Dynaco PAS 3 Hum

I have a hum that is constant and not affected by the volume control.  Both channels, all sources.  What is the appropriate sequence for tracing this issue?  I have check all voltages and they match the spec sheet.  Also checked the tubes with my Hickok and they test good.  Any help will be appreciated.

most likely the power supply, and possibly just the rectifier tube.
Get a new rectifier tube (12X4) and replace the old one.
If that doesn't cure the problem, get a new power supply board, that makes a huge difference in the quality of the PAS3.
Well worth the $54 or so,
https://www.erhard-audio.com/Power_Supply_Kits.html

Roy,

Took your advice and swapped out the 12X4 and installed the Erhard power board on my PAS3, all to no avail. Still have the hum unaffected by input source or volume.

Am I correct that given the constant nature of the hum it has to be associated with the signal path after the volume pot? In other words on the PC5 board, tone control, etc.?

If that be the case, where is it best to start tracing down this problem?

Any suggestions are appreciated.

Bruce

breading wrote:Roy,

Took your advice and swapped out the 12X4 and installed the Erhard power board on my PAS3, all to no avail.  Still have the hum unaffected by input source or volume.  

Am I correct that given the constant nature of the hum it has to be associated with the signal path after the volume pot?  In other words on the PC5 board, tone control, etc.?

If that be the case, where is it best to start tracing down this problem?

Any suggestions are appreciated.

Bruce

Check the wiring, especially the grounding. Any changes from the manual might inject hum.
I also would replace the two tubes on the line board, even if the filament is DC it's
not free from hum.
Also, all covers must be mounted.
The PAS and the power amp could have ground issue, try a coarse wire
between the PAS chassies and the power amp's chassie.

are all the boards grounded to the same ground? Maybe a bad tube in the line stage?

Well, so far I have incorporated all suggested cures for the right channel hum in my PAS3, as well as several of my own doing, all to no avail. They are, in no particular order:

Replaced 12x4
Installed new power board
Replaced all 12ax7s
Checked all ground connections
Connected PAS3 as a phono only preamp through the tape out to my SP14 and found doing so results in dead quiet operation, hum is on line inputs only (PC5?).
Switched 12ax7s on PC5 from left to right and found the hum remains on the right channel.
Checked all resistors and found them to be within +/- 20% of tolerance.
Checked capacitors with 9 volt meter and found them to generally be within range.


My conclusions, rightly or not, are as follows:

Grounding is not the issue since phono only operation results in dead quiet.
It is not tube related.
Based on my ear, the hum appears to be 60 cycle, not 120.

Any suggestions on where next to go in my journey?

breading wrote:Well, so far I have incorporated all suggested cures for the right channel hum in my PAS3, as well as several of my own doing, all to no avail.  They are, in no particular order:

Replaced 12x4
Installed new power board
Replaced all 12ax7s
Checked all ground connections
Connected PAS3 as a phono only preamp through the tape out to my SP14 and found doing so results in dead quiet operation, hum is on line inputs only (PC5?).
Switched 12ax7s on PC5 from left to right and found the hum remains on the right channel.
Checked all resistors and found them to be within +/- 20% of tolerance.
Checked capacitors with 9 volt meter and found them to generally be within range.


My conclusions, rightly or not, are as follows:

Grounding is not the issue since phono only operation results in dead quiet.
It is not tube related.
Based on my ear, the hum appears to be 60 cycle, not 120.

Any suggestions on where next to go in my journey?

Again, make sure the grounding and B+ cabling is done as per the manual ( yes the power board changes this).
In general i have found that shortening the wiring and locating ground and B+ on top
side shortens the cables, this when using power boards.
Avoid the area around the transformer, make sure the line cables are as far into the
chassies corner as possible.

Peterh,

Thank you for your suggestions. Please excuse me, but I am a complete neophyte with regard to these matters. Am I correct that grounding issues effect both channels. It seems from looking at the schematics and the physical layout, that the grounds are common to both channels. If that be the case, is it possible for a grounding issue in my PAS3 to manifest itself as a hum in one channel only? Just trying to better understand this challenging subject.
Bruce

breading wrote:Peterh,

Thank you for your suggestions.   Please excuse me, but I am a complete neophyte with regard to these matters.  Am I correct that grounding issues effect both channels.   It seems from looking at the schematics and the physical layout, that the grounds are common to both channels.  If that be the case, is it possible for a grounding issue in my PAS3 to manifest itself as a hum in one channel only?  Just trying to better understand this challenging subject.
Bruce

The line board is close to the transformer, one channel is closer. Try
move all ground & B+ wires as far as possible from the transformer. Also
make sure that the filament wires are well twinned and does not come close to
the board. The sketch in the manual suggests routing.