Dynaco PAS restoration issues.

Hello. I was gifted an old Dynaco PAS preamp from a friend. It was sitting unused for the last 15 years or so. We originally replaced the can cap with one sourced from dynakit. The Filament heater caps were replaced, but not the selenium rectifier. the 12X4 rectifier tube was replaced. The original 12AX7tubes all tested bad to various degrees. The system was then tested. Line stage functioned properly, Phono section functioned OK, the sound as good, but slightly unbalanced due to the old, unmatched tubes ( I am guessing ). After it played for an hour or so, the phono stage tube nearest to the front end switches and knobs began glowing brighter than the other tube, and the right side had developed a brighter sound. This was confirmed using a mono pressing.

It was then suggested to recap the phono board. all caps were replaced with hi end ones of the same value. At that time, we installed new issue matched Mullards in the phono stage. The next test produced a TON of hiss, and what I think was the documented impedance issue I had read about. Replacing with the old tubes took away the hiss, but issues started developing with the left and right outputs, with one louder than the other..

It was then suggested to replace the selenium rectifier. A rectifier board was sourced from dynacodesigns.com At that time, a new RCA board was sourced from Dynakit. To resolve the impedance issue, a matching impedance board was purchased from dynacodesigns. Those parts were installed and the phono stage tested. hiss was gone, but a new hum was introduced. All wiring instructions were followed by a qualified tech to the letter. After the impedance matching board was installed, there was a single ground line left from point 1 on the line board that was originally terminated at the RCA board. None of the supplied instructions for either the filament heater, or the impedance board mentioned reattaching this wire. It was attached to the original ground spot on the rca board. testing again, we had no sound at all thru t he phono section. the tube in that board closest to the rca board glowed extremely bright and was too hot to touch after we powered it down. the other tube, did not appear to glow much or at all.

We attached a line source and it appeared to play fine. we shut down the dynaco, and it still passed sound. i think we may have been on the tape out. all tubes were removed and it still passed signal when powered off. Is that tape out just a pass thru for the various inputs?

Anyway, my tech is at loss where to go from here. I am awaiting a reply from the gentleman at dynacodesigns dot com to see if he has any ideas since the filament board and impedance board were sourced there.

Any ideas on what may have caused the original hum? Im inclined to have my guy remove the impedance board and retest things. I'd like to use the tone controls if possible so bypassing them isn't an option unless all else fails. Any advice would be appreciated. Thanks

Sorry for a lengthy first post

Freddie_Freekowtski wrote:Hello. I was gifted an old Dynaco PAS preamp from a friend. It was sitting unused for the last 15 years or so. We originally replaced the can cap with one sourced from dynakit. The Filament heater caps were replaced, but not the selenium rectifier. the 12X4 rectifier tube was replaced. The original 12AX7tubes all tested bad to various degrees. The system was then tested. Line stage functioned properly, Phono section functioned OK, the sound as good, but slightly unbalanced due to the old, unmatched tubes ( I am guessing ). After it played for an hour or so, the phono stage tube nearest to the front end switches and knobs began glowing brighter than the other tube, and the right side had developed a brighter sound. This was confirmed using a mono pressing.

It was then suggested to recap the phono board. all caps were replaced with hi end ones of the same value. At that time, we installed new issue matched Mullards in the phono stage. The next test produced a TON of hiss, and what I think was the documented impedance issue I had read about. Replacing with the old tubes took away the hiss, but issues started developing with the left and right outputs, with one louder than the other..

It was then suggested to replace the selenium rectifier. A rectifier board was sourced from dynacodesigns.com At that time, a new RCA board was sourced from Dynakit. To resolve the impedance issue, a matching impedance board was purchased from dynacodesigns. Those parts were installed and the phono stage tested. hiss was gone, but a new hum was introduced. All wiring instructions were followed by a qualified tech to the letter. After the impedance matching board was installed, there was a single ground line left from point 1 on the line board that was originally terminated at the RCA board. None of the supplied instructions for either the filament heater, or the impedance board mentioned reattaching this wire. It was attached to the original ground spot on the rca board. testing again, we had no sound at all thru t he phono section. the tube in that board closest to the rca board glowed extremely bright and was too hot to touch after we powered it down. the other tube, did not appear to glow much or at all.

We attached a line source and it appeared to play fine. we shut down the dynaco, and it still passed sound. i think we may have been on the tape out. all tubes were removed and it still passed signal when powered off. Is that tape out just a pass thru for the various inputs?

Anyway, my tech is at loss where to go from here. I am awaiting a reply from the gentleman at dynacodesigns dot com to see if he has any ideas since the filament board and impedance board were sourced there.

Any ideas on what may have caused the original hum? Im inclined to have my guy remove the impedance board and retest things. I'd like to use the tone controls if possible so bypassing them isn't an option unless all else fails. Any advice would be appreciated. Thanks

Sorry for a lengthy first post

Sorry to hear.
In my opinion you have fallen victim of "too much work and too little diagnose"
As state is now it difficult to come back to a working PAS ( but not impossible)

I suggest you get a copy of the PAS manual, start back tracking what you have done and try to get it back to original form.

Hum will easily occur if ground connections are done different then the manual.

As for channel unbalance, the most common cause is the volume pot. As the
amp uses NFB it should be very insensitive for tube or component properties.
Get a new 250k pot, but wait with the replacement until the hum issue is solved.

As for tubes my guess is that all of them works fine if you use them in
your PAS ( ignore "tubetesters" )

What do you mean with "documented impedance issue" ?

Remember - think first, make a theory, then do something ( replace etc) and
figure out if it supports the theory. If not go back. Don't wildly replace
stuff !

Quoted from Bill Thompson audio website:

The biggest issue with the old PAS-3 is that the original tone control circuit depends on having exactly a 470k Ohm load on the preamp output to get flat response. A lower impedance causes rolled-off bass response, higher causes a bass boost. This makes it impossible to use this preamp with modern solid state power amplifiers that typically have from 10k to 50k input impedance.

The rolled off bass was one of the things that happened on the phono stage. if I switched to the tape out jacks next to the amp out jacks, it bypassed the volume and tone sections and the new tubes did not exhibit this bass rolloff.

I still have the original Black Cat caps from the phono board. The original RCA board was thrown out.

.

PeterCapo wrote:Agree with peterh and echo his question of what you mean by "documented impedance issue," especially since you refer to it in the context of a hiss problem(?).  Yes, we do know about an output impedance issue that occurs in some cases, but it would be good to clarify what you are thinking of.



Sounds like the waters are muddied.  It's a fine idea to think about putting it back to the original circuit design, but if you can't do this yourself, you'd need to find a tech who understands (and respects) the original PAS circuit.

In any case, not quite clear on what the problem is at this point.  Is it just the phono section?  Yes, I believe the inputs will show up at the tape out jacks even with the power off, with the exception of the phono stage which needs power in any case.

What other equipment do you have connected to your PAS?

This: Quoted from Bill Thompson audio website:

The biggest issue with the old PAS-3 is that the original tone control circuit depends on having exactly a 470k Ohm load on the preamp output to get flat response. A lower impedance causes rolled-off bass response, higher causes a bass boost. This makes it impossible to use this preamp with modern solid state power amplifiers that typically have from 10k to 50k input impedance.

The rolled off bass was one of the things that happened on the phono stage. if I switched to the tape out jacks next to the amp out jacks, it bypassed the volume and tone sections and the new tubes did not exhibit this bass rolloff.

I am using a really nice MCS 6700 table with an ortofon 2m red. MCS 3249 receiver. Did have a line out from my laptop running on "spare" and was thinking of adding a DAT machine for playback of some old tapes of mine, but the primary use will be the turntable.

The line section/board never had any issues and was untouched other than new tubes which sounded fine to my ear

.

.

PeterCapo wrote:

Freddie_Freekowtski wrote: ... The rolled off bass was one of the things that happened on the phono stage. if I switched to the tape out jacks next to the amp out jacks, it bypassed the volume and tone sections and the new tubes did not exhibit this bass rolloff. ...

Picking the audio off of the tape out bypasses more than the volume and tone sections.  It bypasses everything but the phono section.  Putting a low impedance input on the tape out should roll the bass off from the phono section, regardless.  Your reported results are puzzling.

Freddie_Freekowtski wrote: ... I am using a really nice MCS 6700 table with an ortofon 2m red. MCS 3249 receiver. Did have a line out from my laptop running on "spare" and was thinking of adding a DAT machine for playback of some old tapes of mine, but the primary use will be the turntable. ...

How are you integrating the PAS into this system?  Are you using either the PAS tape out or pre-out into either a high-level input or a power amp input of the receiver?  Is your turntable going directly to the PAS or are you doing it some other way?  Kind of confusing.

turntable out to phono in on the PAS. Amp out ( or whatever the last RCA's are called ) to the aux in on my stereo. When the phono section started having the issues. I was able to insert the PAS into the tape loop of the receiver and use the line stage with no issues. All testing that was done at the techs was using the phono stage only and where all the issues other than the brightness issue that develops after extended play occurred.

Im digging this advice, thanks everyone. I will have my tech download the manual from curcio audio. As far as the line stage goes, I can leave it unused until I it's needed, this is primarily for my rekkids

.

PeterCapo wrote:Do you mean that you connect the main preamp output (not the tape out) of your PAS to the AUX input of your receiver?  If so, do you have the specifications for your receiver, in particular, the input impedance of its high level inputs?

Main preamp output from any preamp should neither be connected to a high level input nor the tape loop of a receiver.  In both cases, the receiver could be over-driven.

Does your receiver have pre-out/main-in jacks joined by jumpers?  If so, that's the proper way to connect an external preamp, though, in the case of the PAS, you'd probably have to adjust the resistors at its outputs, as indicated in the Audio Regenesis articles.


If you can post a series of photos of your PAS, it could be helpful.

I do not have the specs of the amp, but it is an MCS 3249 which was made by NEC for JC Penney and is from around 1980. It does not have any jumpers.

.

PeterCapo wrote:

Freddie_Freekowtski wrote: ... To resolve the impedance issue, a matching impedance board was purchased from dynacodesigns. Those parts were installed and the phono stage tested. hiss was gone, but a new hum was introduced. All wiring instructions were followed by a qualified tech to the letter. After the impedance matching board was installed, there was a single ground line left from point 1 on the line board that was originally terminated at the RCA board. None of the supplied instructions for either the filament heater, or the impedance board mentioned reattaching this wire. It was attached to the original ground spot on the rca board. ...

Trying to sort this out... I just looked at the Dynaco Designs website, and, unless I missed it, I didn't see an "impedance board."  Are you referring to a small add-on board with transistors on it that is supposed to make the PAS more compatible with amplifiers having a low input impedance?  I have seen things like this for sale on eBay, but I don't recall Dynaco Designs having one, though, again, I might have missed it.  Or, are you perhaps referring to a new back panel I/O board with RCA jacks?

Freddie_Freekowtski wrote:

PeterCapo wrote:Do you mean that you connect the main preamp output (not the tape out) of your PAS to the AUX input of your receiver?  If so, do you have the specifications for your receiver, in particular, the input impedance of its high level inputs?

Main preamp output from any preamp should neither be connected to a high level input nor the tape loop of a receiver.  In both cases, the receiver could be over-driven.

Does your receiver have pre-out/main-in jacks joined by jumpers?  If so, that's the proper way to connect an external preamp, though, in the case of the PAS, you'd probably have to adjust the resistors at its outputs, as indicated in the Audio Regenesis articles.


If you can post a series of photos of your PAS, it could be helpful.

I do not have the specs of the amp, but it is an MCS 3249 which was made by NEC for JC Penney and is from around 1980. It does not have any jumpers.

If no specifications or other documentation are available on your receiver, and if your receiver does not have pre-out/main-in jacks, then I think it would be a good idea to get a tube amp to use with your PAS and then have your PAS restored to the original Dynaco configuration - once you identify which PAS you have, of course.

It was item number 303534661996 on ebay. it says currently 0 available. Brand was listed as Dynacodesigns and this was the descrption. I think it is what you were talking about.
The PAS preamp was designed to work with power amps
having high input impedance, limiting it's ability to
drive later amps of that time period, and modern day
amps as well. That resulted in poor low frequency
response and an impedance mismatch with any power
amp having an input impedance under 50k.
The fix was to remove the tone controls allowing the
PAS preamp to drive any power amp.
Some people (myself included) don't want to give up
their tone controls.
This module makes the PAS preamp compatible with all
power amps old and new. And you can keep your tone
controls!
It can be installed in less than fifteen minutes, and
will have a noticeable impact on the sound quality of
your source material.

.

Examining that Audio Regenesis document, the bass and treble pots are 400k with 1 notch

Freddie_Freekowtski wrote:Examining that Audio Regenesis document, the bass and treble pots are 400k with 1 notch

bass is 750k treble 400k

.

PeterCapo wrote:Sounds like you have either a PAS-2 or a PAS-3, without Dynaco's X upgrade.  If brass face-plate, it's a PAS-2.  Aluminum face-plate, it's a PAS-3.

Your version of PAS also has a some DC offset on its main preamp outputs that might account for some of the difficulty you've experienced running it with your receiver.  In any case, it sounds like there are a few things that would need to be addressed.

If you could post a series of images, i]t might help us to come up with some suggestions on which direction to move in.

wont let me post any photo links. faceplate is aluminum, sticker on back just says PAS

My tech was instructed by one of the board sellers to remove his mod, and restore stock config. That was done and the hum vanished, all issues vanished save 1. The right side is noticeably brighter. this was confirmed using a mono pressing and swapping cables, the brightness did not follow. would one of those tone pots possibly be the culprit, or should we check the values and tolerances of all the caps & resistors in the controls section?

.

Freddie_Freekowtski wrote:My tech was instructed by one of the board sellers to remove his mod, and restore stock config. That was done and the hum vanished, all issues vanished save 1.

That "impedance matching" board is called an output buffer. Sounds like it was either defective or not installed properly. Glad you were able to remove it successfully. One less variable.

What about the tube that was glowing so bright and was too hot to touch? Surely this was not caused by the output buffer.

Unfortunately it sounds like you've got someone working on your unit who doesn't seem to know the PAS that well.  Slow down and read. You're in good hands here. The suggestion to study the Dynaco manual and audioregennesis guide is good advice.

rjpjnk wrote:

Freddie_Freekowtski wrote:My tech was instructed by one of the board sellers to remove his mod, and restore stock config. That was done and the hum vanished, all issues vanished save 1.

That "impedance matching" board is called an output buffer. Sounds like it was either defective or not installed properly. Glad you were able to remove it successfully. One less variable.

What about the tube that was glowing so bright and was too hot to touch? Surely this was not caused by the output buffer.

Unfortunately it sounds like you've got someone working on your unit who doesn't seem to know the PAS that well.  Slow down and read. You're in good hands here. The suggestion to study the Dynaco manual and audioregennesis guide is good advice.

It certainly looks that way. Once the buffer was removed, that tube stopped drawing the high current. Voltage across pins 5 was 28.66 VDC
Waiting on the gentleman from Dynacodesigns to reply on his module. Tech will check the front end pots, resistors and caps to see if any are too far out of spec and might be causing the brightness I hear.

Freddie_Freekowtski wrote:

rjpjnk wrote:

Freddie_Freekowtski wrote:My tech was instructed by one of the board sellers to remove his mod, and restore stock config. That was done and the hum vanished, all issues vanished save 1.

That "impedance matching" board is called an output buffer. Sounds like it was either defective or not installed properly. Glad you were able to remove it successfully. One less variable.

What about the tube that was glowing so bright and was too hot to touch? Surely this was not caused by the output buffer.

Unfortunately it sounds like you've got someone working on your unit who doesn't seem to know the PAS that well.  Slow down and read. You're in good hands here. The suggestion to study the Dynaco manual and audioregennesis guide is good advice.

It certainly looks that way. Once the buffer was removed, that tube stopped drawing the high current. Voltage across pins 5 was 28.66 VDC
Waiting on the gentleman from Dynacodesigns to reply on his module. Tech will check the front end pots, resistors and caps to see if any are too far out of spec and might be causing the brightness I hear.

*deleted*

peterh wrote:

Freddie_Freekowtski wrote:

rjpjnk wrote:

Freddie_Freekowtski wrote:My tech was instructed by one of the board sellers to remove his mod, and restore stock config. That was done and the hum vanished, all issues vanished save 1.

That "impedance matching" board is called an output buffer. Sounds like it was either defective or not installed properly. Glad you were able to remove it successfully. One less variable.

What about the tube that was glowing so bright and was too hot to touch? Surely this was not caused by the output buffer.

Unfortunately it sounds like you've got someone working on your unit who doesn't seem to know the PAS that well.  Slow down and read. You're in good hands here. The suggestion to study the Dynaco manual and audioregennesis guide is good advice.

It certainly looks that way. Once the buffer was removed, that tube stopped drawing the high current. Voltage across pins 5 was 28.66 VDC
Waiting on the gentleman from Dynacodesigns to reply on his module. Tech will check the front end pots, resistors and caps to see if any are too far out of spec and might be causing the brightness I hear.

Funny , i do not find any buffer on the dynacodesigns.com info page. Do you have
any details of it ?

He was selling it on EBay and it currently shows 0 available. He had shipped me a newer model that had socketed transistors for easy removal and upgrades. Maybe I was a guinea pig and his design isn't solid.

I contacted the seller "re-engineer" and asked if the potentiometers in the
pas potentiometer kit. Did they have correct values .
The answer was not to my satisfaction :

The question you're really asking is:
Are they the same resistance as the
originals.
The answer is:
If they were, there would be no need for the
kit.
Thanks for stopping by...C


In other words he refuses to tell what he sells.
Maybe better to skip this seller.

peterh wrote:I contacted the seller "re-engineer" and asked if the potentiometers in the
pas potentiometer kit. Did they have correct values .
The answer was not to my satisfaction :

The question you're really asking is:
Are they the same resistance as the
originals.
The answer is:
If they were, there would be no need for the
kit.
Thanks for stopping by...C


In other words he refuses to tell what he sells.
Maybe better to skip this seller.

He was quite problematic during communication. His instructions for troubleshooting werent exactly clear, and they did did change...I had reported a value across pins 5 that was in the 30's and he freaked. sent back explicit step by step instructions which differed from the first set which stated to take out the tubes  and power unit off....which then changed to remove rectifier and 12ax7's in the phono board, power on and measure. We then got an acceptable value, at least to him, of 28.66 VDC. He has not replied since we got the phono stage working by bypassing his buffer. We followed his supplied instructions to the letter and still had a wire that originally ran from point 1 on the line board to RCA ground. His instructions never mentioned what to do with it.

The brightness issue still exists. When I was listening, it seemed to become more of an issue as the thing was running. The line stage never experienced that issue. Another poster in a different forum said the selector switch could be at fault. I wouldnt be surprised if it was. It was literally frozen in place when I got the unit. My guy had to soak it in something to dissolve the gunk that crippled it. I am not a fan of its feel, it doesnt seem to sit properly in the phono spot. slight turns to either the left or right can cause a channel to drop out completely. The line stage, on spare, seems to feel like it rides its groove better, but you can still cause noise and dropouts by the slightest turn. Edited to add that all resistors were within 10% tolerance of their original values. Bass pots read 71.2k for left and 71.5 k for right..now treble is 368.6k for left and and 345.2k for right.