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Original Message

PAS-3 hum (I know, I know)

Posted by CaseyTheLibrarian on January 17, 2017 at 08:34:08:
Good morning everyone :) I am hoping to solve the pesky hum in the right channel problem people seem to have with the PAS-3.

I recently picked up a ST-70 and PAS-3, both completely gone over, recapped, some mods added, etc. I also picked up a Parks Audio Budgie phono pre so my system looks like this:

Dual TT > Budgie > PAS3 >ST-70 >Boston Acoustic A70s.

Now, onto the issue.

There is a very low and nearly inaudible hum in the right channel. Once the music begins, it's a non issue but it is definitely there.

The ST-70 has 2 individual gain/volume controls. When the ST-70 and PAS-3 are fired up, that is when the hum begins. If I reduce the power in the right channel on the ST-70, I can effectively get rid of the hum. But I have to reduce the gain by about 75%. It becomes audible right around when I hit half-way on the dial. When the PAS is off, the ST-70 is dead quiet.

Things I've tried: switching ICs, reversing plugs, moving the units.

I brought the unit to a guy in town who works on things and is an electrical engineer and had him take a look. He verified that there was a hum (which made me happy that I wasn't crazy and dreaming it up) but was unable to find a solution. I asked him to email me the things he did and this is what he sent:

1. Swapped inputs to the second stage amp. Hum stayed in right channel. This proves the problem is in the second stage amp.
2. Temporarily installed very large value filter caps in all three power supplies - High voltage, and the Plus/Minus heater supply. Hum stayed in right channel.
3. Hum goes away if I unplug, and let the amp run on the residual voltage in the power supply. This leads me to believe it's a power supply issue. Took aluminum foil, and shielded the entire power supply. Hum stayed in right channel.
4. Followed the advice from Dynaco, concerning the grounding issue using temporary jumpers. No change, hum stayed in right channel.
5. Swapped ALL capacitors between the two channels in the second stage amp. Hum stayed in right channel.
6. Swapped the tubes...no change
7. Checked ALL components...all are good.

So that is where I am right now. I've read some old posts on this issue here but was hoping someone could chime in with a possible solution.

I am an absolute electronics dummy, so any additional information provided will be processed through that filter :) I can attach photos of things if that is of help.