[Bill Thomas]

     Hello again!  After the ROUSING success of my first Stereo 70 rebuild, (See http://www.audiocircle.com/index.php?topic=58338.0), I had quite a few people who asked whether I am ready to re-build another Stereo 70.  Well, here it is!  I've named THIS project, "Don't Call Me 'Dimples'!"  I'll explain.

     If you read about "Nothin' Fancy", you'll notice that I mentioned another Stereo 70 that had been "punched" at the fuse-holder.  Well, THIS is that amplifier.  But before I began construction, my Brother and I spent a bit of time doing a little "chassis straightening."  Considering the initial damage, I had pretty low expectations that we could save this chassis.  The final result of the straightening, while not *perfect*, is certainly VERY acceptable.



     The chassis was VERY dull and had a "mottled" appearance.  It took some SERIOUS polishing to bring this one "back from the dead".  Here's a picture.



Once the polishing was done, it was time to perform mechanical assembly.  This amplifier is a bit different from "Nothin' Fancy" in that I am using ceramic tube sockets instead of Micanol this time.  Here is an under-chassis picture of the initial mechanical assembly.



Note the new sockets, the new fuse holder, the new Filter Choke and the new terminal strips and RCA Input Jacks.  Also note the Spectrol 10-turn potentiometers I am using to replace the original single-turn bias controls.  Once again, I have re-used two of the original black plastic tube sockets for the front panel "biaset" sockets for aesthetic reasons.  Here's a picture of the front and top of the chassis.



In my next update, I will cover the wiring of the chassis.  Then, it'll be time to completely rebuild and install the Driver board.  Details to follow!

Sincerely,

Bill Thomas

[Bill Thomas]

8/20/2008 Update:

     Well!  A LOT has taken place since the last post.  Although the amplifier isn't *quite* finished, it's getting close.  We've gone from the bare-bones mechanical assembly, to completely wired chassis - just waiting for the addition of the Driver Board.  Here's what it looks like at this point:



     The "flying wires" you see are for connection to the Driver Board.  Here's a picture of the under-side of the chassis:



     I wish I had been able to take some "before" pictures, unfortunately I was quite a distance from my camera when it became necessary to strip the chassis to do more straightening.  You'll notice in the above picture that there is some "marring" of the metal "lip" near the fuse holder.  It was necessary to use some serious force there and then re-shape the lip, in order to keep the back panel as damage-free as possible.  I think it was a reasonable trade-off.

     Now, on to the Driver Board.  I *did* manage to snap a "before" picture of the Driver Board.  It looked like this:



     Looks like it's just a little dusty, right?  Well, take a look at THIS picture:



     This is the same board with the parts removed.  But the "dust" is still there!  That's because it isn't just dust!  It seems to be a combination of dust, dirt and grease.  It *almost* has the qualities of tobacco "tar", but it doesn't smell like tobacco.  It just smells "old" and "crusty" to me.  It sure ACTED like tobacco tar.  It was VERY difficult to remove it from the surface of the Driver Board.  But *finally* it yielded to provide THIS picture:



     Quite an improvement!  Now it's time to repopulate the board with new components.  Again, the 1 Watt resistors checked out just FINE and very close in value.  The 1/2 Watt resistors were all over the place.  After a round of serious "resistor matching", here are the NOS Allen-Bradley resistors that will populate our refurbished Driver Board:  (Sorry about the colors being a little "off" in this picture.)



       And that brings us up to date on the buildup of this Stereo 70, "Don't Call Me 'Dimples'!".  We'll cover the buildup and installation of the Driver Board next.  Then, it's just a matter of restoring and installing the Power Cord, testing the unit, and FINALLY, we'll get to "just LISTENING" for a while, as we burn-in the amplifier.  More to come!

Sincerely,

Bill Thomas

[jon_010101]

You're doing some great work here!   

What chrome polish did you use?

[Bill Thomas]

     Thanks for the kind words, Jon.  Since I am doing the polishing by hand, rather than machine, I went to my local auto parts store and got a can of Eagle One "Nevr-Dull."  It requires a LOT of effort and NUMEROUS applications and rubdowns, BUT it seems to be a REALLY good product for this purpose.

     When Dynaco originally made these things, they were doing EVERYTHING they could to keep the costs down to a bare minimum.  That included having the chassis nickel-plated, rather than chrome-plated.  Nickel-plating is a two-stage process.  The bare steel is first plated with a fairly heavy layer of copper.  Then it is plated with a layer of nickel.  *If* the chassis were to be chrome-plated, it would then have a very brief dip in the chromium plating bath.  The chromium is mainly a protective layer.  It is the nickel that provides the real luster and shine.  The chromium protects the nickel layer from negative effects from the atmosphere.  Dynaco didn't add that final chromium layer, so the original Stereo 70 chassis' were all just nickel-plated.  They looked BEAUTIFUL (when they were new), but unless they were polished and waxed on an almost WEEKLY basis, they would all eventually suffer from pitting and discoloration.  (After all who ever expected a Stereo 70 to remain in service 46 years later?)

     Nevr-Dull has turned out to be a GREAT product to minimize the effects of the pitting.  But it takes a LOT of work to get there.  On average, I'm spending two or three evenings on each chassis to get them to an "acceptable" point.  I have also used Nevr-Dull on the earlier brass faceplates on the PAS-2 and FM-1 tuners and it takes even MORE effort to make them shine, but when they DO get shiny, they are simply STUNNING!  I'll take pictures of one of the plates as soon as I finish it completely.

     Brasso also works well on the nickel chassis, and REALLY works well (albeit with a lot MORE effort) on those old brass faceplates.  I have tried regular polishing compounds, but they don't seem to work as well and require even MORE effort to work at all.  Rubbing compound does NOT work well.  It tends to cause more damage than it can fix.  I understand Simichrome Paste works well, but I don't happen to have any in my "polishing" arsenal.  If anyone has any other ideas or tips that might work easier or faster, PLEASE let me know!  (My right arm is beginning to resemble Popeye's!)

     We're in the "Home Stretch" with this amplifier.  The latest "Update" follows next.

Sincerely,

Bill Thomas

[Bill Thomas]

     TWO updates in one day?  Yep!  Here's the latest!

     When last we left our Stereo 70 rebuild, we were at the point of mounting all the parts on the newly cleaned Driver printed circuit board.  Well, here is the completed Driver Board, ready for installation:



     All we have left is to mount the board on the chassis, connect 23 wires, wire the Input Jacks and Stereo-Mono Switch, and then restore the Power Cord and install it.  Piece of cake!  I'll report back once the amp has made it through the testing phase.

Stay Tuned for the "Grand Finale!"

Sincerely,

Bill Thomas

[Bill Thomas]

August 21, 2008 - Update:

     This one could be titled "For Want of a Screw..."  Here's our (delayed) installment - and the SILLY reason *why* we're delayed!

     Believe it or not, two 4-40 x 1/4" slotted pan head screws (or the lack thereof) caused a five-hour delay in firing up "Don't Call Me 'Dimples'!"  Since I'm located on the side of a mountain, away from the hustle and bustle of ANY town that *might* have the "correct" fasteners, I wound up having to drive over a hundred miles to find the last two screws needed to mount the Driver Board to the chassis.  I originally had *just* enough of the correct fasteners to completely rebuild two Stereo 70's in STOCK configuration.  The only problem is, in eliminating the selenium rectifiers, I needed to use two of the 1/4" screws to mount the new terminal strips for the silicon diodes.  That left me "two screws short."  (Insert your own "screw-loose" joke here.)

     A few days ago, I made a 54 mile pilgrimage North to the nearest *fairly major* town to search for the "correct" screws.  No dice!  All they had was Phillips-head screws.  Sure, I *could* have used them, but that sure would have made the amplifier look "non-stock" and I simply didn't want to do that.  It wasn't really a priority then because I *thought* I had enough of the "correct" screws to finish this amplifier.  Wrong-O!

     That brings us to this morning.  After searching and searching for just *two more lousy $%#@*^ screws*, rather than use some old screws (which were pretty rusted, actually), I "bit the bullet" and drove SOUTH this time, to a substantially *larger* town where I have always had good luck finding the oddest of fasteners.  (If anyone is interested, just e-mail me and I'll tell you the name of the "Nut-Bolt and Tool Company" who stocks the necessary fasteners.)  I hadn't bought anything from them in over six years, but they came through again with EXACTLY the correct-looking screws!  (Yes, I bought a BUNCH of 'em!)

     After I got back to "The Bench" with the correct fasteners, I added the final two screws and mounted the Driver Board back onto the chassis.  It took a couple of hours to complete the wiring of the Driver Board and finish the amplifier.  Why two hours?  Because, of all the silly things, I found out that the original Power Cord had some SERIOUSLY compromised insulation and needed to be replaced.  Since I wanted this amplifier to use an original beige Dynaco Power Cord, I had to dig through my carcasses to find a suitable "donor."  Here is a picture of the Power Cord I found - BEFORE restoration:



     Pretty grubby looking, isn't it.  Now, here's the same Power Cord AFTER restoration:



     Now THAT'S more LIKE it!  The perfect "Finishing Touch" for "Don't Call Me 'Dimples'!"  Once the cord was installed, it was time to take this amp on a test drive.  (I'll give you a hint: This Stereo 70 fired up just like "Nothin' Fancy" did and made some pretty music on the first pass!)  But we'll wrap up *this* restoration/re-build with one final installment - complete with pictures of the "blessed event."

     The "burn-in" process is going quite nicely.  One hour has passed since firing it up and bias is steady, the sound is "Classic Stereo 70" and there is not a TRACE of hum or noise - electrical OR mechanical.  Stay tuned for our FINAL installment Tomorrow.  Thanks for sticking with me this far.  We're *almost* done!

Sincerely,

Bill Thomas

[Bill Thomas]

August 22, 2008 - Update:

     I know I said we were going to wrap up "Don't Call Me 'Dimples'" in this update, but we need to change one resistor value first, in order to allow a broader range of EL34's to be used.  We need to change the value of the 4.7K resistor (The value used in the "bias expansion" update) to a somewhat higher resistance.  I suggest going to an 8.2K resistor instead.  This still allows SOME expansion to the bias range, but not TOO much.  It makes it easier to set the idling current of certain EL34's to a low enough value.  You might have to "juggle" that value a bit.  In your case, you might have to go back to the original 10K resistor at that location, OR you might have to go to a 7.5K or even 6.8K resistor.  But 8.2K is a GOOD compromise, and a good place to start.

     I just wanted to head off any potential frustration you might have with you OWN Stereo 70 rebuild.  Especially if that frustration were due to my "rebuild chronicles."  Our regularly scheduled "update" will follow later today.  Thanks for your understanding.

Sincerely,

Bill Thomas

[pbrstreetgang]

Very nice Bill!! Just curious I know you werent interested in mods but was there a reason not to choose a 3 prong for any safety stuff?

[Bill Thomas]

     Thanks again for some very kind comments.  As for the 3-wire Power Cord, the answer is: Yes, a 3-wire Power Cord was purposely NOT used.  It all has to do with "Ground Loops"  and minimization of induced hum and noise.  If a 3-wire cord were used, the chassis would have been directly connected to the "Earth Ground" of the electrical system at the Service Entrance to the house.  Sounds like a GOOD idea, but as they say, "T'aint necessarily so."

     Let's add a preamp (WITH a 3-wire Power Cord), a Tuner, a CD player, and the audio Output of a Cable Box.  All of these units use unbalanced audio connections, just like the Stereo 70.  Once everything is connected together, we'll have MANY different "grounds"; and many multiple ground connections between the gear.  While the Stereo 70 uses 10 Ohm resistors between the shell of the RCA jacks and the chassis "ground" (mainly to avoid a "hard current" situations where ground currents will be able to circulate unimpeded), connecting the preamplifier will most likely short out those resistors.  This may, or may not cause hum or noise, depending on the "quality" and "integrity" of the equipment ground, but sending an audio ground through the 3rd wire in the Power Cord, back to the Service Entrance, and then back to another piece of gear with a 3-wire Power Cord, will almost *guarantee* an increase in noise and hum.  Meanwhile, the cable box has a totally *different* ground; provided by the shield on the coax coming into the house.  Plug in the cable box audio connectors, and you often have a large "hum" induced into your entire system.  Grounding the chassis to the Service Entrance Ground makes all this worse.

     With unbalanced audio connections, it is almost IMPOSSIBLE to eliminate all multiple grounding connections between units, but in most cases, these are short connections and *usually* wind up being similar distances, so the effect is (usually) minimal.  (Don't expect the effect to be minimal in a high RF environment though.)

     You get the idea.  The third wire ground is a great "safety idea" in general, but in the audio world, it tends to make us use "cheater" adapters to lift the ground in order to eliminate hum.  Dynaco's Stereo 70 designers (Ed Laurent and Bob Tucker) didn't have to face this problem because 3-wire grounding was still VERY uncommon when the Stereo 70 was designed, and it wasn't Federally mandated then.  Since this is a "Legacy Product", we can still avoid the additional hum and noise potential caused by the 3-wire ground because this is a "refurbishment" rather than "new production."

     This is a long and incomplete explanation, but I hope you get the idea.  By the way, I have NEVER heard of a Stereo 70 Power Transformer failing in such a way as to "energize" the chassis with current from the Primary of the Power Transformer.  I'm not saying it CAN'T happen, but I've never seen it (or even HEARD of it) happening.

     I hope this makes *some* sense.

Sincerely,

Bill Thomas

P.S. I realize that our final update on "Don't Call Me 'Dimples'!" has been delayed a day, but I'll be posting pictures of the "finished product" later today.  (I was called out of town yesterday and had to put everything "on hold" for one more day.)  Thanks for your patience.

[Bill Thomas]

8/24/2008 - FINAL Update!

     "Don't Call Me 'Dimples'!" is now complete!



     I don't know about you, but *I* happen to think the classic design of the Stereo 70 is a mighty good looking piece of equipment - in an "industrial" kind of way.  "Don't Call Me 'Dimples'!" was posed with a NOS Mullard-manufactured, GE-branded GZ34/5AR4, two NOS Sylvania 7199's and a quartet of NEW Electro-Harmonix EL34EH's.  This is the configuration I used when testing and "burning-in" the amplifier.  Throughout the "burn-in" phase, "Don't Call Me 'Dimples'!" performed FLAWLESSLY - producing that warm and wonderful sound Stereo 70's are famous for.

     Here's a picture of the chassis wiring:



     Any similarity between "Don't Call Me 'Dimples'!" and "Nothin' Fancy" is purely intentional!  The construction is VERY similar.  Notice the ceramic tube sockets.  They will probably last longer than WE will.

     Here's a look at the ceramic octal sockets with the tubes removed - from the top.



     And here is a look at the rear of the amplifier.  This photo highlights the "Dimpled" area at the fuse holder.  In reality, it doesn't look anywhere NEAR as distorted as the photo makes it appear.



     And FINALLY, here is a photo of "Don't Call Me 'Dimples'!" with the tube cage/cover installed.



     Notice that this is an earlier production unit from 1962 or 1963 (according to the date codes on the transformers).  That's why this Stereo 70 sports the "Dynakit" foil logo on the cover and the cover itself has a "coarser" hole structure than later models from the 1970's.

     This particular re-build was a little more challenging than our first re-build, "Nothin' Fancy."  The actual condition of the chassis' plating was better than "Nothin' Fancy", but the serious damage around the fuse holder was *almost* a "deal-breaker!"  Fortunately, my Brother enjoys auto body work and managed to pull everything out to an "acceptable" level.  No, it's certainly not as smooth as a mirror, but the IMPORTANT thing to remember when straightening metal like this is to know when to STOP!  We could have worked further on the damaged area, but there would have been MUCH more damage to the finish of the back of the chassis - something I preferred NOT to do!  So, we wound up with a *slight* "Dimple" around the fuse holder.  I think it was an acceptable trade-off.  (And, it gave us a NAME for this project!)

     My SINCERE thanks to the members of Audiocircle who have e-mailed me, or posted comments regarding this re-build.  My DEEPEST thanks go out to an Audiocircle member who shall remain nameless.  He is the new caretaker of "Nothin' Fancy" and was invaluable in helping me "tweak" the values of the resistors in the Bias Supply in order to accommodate a broader selection of EL34's.  He is a retired member of the US Military and has my UTMOST admiration, respect and my SINCERE thanks for his service to our Country.

     I think it's safe to say that THIS re-build is complete!  Our NEXT project is going to be something different and VERY special.  As a matter of fact, it will be a Dynaco PAS-3X preamplifier - complete with those pesky "special" tone controls.  I just happened to find a complete set of treble and bass controls that have never seen a soldering iron.  They will soon!  Stay tuned!

Sincerely,

Bill Thomas

     

[salb203]

Hi Bill,
What kind of quad capacitor did you use? What are the specs of it?

Thanks,
Sal

[Bill Thomas]

It's an 80-40-30-20 at 525.  It's the forerunner to the current CE version that was made by Aerovox.  Basically the same cap.

Bill

[salb203]

Thanks Bill.

Sal

[Listens2tubes]

  Nice attention to detail, you are the man Bill!