PAS 2 Resurrection issues.... ideas?

Recently have been working on bringing a ST70 and PAS2 back from retirement.  (Both purchased in pretty bad condition on eBay).

Did the ST70 first.  Very straight forward (on my third one, so it went quickly)  Fixed broken speaker connectors, switch and fuse holder (why do these always get smashed, annoying!!).  Swapped original PC board for a repop PC3 (7199).  New quad cap, bias caps, replaced selenium rectifier.  Pulled some good tubes from my stash and voila, instant sweet music.

So, was hoping for a similar quick result for the PAS 2.... Not.  

So, here's what I did.  Replaced both PC's with Erhard's repops.  Installed the Erhard PSU board.  Tubes tested well on Jackson tube tester (West German Telefunkens).  

Found some initial startup issues.  Power switch intermittent, pilot lamp blown, all pots noisy.   Replaced power switch, new bulb and cleaned all pots and rotary switches.  So... here's where I'm at (took me long enough to get to the problem, eh?)

Right channel sounds fine.  Left channel appears to have less gain then the right (have to bias balance towards left to get similar outputs). Audio from left channel seems to have muffled mid-range, and left treble has limited effect (it makes a difference, but not like a working PAS).   Preliminary trouble shooting, swapped 12AX7's between channels on the PC-5, swapped the input L/R (to make sure source wasn't the issue) and tried different inputs.  Problem is definitely something with the left line stage.  (Haven't even gotten to test the phono amp yet).  

Not sure where to start, suggestions accepted.  My thoughts were to bypass the tone controls.  I've seen Curcio's instructions (jumper and disconnects) and Audio Regensis (pretty much the same, but add 1uF cap to prevent DC offset to power amp).  Pretty much done working on it tonight, but is that a good first step to track down where the problem is?  I'm thinking something is duff in the tone circuit on that side.  I don't think there is an issue with the PC5.  I did verify all voltages on the board with respect to the PAS2 manual, pretty darn close, so I don't think I muffed up the PSU installation.

Any way, going to step away for a bit, and see what suggestions might come.

Thx

Scott

So.....  now I'm

peterh wrote:Sorry but you are already too far. Fault finding should have started before any repacements.
If the problem is in the line board, check wirings, then change back to the original board.

Well..... seeing as the original boards were basically trash, not much could be done to even try those.
I mean, we have two parallel and identical circuits. It would seem to me there should be a way to isolate sections; bypass things (blend circuit, balance circuit, tone circuit), or even swap left and right channel connections to isolate.

I'm just not sure what / where things can be bypassed

You indicated that all B+ and filament voltages check out, so we can pretty much say that at least the power supply is good.
Also, right channel is working fine.
If you are 100% sure that all components pertaining to the left channel are verified, then again we can pretty much say that at least physically all is good with the line stage pcb.
Just double check all left channel pcb connections/wiring as well.
If the tone control pots were pretty crappy to start with, personally, I would not have bothered cleaning and reuse them, since they are 40+ years old are more than likely worn. Same goes for the caps in the tone control circuit, they are known to drift and leak. Not leak as in dripping leak but leak as in allowing DC to flow through. One can only take this 'wishing to keep it original' thing so far!
It is easy and cheap enough to replace the tone control parts.
Interestingly enough, I have never gone through the exercise of bypassing the tone controls on a PAS even though I work on them a lot, I usually just remove them or replace them depending on which upgrade kit is being used.
But by all means, bypassing them, in theory, should determine if there is an issue with the line stage pcb or somewhere else in the circuit.

erhard-audio wrote:You indicated that all B+ and filament voltages check out, so we can pretty much say that at least the power supply is good.
Also, right channel is working fine.
If you are 100% sure that all components pertaining to the left channel are verified, then again we can pretty much say that at least physically all is good with the line stage pcb.
Just double check all left channel pcb connections/wiring as well.
If the tone control pots were pretty crappy to start with, personally, I would not have bothered cleaning and reuse them, since they are 40+ years old are more than likely worn. Same goes for the caps in the tone control circuit, they are known to drift and leak. Not leak as in dripping leak  but leak as in allowing DC to flow through. One can only take this 'wishing to keep it original' thing so far!
It is easy and cheap enough to replace the tone control parts.
Interestingly enough, I have never gone through the exercise of bypassing the tone controls on a PAS even though I work on them a lot, I usually just remove them or replace them depending on which upgrade kit is being used.
But by all means, bypassing them, in theory, should determine if there is an issue with the line stage pcb or somewhere else in the circuit.

Good point on 'keeping it original only so far .  

Seeing as I'm in the 'sticks', I don't have a ready source for pots, caps and etc readily available, so figured we'd try to see how far I'd get with what was there.  Yes, I understand the concept of 'leaky' caps.  As for voltages, everything was well within 5% of test point values from the Dynaco manual (some were dead spot on), and left / right channels were identical.

So if I'm understanding you correctly, bypassing the tone controls would be a valid diagnostic step.  I'll recheck that I didn't somehow switch something on the pcb.  I believe I had tested (verified identification) of the caps you sent with the kit on my meter, so I'll go with the assumption that the PCB components are good.  

Seeing as this is going to take longer than I thought, I suppose I best swap all the pots and caps on the front panel.  I'll also verify the resistors having drifted to some way off values.  

Fortunately the right side is working so I have a point of comparison. (makes me wish I was at my old job where I'd have access to a signal generator and a scope. then it would be easy to just inject a signal into both channels, and compare going through the amplification stages. Oh well, those days are in my past)

Any recommendations on replacement source for original style volume, balance and tone pots?

There are no direct replacements for the original pots, some of which were specials. Bypass the tone controls on the bad channel as a first step. It's easy to do and you'll know pretty quick if the problem is in the tone controls or on the board.

Check the selector contacts.

If you switch tubes, does the problem follow the tubes, or stay in place?

Had to ask.

boeingpilot wrote:

erhard-audio wrote:You indicated that all B+ and filament voltages check out, so we can pretty much say that at least the power supply is good.
Also, right channel is working fine.
If you are 100% sure that all components pertaining to the left channel are verified, then again we can pretty much say that at least physically all is good with the line stage pcb.
Just double check all left channel pcb connections/wiring as well.
If the tone control pots were pretty crappy to start with, personally, I would not have bothered cleaning and reuse them, since they are 40+ years old are more than likely worn. Same goes for the caps in the tone control circuit, they are known to drift and leak. Not leak as in dripping leak  but leak as in allowing DC to flow through. One can only take this 'wishing to keep it original' thing so far!
It is easy and cheap enough to replace the tone control parts.
Interestingly enough, I have never gone through the exercise of bypassing the tone controls on a PAS even though I work on them a lot, I usually just remove them or replace them depending on which upgrade kit is being used.
But by all means, bypassing them, in theory, should determine if there is an issue with the line stage pcb or somewhere else in the circuit.

Good point on 'keeping it original only so far .  

Seeing as I'm in the 'sticks', I don't have a ready source for pots, caps and etc readily available, so figured we'd try to see how far I'd get with what was there.  Yes, I understand the concept of 'leaky' caps.  As for voltages, everything was well within 5% of test point values from the Dynaco manual (some were dead spot on), and left / right channels were identical.

So if I'm understanding you correctly, bypassing the tone controls would be a valid diagnostic step.  I'll recheck that I didn't somehow switch something on the pcb.  I believe I had tested (verified identification) of the caps you sent with the kit on my meter, so I'll go with the assumption that the PCB components are good.  

Seeing as this is going to take longer than I thought, I suppose I best swap all the pots and caps on the front panel.  I'll also verify the resistors having drifted to some way off values.  

Fortunately the right side is working so I have a point of comparison. (makes me wish I was at my old job where I'd have access to a signal generator and a scope.  then it would be easy to just inject a signal into both channels, and compare going through the amplification stages. Oh well, those days are in my past)

Any recommendations on replacement source for original style volume, balance and tone pots?

have a look on our web site under PAS upgrades, we do offer an original style tome control upgrade kit

Ok, totally egg on face moment....



Ordered the tone capacitors and got them in. Decided to just replace the left channel (where I was having issues). Still, no joy (low audio, muffled audio, etc). I did notice that at one point it sounded fine, and then started to degrade. Hmmm, thermal?

Went back to electronics 101 and started to look carefully at the PC5, especially the left channel components. Found a couple of solder joints I wasn't too proud of. And found one in particular that looked like I had definitely had not flowed enough solder to. Reflowed all left channel solder connections. Voila..... that wonderful Dynaco sound!

As my old boss (I used to work in medical electronics) would say, did you check the power supplies, did you check the solder joints......

I did sand the front panel and reapplied the lacquer. Not perfect, but wow, instant Art Deco Dynaco. I feel likes it's 1961 again!

Congrats, nice look and you got it working again!