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Let's look at a Dynaco PAS-3x

8xlaxx

8xlaxx

Have you ever seen a spleen that large?
Seeking expertise for my PAS-3x with some symptoms it's having and where I should start in resolving issues.

This pre is new to me and I have no knowledge on the back history. For the most part it looks stock, powers up and plays but there are a couple changes on it. In using the phono inputs and the outputs to the amp, there was no sound coming out of the right channel. Sprayed and worked the balance and volume pots with D5. The selector switch was soft and 'gooshy' in rotation so it was cleaned up w/ D5 and Fader Lube. The selector switch now makes solid contact and has an audible click at each input.

The right channel was still not working. I pulled the original Telefunken 12AX7 tubes out and checked them all on my TV-7 tester. One of the 12AX7 tubes was weak on the phono stage board, registered a 28 (minimum is 32 for an acceptable tube). Another 12AX7 tube on the output side PC-6 registered 20! The other 2 original, Teles both registered 40, so I left those installed but switched them both to the output section on PC-6. I re-installed a matched pair of NOS Sylvania 12AX7s into the PC-5 phono section. Success!,... music from the right channel now.

Here is the problem: on the RIGHT channel only, the bass frequencies are extreme. Just turning the BASS pot to the right of the mid-point throws in a ton of low end to the point of distortion and muffle. On the LEFT channel there is very little bass even when the pot is turned all the way to the right. The LEFT channel is actually quite a bit pinched and is 'tinny'. There are 4 yellow, non-original caps that have been swapped out on this PAS-3x. Two of the caps go to the bass controls on the RIGHT and LEFT bass pots,...hmmmm? Both of the yellow cap values are 1.0uF 100V. The other two caps are on the PC-6 output section of the board and are .22uF 630V Seacor branded caps.

There is another cap (see pic) on the underside of PC-6 that I do not think belongs there. It is bulging on the (+) side and looks awful.

Again, looking for some help on this one and am willing to put a little time and effort into it. Thanks for the assistance and for reading my post.


Bulging cap (+) side with crud leaking out,...
FC1A1294.JPG

FC1A1295.JPG

Diodes in for selenium rectifier,...
FC1A1296.JPG

Seacor caps on board. Unknown brand going to bass controls.
FC1A1297.JPG
 
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About 25 years ago I bought a PAS, read the various update suggestions and did the ones that made sense to me. The updates included replacing selenium rectifier with silicon, adding power supply filter board, removed tone controls, upgrading resistors and capacitors with some value changes, replacing RCA connectors and selector switch. The results were surprising, it sounded better to me, than some preamps that cost a heck of a lot more.

My suggestion is to do similar, rather than to troubleshoot the existing problems. I'm guessing that doing so will resolve the problems and you will be happy with the results.
 
My suggestion is to start with "blueprinting" .
Get the original schematics. Compare voltages , fix if they fail.
Then check most ( all) resistors, and i think most of them can be measured in place without desoldering.
Also check the pots, note they are unique and unobtainable. But check them to identify if they fail or pass.
You already got the selector working.
Do check that caps has original values everywhere.
Tube conditions does not really matter as the amp is in a feedback mode, any working tube will do.

When the above is done you should have a working pas.
 
8xlaxx said:
Seeking expertise for my PAS-3x with some symptoms it's having and where I should start in resolving issues.

This pre is new to me and I have no knowledge on the back history. For the most part it looks stock, powers up and plays but there are a couple changes on it. In using the phono inputs and the outputs to the amp, there was no sound coming out of the right channel. Sprayed and worked the balance and volume pots with D5. The selector switch was soft and 'gooshy' in rotation so it was cleaned up w/ D5 and Fader Lube. The selector switch now makes solid contact and has an audible click at each input.

The right channel was still not working. I pulled the original Telefunken 12AX7 tubes out and checked them all on my TV-7 tester. One of the 12AX7 tubes was weak on the phono stage board, registered a 28 (minimum is 32 for an acceptable tube). Another 12AX7 tube on the output side PC-6 registered 20! The other 2 original, Teles both registered 40, so I left those installed but switched them both to the output section on PC-6. I re-installed a matched pair of NOS Sylvania 12AX7s into the PC-5 phono section. Success!,... music from the right channel now.

Here is the problem: on the RIGHT channel only, the bass frequencies are extreme. Just turning the BASS pot to the right of the mid-point throws in a ton of low end to the point of distortion and muffle. On the LEFT channel there is very little bass even when the pot is turned all the way to the right. The LEFT channel is actually quite a bit pinched and is 'tinny'. There are 4 yellow, non-original caps that have been swapped out on this PAS-3x. Two of the caps go to the bass controls on the RIGHT and LEFT bass pots,...hmmmm? Both of the yellow cap values are 1.0uF 100V. The other two caps are on the PC-6 output section of the board and are .22uF 630V Seacor branded caps.

There is another cap (see pic) on the underside of PC-6 that I do not think belongs there. It is bulging on the (+) side and looks awful.

Again, looking for some help on this one and am willing to put a little time and effort into it. Thanks for the assistance and for reading my post.

There is definitely, changes made but maybe it is better to fix the bad stuff and see if that cures most of your problems. Leaking electrolytics are a definite change out. I rebuilt a PAS 3 to the original schematic, and there were no 1.0uf caps in the original circuit. The phono section has .22uf caps, if i am correct.
I
Click to expand...
 
Nice PAS-3X! I refreshed my own PAS-3X (and my PAS-2) by replacing the Black Cats on both circuit boards with Orange Drops. I replaced the selenium rectifier in my PAS-2 with silicon diodes (haven't gotten around to doing the same with my PAS-3X yet), and both of them have had new electrolytic capacitors installed in the filament supply. I used big fat Solen 1µF capacitors to replace the bass control caps in my PAS-3X (the PAS-2 doesn't have those capacitors).

The leaking/bulging (can't really see it from your pics) capacitor hanging off of the circuit board is an add-on; not sure off-hand what its intended function is. As others have said, the assembly manual is your friend. Use that as your guideline to return the unit to factory original, then go from there with any possible modifications you may perform. Good luck!
-Adam
 
Thanks for the replies. I'd like to get to the bottom of why the additional caps were placed in this preamp.

Here is a better pic of the bulging cap:

EDIT: This is a 315V 22uF cap.

FC1A1304.JPG

This cap is tied in between points 11 and 16.

There are two other caps I see on the board here:

FC1A1306.JPG

and here:

FC1A1305.JPG

This 'extra board' is tied in to the quad cap and has a blue cap in series:

FC1A1307.JPG

The pots are the stock original ones with the indents that end at the 8 and 4 o'clock positions. They are working. I'll take some voltage readings from page 11 and see what happens. Also, if a new quad cap is needed this one has the original Dynaco 30 20 20 10 inside. What is the recommended replacement quad cap that will fit with the cover installed?
 
Last edited:
I'm not seeing the phono stuff hanging off the selector? I know a popular mod for these was to move it off the switch and onto the phono board. Like said above, I'd just start at the beginning of each section at a time, power supply, phono board, and linestage. You can look around the web and find ways to fix the phono IIRA which is off some in the stock PAS. I put a phono section out of the RCA tube manual in the last PAS I did, with good results I think.
 
Here are the voltages that I have observed:

Front 12AX7 on PC-6 voltage readings:

Pin (all are VDC) w/ *****spec voltages given in owners manual to the right******
1) 114.........115
2) 0.0..........0.0
3) 0.7..........0.7
4) 0.0..........0.0
5) -12.8.......+/- 11
6) 134.3......135
7) 0.0..........0.0
8) 0.8..........0.8
9) -6.4.........+/-5.5

Rear 12AX7 on PC-6 voltage readings:

Pin (all are VDC)
1) 110.4.........115
2) 0.0.............0.0
3) 0.7.............0.7
4) 0.0.............0.0
5) 12.3..........+/- 11
6) 138.7........135
7) 0.0............0.0
8) 0.8............0.8
9) 6.4............+/-5.5

Left 12AX7 on PC-5 voltage readings:

Pin (all are VDC)
1) 170.4........175
2) 0.0............0.0
3) 1.8............1.45
4) 0.2............<1.0
5) 12.3..........+/-11
6) 188.2........200
7) 0.0.............0.0
8) 1.3.............1.25
9) 6.2.............+/- 5.5

Right 12AX7 on PC-5 voltage readings:
Pin (all are VDC)
1) 172.6........175
2) 0.0............0.0
3) 1.8............1.45
4) 0.2............<1.0
5) -12.7..........+/-11
6) 184.7........200
7) 0.0.............0.0
8) 1.3.............1.25
9) -6.1.............+/- 5.5

12X4 observed readings w/ *****spec voltages given in owners manual to the right*****
1) 343.9 VAC........335
2) 0.0..................0.0
3) ----|
10.8 VAC......10.5
4)----|
5) 0.0...................0.0
6) 343.7 VAC.......335
7) 423.2 VDC.......405 VDC

Quad Cap observed readings w/*****spec voltages given in owners manual to right*****
(square) 424.6 VDC..................405
(triangle) 362.6 VDC.................355
(---------) 327.9 VDC..................330
(half dome) 424.4 VDC.............210

Half dome is double of what is should be. All the other readings look pretty good. Suggestions welcome.
 
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Bulging cap is a Rippon Chemi-Con 315V 22uF cap. I don't think it is suppose to look like that?

FC1A1308.JPG
 
8xlaxx said:
Bulging cap is a Rippon Chemi-Con 315V 22uF cap. I don't think it is suppose to look like that?

View attachment 1158285
Click to expand...
That pinch in the positive end is normal. It's another way of indicating the polarity of the capacitor, besides the arrow stripe. Nevertheless, I would excise it from the circuit, as it's not supposed to be there, and return things to normal.
-Adam
 
If the black caps are Black Cats, I believe they are Mylar (polyester) and don't need to be changed. Check the carbon comp resistors for value and change any that are on the plates of the tubes. They contribute noise. Check all resistors. Remove what isn't supposed to be there and get back to stock. I think they're wired with solid wire, so check for breaks.
 
QUOTE="8xlaxx, post: 11500910, member: 229635"]Seeking expertise for my PAS-3x with some symptoms it's having and where I should start in resolving issues.

This pre is new to me and I have no knowledge on the back history. For the most part it looks stock, powers up and plays but there are a couple changes on it. In using the phono inputs and the outputs to the amp, there was no sound coming out of the right channel. Sprayed and worked the balance and volume pots with D5. The selector switch was soft and 'gooshy' in rotation so it was cleaned up w/ D5 and Fader Lube. The selector switch now makes solid contact and has an audible click at each input.

The right channel was still not working. I pulled the original Telefunken 12AX7 tubes out and checked them all on my TV-7 tester. One of the 12AX7 tubes was weak on the phono stage board, registered a 28 (minimum is 32 for an acceptable tube). Another 12AX7 tube on the output side PC-6 registered 20! The other 2 original, Teles both registered 40, so I left those installed but switched them both to the output section on PC-6. I re-installed a matched pair of NOS Sylvania 12AX7s into the PC-5 phono section. Success!,... music from the right channel now.

Here is
the problem: on the RIGHT channel only, the bass frequencies are extreme. Just turning the BASS pot to the right of the mid-point throws in a ton of low end to the point of distortion and muffle. On the LEFT channel there is very little bass even when the pot is turned all the way to the right. The LEFT channel is actually quite a bit pinched and is 'tinny'. There are 4 yellow, non-original caps that have been swapped out on this PAS-3x. Two of the caps go to the bass controls on the RIGHT and LEFT bass pots,...hmmmm? Both of the yellow cap values are 1.0uF 100V. The other two caps are on the PC-6 output section of the board and are .22uF 630V Seacor branded caps.

There is another cap (see pic) on the underside of PC-6 that I do not think belongs there. It is bulging on the (+) side and looks awful.

Again, looking for some help on this one and am willing to put a little time and effort into it. Thanks for the assistance and for reading my post.
xx- I had the same experience with my PAS3x at first. The Bass controls are very touchy in this preamp. Just 1/8 turn to the right boosts about 5 dB! Maybe it was because speakers in circa 1958 had poor bass response and needed a sharp filter, or the controls have drifted in value drastically over the years but after fiddling with the tone too many times, I finally swapped out the pots. I don't think I could get 750k Ohms at the time so, I used 500k Alphas. That does change the turnover freq. but I didn't hear a negative difference and the controls work MUCH BETTER!! So smooth and responsive. Here's a tip, ebay has Clarostats, long shaft 750k, Linears for just $10 if you are interested.

Bulging cap (+) side with crud leaking out,...
View attachment 1157846

View attachment 1157847

Diodes in for selenium rectifier,...
View attachment 1157850

Seacor caps on board. Unknown brand going to bass controls.
View attachment 1157851[/QUOTE]
 
Voltages seems ok ( except for the final quad/hald dome).
The extra caps : cut and throw away. Cut anything that is not in the manual.
As for the quadcap, new ones may be obtained from triode. But i suggest you get a power board like this one :
http://tubular-well.se/index.php/webshop/reservdelar/tubes4pro-psu-detail ( it's in sweden so the shipping costs alone
prohibits buying this ) Simular ones may be obtained from http://www.erhard-audio.com This will replace the quadcap
and all components for recitfying and filtering the filament at a cost comparable to the quad cap. But wait with
that until the audio problem is fixed.
The fix is most likley to find in a out-of-spec resistor. Check them one-by one. Any changed caps might also
be treated with suspection, replace if quastinable. Don't "remove tonecontrols" or other "garden tip's" , this will
transform yourPAS3X to some garden variety bastard that has low value on the after matrket. A well functioning
PAS3X however is a valuable item that still has it's place.
http://www.audioregenesis.com/ has an in-depth analasys of PAS, and some extra reading if you want.
 
A good replacement power supply board is from Triode electronics. PN SDS-PAS-PARTS, I use one in mine. There is only one caution about it. They spec'd. in a 47u cap for the first value from the rectifier. This is too much for a 12X4. The spec. is 10uF. I used 22 uF and it works fine. The cost is $63 with the parts.
 
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