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Originally posted by g1
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Switching on the chorus makes the hum go away.
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Ok, so I removed Q1 and Q3 (just in case) and the hum is still there, but it gets worse if I snip R20 at the R21 end. Pushing the snipped leads together makes the hum less. It's hard to know what to make of this experiment. I rechecked everything in the U1B section, and also checked underneath the board for continuity between components via the PCB.
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Switching on the chorus makes the hum go away. -
I was thinking that since the hum goes away when switching to the drive channel, and to the chorus while on the clean channel, that the problem is something to do with the switching of channels and chorus. If the hum is not there on the clean channel when the chorus is on, it can't be coming from the clean channel, can it? Thinking aloud here. The amp has gone away for a few days for rehearsals and a gig. When it comes back I'll be looking at the circuit around U10 and U11, also U6 and U7. I've ordered a bunch of transistors to replace the J111 type switching ones, in case any of them are problematic. Can UJTs became leaky like bipolar ones do? All suggestions still welcome! Thanks.
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Have been trying to think of anything that could be common to those things. Maybe the low voltage supply rails? The chorus just runs on the +16 I think, so maybe it could offset some kind of imbalance?
Check + and - 16V supplies, both AC and DC voltages, with and without the hum.Originally posted by Enzo
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I'll check them when the amp comes back. I think there is a small difference between +16V and -16V. Can't see any ripple there with the scope, but will check it again. I thought of replacing the zener diodes to help regulate it better. I'm wondering what else happens when switching channels or chorus that could cause the hum. Thanks for the input!Originally posted by g1 View PostComment
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U10 is common to both the channel and chorus switching circuits.Originally posted by Enzo
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Yes, that's right and it's the next area I shall investigate when the amp comes back next week. I will check every component and I have a spare IC.Originally posted by g1 View PostComment
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Season's greetings! I got back to the amp in the last few days. I found no faulty components around the U10 area, but replaced a few caps and the IC itself. I restarted tracing the hum and did find a very small ripple on my scope in some places in the area of U4 and U5. I checked every component in these areas, replaced a few caps around U5, and replaced both U4 and U5. The ripple was small, about 10mv P-P. But it did drop to about 4-5mv when switching to the drive channel, so seems to be a symptom of the problem. What I have now done is cut the track between Q3 and R53/U4, pin2. I have added an 82K resistor from the R53 end to earth. I turned the amp on and the hum is gone, no more noise or hum on clean or drive channels, and the ripple has disappeared from U4 and U5. Also the chorus is quiet on both channels. It was noisy before on clean.
There is still ripple showing at the junction of R20 and R21. It is less than before I cut the track, maybe 6mv P-P, so it has dropped from about 10mv (which it was before cutting the track). So the hum seems to originate in the second stage of U1, but I can't find a cause. Also something down the line seems to make the hum worse, as I also found before when I snipped the end of R20.
Anything stand out as a possible cause or solution? I'm not sure what else to try.
By the way, I did look at the + and - 16V supplies and there was up to 1V difference in them. I added some more zeners in parallel with the existing ones and the voltages are now closer to the same, and also smoother. I don't think it made any difference to the hum, but was worth a try.
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