Right channel noisy. Help troubleshooting

[Paul Birkeland]

If the noise is there with terminals 4 and 5 connected, that indicates the PC boards are fine and that you should not remove the shunt regulator.

I would swap 2A3s just to rule out the most obvious possibility.  You can leave the jumper in for now.

[denti alligator]

OK, 2A3s swopped. Same noise. Same channel. It’s immediate now when I turn it on.

[Paul Birkeland]

Now try it with no 2A3 at all.

[denti alligator]

Now try it with no 2A3 at all.

No noise.

[Paul Birkeland]

Is there any green staining around the hum pot?

Do the DC voltages wander around when the noise creeps in?

[denti alligator]

I don’t see any.

I haven’t tested the voltage with speakers attached. I can try that tomorrow. But I’m assuming I would have seen the wandering when I did the voltage test earlier, and I didn’t.

I did notice, though, that the voltages were slightly lower on the right channel. I think. I can double check tomorrow. Nothing dramatically different, but still.

[2wo]

Just a thought, run the hum pot on that side back and forth a number of times and reset for lowest noise, can't hurt....John

[denti alligator]

I tried turning the hum pot when I first was troubleshooting. Made no difference. I can try again.

Paul, when I test the voltages again later today do you want me to leave the jumper in or remove it first.

[Paul Birkeland]

You can leave the jumper there.

[denti alligator]

OK, no obvious fluctuation in voltages when the noise comes in. No green visible anywhere.

Here are the voltages:

1:382
2: 399

6: 53.8
7:216.2

21: 54
22: 54.6
24: 54.7
25: 54.1

For comparison (the left channel, which works fine):

14: 210
15:56.5
19: 398
20: 370

36: 56.8
37: 57.4
39: 57.4
40: 56.7

[Paul Birkeland]

At this point I would suggest running the amp for a week straight without turning it off, then revisiting the issue.  If there is some partially conductive goo still in there somewhere, it may burn out over time or make itself more obvious if the amp is allowed to run for a considerable time period.

[denti alligator]

At this point I would suggest running the amp for a week straight without turning it off, then revisiting the issue.  If there is some partially conductive goo still in there somewhere, it may burn out over time or make itself more obvious if the amp is allowed to run for a considerable time period.

Will do. I’ll report back next week. Or sooner, if it somehow disappears before then.

(It’s quiet now … was noisy a few minutes ago… )

[denti alligator]

Quick update: nothing definitive yet, but it’s been on for 16+ hours now and although I can’t right now sit next to the speaker (or even in the same room), I was just downstairs and sat there for about 5 minutes (it’s rarely silent for more than a minute) and it was completely silent.

I remember when I first got it back from you, Paul, I turned it on and immediately put music on, so I didn’t hear the noise right away (I was listening to LPs, so easy to take the noise for static on the record). That night I accidentally left it on and in the morning I did not notice any sound. It wasn’t until I turned it off and then later back on that I heard the noise.

Also, you said you ran it for 40 hours straight with no noise. So maybe it goes away as the amp gets really warm?

Does this helps identify the possible source?

[Paul Birkeland]

This would be consistent with the contaminant hypothesis.  If the contaminant makes semi conductive pathways in the amp but these burn off as the amp runs, you may be able to just cook this off by leaving the amp running continuously for a week. 

[denti alligator]

Fingers crossed