "Mini-Update!"  I'll try to finish up the SCA-35 shortly, but be advised that since no one decided to purchase "The 38 Year-Old Virgin" two days ago, my ability to continue providing these articles, (as well as "reasonably priced" Dynaco equipment), has been SEVERELY limited.  (It's a LONG story!)

     Sad, but true.

Sincerely,

Bill Thomas

cryoparts

 :rock:  You are the man!

Peace,

Lee

joeriz

Quote
I'll try to finish up the SCA-35 shortly, but be advised that since no one decided to purchase "The 38 Year-Old Virgin" two days ago, my ability to continue providing these articles, (as well as "reasonably priced" Dynaco equipment), has been SEVERELY limited.  (It's a LONG story!)


Sorry to hear that, Bill.  I hope it's nothing serious.  I love these threads!

By the way, I'll tell you what else does a great job on those older die-cast knobs: Nev-R-Dull.  Works like a charm!  I usually give them a good rub-down with a wad of Nev-R-Dull and then, to clean all of the solvent off, a few spritzes of Windex and a good drying with a paper towel.  Makes them look brand new (except for any scratches, of course).

Joe