Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"

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Bill Thomas

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August 24, 2008 - "A New Beginning"

     Rather than keep you in suspense, I thought I'd whet your appetite a bit by showing you the "bare beginnings" of our next rebuild/restoration project.  This will be as close as you can get to a new PAS-3X preamplifier; complete with NOS "special" tone controls.  But let's REALLY start at the beginning.  Here's a picture of the "bare bones" re-plated PAS preamplifier Front and Rear panels:



     Silver cadmium is a difficult finish to get "perfect" and this plating company didn't.  (No surprise there!)  Still, it's a LOT better than the funky old chassis' you typically find.  I'd give them an A- on the re-plating job.  Here's a picture of the Main Chassis and the Bottom Cover Plate:



     Again, not a "perfect" re-plating job, but not *too* bad overall.  Again, MUCH better than the typical PAS preamplifier metalwork you'll see these days.  And finally, here is a picture of two VERY rare birds:



     These are two Epoxy-Glass circuit boards.  They are the "guts" of the PAS preamplifier series.  These two boards are rare because MOST of the PAS preamplifiers were equipped with phenolic circuit boards.  These boards are MUCH more durable and should turn into a FINE PAS-3X preamplifier.

     So... have I whet your appetite?  I *thought* so!  Next, we'll begin the mechanical assembly of the Front Panel and then the initial wiring of the Front Panel.  Stay Tuned!

Sincerely,

Bill Thomas

Bill Thomas

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #1 on: 29 Aug 2008, 04:28 pm »
August 29, 2008 - Update:

     We're *just* beginning our build of a Dynaco PAS-3X preamplifier. 



     It all starts by mounting the Slide Switches to the Front Panel.  Make SURE you mount the Power Switch so it is "ON" when switched to the LEFT!  The rest of the Slide Switches can be mounted in either direction.  Also mount the Lamp Holder.  Here is a picture of the back of the Front Panel:



     It is important to center the switches in the rectangular cutouts.  (I must have forgotten that when I mounted the Tape Monitor Switch.  Don't worry, I've already addressed that little oversight.)  We'll be mounting the Tone Controls when we continue with the Mechanical Assembly of the Front Panel.  Here is a picture of two NOS "Special" Bass Potentiometers.  Solder has never touched these controls.  I truly *doubt* they are the last new ones left on the planet, but they *could* be!



     Note the Dynaco part number: 147754 and the TWO "stops" on the back of the housing.  This POSITIVELY identifies these controls as those VERY "special" potentiometers.  They will probably NEVER be reproduced.  Centralab made the originals.  But before we mount these to the Front Panel, let's switch to the Back Panel and mount the NOS AC Convenience Outlets.  Here's a picture:



     Here's a better view of the mounted sockets:



     That's right.  Solder has never touched these sockets either.  They are absolutely NOS originals.

     That wraps up our "Initial Mechanical Assembly."  We still have a few items to mount before we can get started on the actual sub-assembly wiring.  On the Back Panel, we'll mount the RCA Jack Strips, and on the Front Panel, we'll be mounting the Volume Control potentiometer, a "reclaimed" Blend Switch, "reclaimed" Balance Control and those four NOS Tone Control potentiometers.  THEN we can begin the actual wiring.  Meanwhile, we're waiting on some capacitors to arrive so we can rebuild the Printed Circuit boards and complete the Front Panel sub-assembly.  MUCH more to come.  Stay tuned!

Sincerely,

Bill Thomas

Bill Thomas

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #2 on: 1 Sep 2008, 07:57 am »
September 1, 2008 - Update:

     Well, there's been a *little* progress on "The Last of the Dinosaurs", our build of one FINAL PAS-3X preamplifier.  We've completed mechanical assembly of the Rear Panel.  Here's a picture from the back:



     You can see where Dynaco saved a LOT of money!  The RCA jacks couldn't have cost a Dollar!  But this was pretty common stuff in the 60's.  Take a look at a Fisher preamp, or one from H. H. Scott.  You'll see the same type of RCA Jack assemblies.

     If you're rebuilding a PAS preamplifier, there are a few things to be aware of when installing these Jack assemblies.  The ONLY RCA Jack that is actually grounded to the chassis is the "Main Output" Jack.  Here's how Dynaco did it:



     Simple yet effective.  But make SURE that NONE of the other RCA Jacks are grounded to the chassis, or you'll have serious hum problems.  There's not a lot of clearance, but there's enough "wiggle room" to allow *just* enough clearance.  The "Phono Input" is a "close call", but it *just* clears enough. See?



     Here's a picture of the RCA Jack field in its entirety.  Just two Jacks are grounded to the chassis, the others must NOT ground to the chassis or serious hum problems will ensue.



     Here's a picture from the inside:



     Nothing fancy or exotic here.  Just simple, effective and CHEAP!  That was the Dynaco Way!  Wherever they could "do the same for a penny less!" they did it.

     That brings us back to the Front Panel.  Here's a picture of the back of the Front Panel:



     Notice that one of the "special" potentiometers is missing in this picture.  That was due to a "Duh!" moment when I casually misplaced the thing (for a couple of days)!  Here's a shot from the front:



     This is the "business end" of the PAS-3X preamplifier.  It doesn't look like much now, but it will "blossom" into something very attractive soon enough.

     More to come.  Stay tuned.

Sincerely,

Bill Thomas

salb203

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #3 on: 4 Sep 2008, 01:50 pm »
Bill,
Great job on the PAS-3X build! I am looking forward to this series as I will be rebuilding a PAS-3 and PAS-3X later this year.  On the PAS-3 I use daily I removed the filament voltage doubler selenium rectifier and capacitors (low filament voltage) and built a voltage doubler circuit on a PC board. Thankfully the DC going to the 12AX7 filaments are hovering about 24.5 volts not higher, I was thinking putting a 1N4001 in series to drop the voltage another .6 volts.

Sal

Bill Thomas

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #4 on: 4 Sep 2008, 06:35 pm »
Dear Sal,

     I consider that "high praise" coming from you, Sal.  I have appreciated the quality of your work for quite a while now.  It's nice to know that there are a few of us old Dynacoheads still hanging around.  I wouldn't worry about 24.5 Volts on the 12AX7's.  In reality, that is *still* operating them *slightly* below their rated (combined) Voltages.  Remember that 6 Volt filaments are actually rated at 6.3 Volts.  12 Volt tubes are rated at 12.6 Volts.  In series, that works out to 25.2 Volts.  See?  You're STILL running them a little below rated Voltage.  Of course, in stock form, a PAS preamp runs them around 20 to 21.5 Volts (depending upon the condition of the selenium rectifier), so the ol' Telefunken's tend to last FOREVER.  (Not a BAD thing these days!)  But with 24.5 Volts on the filaments, you will be able to use some of the "leftover" Telefunkens a good deal longer.  Again, not a bad thing.  (I just *LOVE* it when EVERYBODY wins! (lol))

     Keep the faith, my friend!

Sincerely,

Bill Thomas

P.S. I'll be having an "Update Moment" sometime later in the day.  Right now, I'm midway through the line stage board rebuild.  The HARDEST part was finding new .0075 uF capacitors.  I *found* some, but I wouldn't count on finding others.  More to come.

Bill

salb203

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #5 on: 5 Sep 2008, 01:37 am »
Hello Bill,
I checked justradios.com as he has a very nice selection of capacitors at reasonable prices and shipping but this afternoon he shows he was out of stock on the .0075uf capacitors and this evening when I got home from work, they are not on his spreadsheet anymore. I will email him to see if he can get more. He has .0070uf and .0005uf caps you can parallel to get the capacitance you need.

Thank you for the kind words Bill, if I knew 10 percent of what you know about Dynaco amps I would be very happy, and I mean that! I have been following your posts for a while.

Keep on soldering... :D

Sal

PS: If you need any new square sockets (outlets) I have a box of them.




Bill Thomas

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #6 on: 5 Sep 2008, 03:12 am »
Thanks, Sal,

     I have plenty of those little suckers.  But I really appreciate the offer.  As for the .0075's, I have 'em.  Hard to find though.  I knew Dave was out.  There are a *few* places that still list them, but not very many.  Still dealing with internet troubles tonight.  Hopefully, the cable folks will get their act together soon.  Until they do, my latest update has to wait.  The way my connection is going, I'd be here until tomorrow uploading the first photo.

     Well, as Scarlet once said: "Tomorrow is another day."  (Well, Duh!)

Talk with you soon.

Sincerely,

Bill Thomas

Bill Thomas

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #7 on: 7 Sep 2008, 09:50 pm »
September 6, 2008 - Update! - "Cleanliness is next to Godliness.  AND, it's next to impossible!"

     At long last, we have some *meaningful* progress to report on our "last" PAS-3X build.  (Actually, progress was being made, but the *reporting* of that progress has been hampered by "technical difficulties" my local cable company has been suffering.)  Enough!  Let's take a look at the latest "stuff."

     Allow me to introduce you to "Pubic Enemy #1" in the PAS-preamplifier world:



     This is the main Selector Switch.  It probably cost Dynaco around $3.85.  It is worth every penny - and probably not a whole lot more.  ALL of the audio travels (in one way or another) through contacts in this switch.  Here is the way it will probably look when you encounter it inside your preamplifier:



     Doesn't look too good, does it?  But looks can be deceiving.  Underneath this surface of oxidation and crud, we'll find silver-plated contacts.  But there's NO way to tell what the actual condition of those contact surfaces might be - yet.  Let's tear down the switch like this:



     This allows us full access to both sides of each wafer.  But before you take the switch apart, mark the wafers and the rotor positions so you will be able to tell how the assembly goes back together properly.  Once the wafers have been separated, you will see something like this:



     While you have the frame of the switch dis-assembled, clean out the old grease around the detent assembly.  Apply white grease to the area where the detents slide.  While it might be tempting to use cotton swabs to clean the contacts, a MUCH better choice is to use a firm bristle brush.  Trying to get all the cotton fibers out of the "guts" of the switch gets old pretty quickly.  A bristle brush lets you get to the buried crud without the danger of snagging and bending a contact.  After our first treatment with de-oxit and a brush, we wound up with this:



     We're certainly not done with the cleaning, but at least we can see that the actual contact surfaces are really in pretty good condition.  After MUCH more cleaning of both the contacts and the phenolic wafers, all the sections turned out pretty well.  After re-assembly, we wind up with this image:



     Looks a LOT better than what we started out with!  See?



     Now before we go a lot further, it should be noted that Silver oxide is conductive!  So MOST of that black stuff we started with on the contacts would reduce the conductivity of the switch contact connection VERY little.  So why bother cleaning the thing?  So we can actually SEE how much wear the contact surfaces have undergone.  As it turns out, this particular Selector Switch shows very little actual wear.  There's still PLENTY of silver on the entire contact surfaces and it appears that there is no real "groove" worn in the rotor contacts.  That means this particular "harvested" Selector Switch will be a good candidate for our PAS-3X build.  We treated the Blend Switch to a similar cleaning.  We started with this:



     And after the first cleaning, we came up with this:



     (You'll note the cotton fibers that have since been removed.  Avoid cotton swabs for this!)  Seriously, I have mentioned that these are "initial" cleanings.  There is a bit of a procedure involved.  It starts by blowing the switch off with a blast of compressed air.  Then de-oxit is applied to the metal contacts.  A stiff bristle brush is used to "assist" in the cleaning process.  A second blast of de-oxit and more brushing results in a pretty clean and de-oxidized surface.  But we're dealing with a phenolic substrate here.  It can absorb the de-oxit and actually wind up creating more "gunk!"  So, once we're satisfied with the contact surfaces, we remove the remaining de-oxit from the phenolic using a paper shop towel DAMPENED with lacquer thinner.  A HEAVY application of lacquer thinner could cause the phenolic to soften, so "dabbing" is best.  Follow that up with a paper shop towel moistened with iso-propyl.  Once everything is completely clean and dry, THEN apply the de-oxit Gold.  But here's something to know about de-oxit Gold, less is better than more.  A *tiny* misting of the contact surfaces is preferred.  Then operate the switch throughout its rotation several times.  The overall goal is to have nice shiny contacts, along with a nice, DRY phenolic substrate, with as little overspray of the de-oxit Gold as you can manage.  Don't get CRAZY about it, just keep the overall goal in mind.

     You can expect to spend about three hours doing a thorough cleaning of these switches.  Or, about 17 hours *IF* you don't mark how the selector switch wafers and rotors align back together, so make sure to mark 'em BEFORE you take them apart.  That wraps up our reclamation of the two Wafer Switches used in our PAS-3X.  Coming up next, our completely rebuilt printed circuit boards, AND some circuit "modifications."

     More to come.  Stay tuned!

Sincerely,

Bill Thomas
« Last Edit: 8 Sep 2008, 07:08 am by Bill Thomas »

Bill Thomas

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #8 on: 7 Sep 2008, 11:01 pm »
September 7, 2008 - Update - "It's Stuffin' Time!"

     There are only a few pictures with this update.  The first is the picture of the Printed Circuit boards we started with:



     And the next picture shows our completely rebuilt circuit boards:



     Some parts values have been changed, but the values around the tone controls are the original values.  The coupling capacitors on the Phono Preamp board have been increased to 1.0 uF.  Likewise, the final Output Capacitors on the Line Stage board have also been increased to 1.0 uF.  This improves the low-frequency response throughout the preamplifier.  Now, though, we get to another bit of "parts juggling" in order to correct some design "compromises" in the Phono Preamp board.

     Back on December 21, of 2005, Joe Curcio, the "Dynaco Doctor" posted a "fix" for the PAS preamplifier's "excessive deviation" from the standard RIAA Phono reproduction curve.  You can read his post at this link: http://www.audioasylum.com/cgi/t.mpl?f=dynaco&m=7492.  It's a simple, straightforward modification that will yield a tenfold improvement in RIAA accuracy.  This is one modification that really SHOULD be made, so it has been incorporated in our PC-6 rebuild.  That's why you'll see a couple of polystyrene capacitors paralleling a couple of dipped silver-micas on our rebuilt board.  It may look a little strange, but it will complete the "tightening-up" of the RIAA response.

     Few people really have the need for more than one RIAA equalized Phono Input, so for our "final" PAS-3X preamplifier, we are revising the Selector Switch wiring to add two more line-level inputs in place of the "Tape Head" and "Special" inputs.  (Do you REALLY need a 7 1/2 ips equalized Tape Head preamplifier?  I didn't think so!)

     There is one FINAL deviation from "stock" form.  Take a look at the typical wiring of the input connections and the Selector Switch:



     It should be pretty obvious why Dynaco never published any crosstalk specifications between different line-level inputs.  The Selector Switch *usually* gets blamed for the high crosstalk, but in reality, it's the unshielded wiring that is bundled together that is *really* responsible.  We will tackle that by shielding the Input Wiring to the Selector Switch.  (At least, that's the PLAN anyway.)  But first, it's time to get down to the actual wiring of the front panel, the chassis and the rear panel - all of which is coming up in our NEXT installments.

     Stay Tuned!

Sincerely,

Bill Thomas
« Last Edit: 9 Sep 2008, 09:22 am by Bill Thomas »

Bill Thomas

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #9 on: 9 Sep 2008, 10:48 am »
September 9, 2008 - Update:  "Shields UP!"

     Our PAS-3X preamplifier is beginning to come together now.  Sharp-eyed viewers will notice a different Balance Control than the one I had originally planned to use.  That's because I located a NOS, late-production Balance Control in my "stash of NOS Dynaco parts."  The Front Panel has been pre-wired according to the Dynaco manual, with a few MAJOR exceptions:  In order to reduce noise and crosstalk, we are using shielded wiring to and from the Monitor Switch, and from the Volume Control to the PC-5 Line Stage board.  Here's an overall picture of the pre-wired sub-assembly:



     Take a closer look at the connections at the Monitor Switch.



     Notice that there are no shield connections at the Monitor Switch.  The shields are connected at ONE END ONLY!  We don't want to *create* any new Ground Loops inside our preamplifier, but we DO want the benefit of additional shielding.  HERE is where we connect our shields:



     It's pretty crowded around the Blend Switch and the Volume Control, but there is *just* enough room to connect our shields to the ground points at the controls.  This should improve the crosstalk between channels a bit and it will reduce the possibility of picking up noise or hum from inside the preamplifier itself.

     Now that the Front Panel has been pre-wired, it's time to complete the Initial Mechanical Assembly of the Main Chassis.  Here's the result:



     Notice the absence of a selenium rectifier on top of the filament supply capacitors.  Instead, we have mounted a terminal strip so we can replace the selenium rectifier with modern silicon diodes.  But first, we need to mate the Front Panel Subassembly to the Main Chassis like this:



     It's actually *beginning* to look like a preamplifier!  But we still have quite a lot of wiring to do.  Next up:  We'll be building our Filament Supply and our High Voltage (B+) Supply and then we start connecting this all together.  Once that is done, we'll move on to pre-wiring the main Selector Switch and the Rear Panel.  More fun to come!  Stay tuned!

Sincerely,

Bill Thomas

MaxCast

Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #10 on: 9 Sep 2008, 01:52 pm »
I wish you were my neighbor.  :D

salb203

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #11 on: 10 Sep 2008, 01:19 am »
Bill,
Once again, you are doing an awesome job with the PAS-3X build. Nice job on the switch assm and for using shielded cables for the inputs. Looking forward to this thread and the outcome.

Regards,
Sal

cryoparts

Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #12 on: 10 Sep 2008, 02:12 am »
Great work!   :thumb:

Peace,

Lee

ST-2A3

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #13 on: 10 Sep 2008, 02:21 am »

Bill,

Could you give me some help in bringing the "Phono OUT" to a set of unused RCA jacks? One that is not full of noise. I want to be able to feed the output directly from the phono amp (bypassing the PAS preamp stage) to my 2A3 amp.

Thanks

Robert

Bill Thomas

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #14 on: 10 Sep 2008, 09:27 am »
Wow!  Thanks, Guys!

     To Maxcast:  No you don't!  (Just ask my *current* neighbors!) (lol)

     To Sal:  Thanks!  To be honest, *I'M* looking forward to seeing if "theory" and "practice" actually work out the way they *should* work out! (lol)

     To Lee:  Thank you!  I can use all the encouragement I can get! (lol)  I only get one shot at this, so I really DO want this one to turn out well.

     To Robert:  Actually, you should probably try using the "Tape Out" jacks first to see if the Dynaco Phono Preamp stage will work well with your 2A3 amplifier.  (For all practical purposes, this is the same thing as running an Output connection to an "unused" pair of RCA jacks.)  Avoid the temptation to try to use a pair of the "unused" "Tape Head" or "Special" jacks.  This will NOT work unless you change the method of grounding those jacks.  This is a MAJOR reworking of the grounding scheme used at the RCA jacks UNLESS you are starting completely from scratch - as we are on this project.  As it turns out, I will be making a change in the grounding arrangement of those jacks on this particular preamplifier.  It will not be EXACTLY the same arrangement you would use, but it *should* show you HOW to make such a change.  In our preamplifier, we will be changing the grounding of the two sets of "Tape Head" and "Special" input jacks so that the ground will be the same ground as the other input jacks, rather than the ground source used at the "Phono" input.  If you were to use one pair of these jacks as an Output, you would connect your grounds to the same grounding point as the "Main Out" jacks (which just happens to be the same grounding point as the "Tape Out" jacks).  Confusing, isn't it?  That's because the Dynaco preamplifier actually has THREE different "Ground Points" and using the wrong "Ground Source" will lead to a problem with hum and/or noise.

     The MAIN problem with using the Phono Section as a "standalone" unit has to do with the high Output Impedance of the Phono Section.  If your 2A3 amplifier has a high Input Impedance  (which is quite likely), it should work well.  If it doesn't, you will notice that the bass rolls off pretty quickly.

     Of course, there is an added advantage in using the "Tape Out" jacks in that the Selector Switch can then be used to select additional line-level sources as well as the Phono Section.  The main *disadvantage* is that your signal is going through the main Selector Switch before it gets to your 2A3 amplifier.  If you can live with that (and your Selector Switch is in good condition), just give the "Tape Out" jacks a try before committing a permanent change to the RCA jack field.

     I hope this helps.

Sincerely,

Bill Thomas

Bill Thomas

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #15 on: 12 Sep 2008, 11:27 pm »
September 12, 2008 - Update:  "We've made a few changes..."

     We are now (finally) approaching the end of our "LAST of the Dinosaurs" PAS-3X build.  The main chassis and the front panel have now been wired together.  The Rear Panel has been attached, the AC wiring has been completed, and a restored Dynaco Power Cord has been added as well.  Here's a picture of the result:



     We have completed our Filament Supply wiring on the top of the chassis.  Here's a picture:



     We have also completed the under-chassis wiring associated with the B+ Supply and attachment of the Front and Rear Panels.  Here's a picture of the result:



     Ahh, but there's more!  It took a bit of extra planning to decide the best way to pre-wire the main Selector Switch so we can use shielded wiring to the Input and Output jacks.  Here's how that turned out:



     We have also reconfigured the wiring of the Selector Switch to change the "Tape Head" and "Special" low-level, equalized inputs into two more line-level inputs.  Since we decided to eliminate any other equalization curves other than the RIAA phono equalization, we were able to free up the middle wafer of the Selector Switch.  This also allows us to avoid running low-level signals through the Selector Switch.  Now, the Phono Input is connected directly from the Rear Panel RCA jacks, to the Input of the PC-6 PC board.  If you'll look closely at the wiring from the leftmost Input Jacks, you'll notice that this is *one* place where I DIDN'T use shielded wire. 



     I know it seems counter-intuitive, but the reason is simple:  Shielded wiring in this location will add more capacitance to the load the phono cartridge sees, so instead, we have simply used a twisted pair of wires for each channel.  They are routed close to the chassis to keep the likelihood of hum pickup to a minimum.  Yes, the twisted pair will add a *tiny* bit of capacitance, and even running the wiring against the chassis will add a bit more, but it is nowhere NEAR the amount that closely-shielded wires would have added.  Will it make a difference in real life?  Probably not, but you never know...  Better to be safe, than sorry.

     Up next:  Installation of the pre-wired Selector Switch and completion of the Input and Output wiring.  Then, we move on to the final "finishing touches."  This is REALLY getting close now.  I hope to have the next update tomorrow.  Stay tuned!

Sincerely,

Bill Thomas


Bill Thomas

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #16 on: 14 Sep 2008, 08:20 am »
September 14, 2008 - Update:  "All Circuits Are Busy!"

     Yes, it's true!  Electrically, we are now complete!  Remember the picture I previously posted showing "typical" selector switch wiring?  Here it is again:



     Quite a Rat's Nest, isn't it?  Here's a picture of the shielded wiring at the Selector Switch of our "Last of the Dinosaurs" PAS-3X:



     And here is a picture of the shielded wiring at the rear panel RCA jacks:



     Quite a difference, wouldn't you say?  Which wiring arrangement do YOU think will have better crosstalk figures between Sources?  When we get to the testing phase, we'll find out!

     Here's a picture of the Shielded wiring of the Main Outputs:



     There shouldn't be much crosstalk between channels here either.  Zooming out a little, here's a picture of the completed chassis:



     And finally, here's a picture of the underside of the completely wired chassis:



     Testing the unit completely will take the better part of a day, but I can report a few initial findings.  After installing only a #53 miniature lamp, I *gradually* brought the unit up with a Variac while measuring the Voltage of our filament supply.  Unloaded, it produced a little over 30 Volts!  Looks like the doubler circuit is working *quite* well.  Adding a pair of NOS GE 12AX7's to the PC-5 Line Stage board, and another pair of NOS GE 12AX7's to the PC-6 preamplifier board, brought the Voltage down to a more reasonable 24.9 Volts.  That's a good deal higher than a PAS preamplifier with a selenium rectifier, but it's *just* under the 25.2 Volts a series-connected pair of 12AX7's would be rated for.  In other words, "Bingo!"

     Next, I installed a 12X4 rectifier and once again, GRADUALLY brought up the Variac.  As the rectifier warmed up, we began developing B+ Voltage and finally settled in at around 410 Volts at the Input to the first section of our filter capacitor.  Looks like we may have a good B+ supply too.

     Since I'm an impatient sort, I went ahead and connected a CD player to one of our new line-level Inputs, and connected the Main Output to a "Cheap-O" Radio Shack solid-state receiver.  I have no idea what the Input Impedance of the Radio Shack receiver is, but I'm SURE it is a LOT lower than the PAS-3X preamplifier would like.  Still, our new PAS-3X performed VERY nicely!  No noise. No hum.  The Blend Switch and the Selector Switch operate without a hitch!  Apparently, our careful selection and our careful cleaning have REALLY paid off!

     It's ALWAYS nice when a build just WORKS, right off the bat, but there's still a LOT more testing to be performed.  By using the "finger test" (applying a finger lightly to each of the Phono Inputs to see if we get some hum), I DO know that the Phono Preamp section is amplifying.  In our next update, we'll see just how well the Phono Section SOUNDS when connected to a real turntable.  (I have high hopes!)

     We still have a bit more to accomplish before our "Last of the Dinosaurs" PAS-3X preamplifier is finished.  I have selected a nice "Champagne Gold" "new-design" Front Panel and a complete set of late-production aluminum knobs, rather than the earlier cast "pot-metal" knobs (since this is REALLY a late-model PAS-3X!)  I'll give them a good cleaning and mount them on the front of our finished preamplifier.

     There are just a few "loose ends" remaining before we can call this project complete.  We'll finish our electrical testing, finish the "cosmetic" portion of the build and report the results in our next update.  There's just a *wee* bit more to come.  Don't miss our exciting conclusion, coming SOON!  Stay tuned!

Sincerely,

Bill Thomas

Bill Thomas

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #17 on: 14 Sep 2008, 11:27 pm »
September 14, 2008 - Mini-Update:  "Champagne anyone?"

     I know I had hoped to report on the testing phase of the preamplifier, but other commitments took most of my time today.  However, I was able to take a picture of the completed front panel.  The cleanup efforts went quite well.  See what YOU think:



     Personally, I think Dynaco's so-called "new design" front panel looks simply elegant.  It's a far cry from the "generic black" front panels of today's units, isn't it?  The color is described as "Champagne Gold" in Dynaco's literature.  The panel is made from extruded aluminum which was then anodized with the gold color.  The knobs are the aluminum versions that were used in Dynaco's final preamplifiers.  Since this is probably "The LAST (mostly) new PAS-3X preamplifier we will probably see, it seemed fitting to use the final design for the knobs.  Besides, they look a LOT nicer than the original die-cast, pot-metal knobs that tended to discolor upon exposure to the atmosphere.  These knobs don't usually strip the threaded setscrew holes either, unlike the original die-cast knobs.  All in all, this turned out MUCH better than I had hoped for.

     Sorry to make this so brief, but I will report on the final testing phase as soon as possible.  I also have a couple of additional (nice) surprises up my sleeve before we call this preamplifier complete.  Here's a hint:  Don't become "married" to the tubes I am using for testing purposes.  We'll see if the "Tube Fairy" is able to come up with something REALLY special!

     Our final update is coming soon.  Then it's on to our next TWO projects.  We'll be moving away from the "mostly stock" Dynaco realm.  Our next projects will document the build of Frank Van Alstine's "Super PAS Three" preamplifier and "Ultimate Stereo 70" amplifier!  I have never built either of these kits, so I will be documenting the construction of both units, just as I have the previous "builds."  The kits are already "in-house" and I have selected the "donor" chassis' for the projects.  Needless to say, I am SUPER excited and can hardly wait to get started!

     Stay tuned!

Sincerely,

Bill Thomas

Boscodude

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #18 on: 16 Sep 2008, 05:01 pm »
I have a Dynaco PAS-3 and did the Van Alstine mod to it about 20 years ago. feel sick about it now though after looking at these pics and how well u cleaned everything first. All I did was just throw it together. GRRRRR. makes me want to tear it apart and do it right...oh well.
Thanks for the conversation today. Jim

JakeJ

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Re: Dynaco PAS-3X rebuild - "The LAST of the Dinosaurs!"
« Reply #19 on: 16 Sep 2008, 05:45 pm »
Hi Bill,

I would like to add my thanks to all the others for documenting these Dyna rebuilds. You not only provide a window on the quality of your personal rebuilds but do so in a manner that benefits all other DIYers by this instructional thread. Kudos, sir, not just for turning out some beautiful handiwork but also for contributing to the knowledge base here at AC.  :thumb:

I too have PAS-3 (not sure if it's a 3x) I plan to rebuild...someday. My next project is an amp based around the transformers on my Dyna Mark VI amps. Conversations with Roy Mottram of VTA fame convinced me not to butcher the original chassis but build from scratch instead.

Regards,
Jake