AC15 Hum issues

[Bassist001]

I've got a '96 AC15 that I'm fixing that has the dumbest hum haha Can't seem to chase it yet. It's very sporadic, and doesn't happen til the amp is good an warmed up.There are no shorts underneath the PCB... Any ideas? Here's a link to a video of it acting up:

https://www.youtube.com/watch?v=wWVCrp2Kd08

[Tom Phillips]

Based on your video if I was working on that amp I'd check the solder joints under that PC board.

[Bassist001]

Yeah, that's what I thought. I took it all out and touched up the suspects. But still doing the same thing I did notice that when it starts happening. If I turn the the trem pot away from ground, It stops. I also pulled all the tubes and replaced them. The tubes are all still good. The hum did stop when V1 and V2 were both empty (no tubes in). Which could mean that there is a bad part in the first or second gain stage. Just not sure what would do that. Would a bad cathode cap do this?

[Tom Phillips]

The intermittent contact could indeed be a part problem such as a bad lead to part body attachment or a cracked resistor body. It could also be a hairline crack in a PCB trace but that's less likely. I use lots of good lighting and magnification to try an find a definite problem before changing anything if at all possible. You may be able to isolate the problem by gently chop-sticking individual components.

Frustrating work but at least you have a fault that is reproducible on the bench.

[Bassist001]

Well, almost reproducible haha take an hour os so before it will start doing it. Making me think it has to do with a part that's overheating or something and not just a crack. But, I throw her back on the bench tonight and see what I can find.

[Tom Phillips]

Maybe help it along with a heat gun?

[g1]

Have you tried cleaning and resoldering the trem pot? Also check that it's nut is tight (if it has one).

[Bassist001]

Have you tried cleaning and resoldering the trem pot? Also check that it's nut is tight (if it has one).

When I took the PCB out to check for shorts. I touched up all the main junctions, to see if that would fix it. I also did find a small microphonic cap near the trem circuit. Which could be causing some issues. But, It didn't really seem like it was connected to the issue.

[Bassist001]

Maybe help it along with a heat gun?

That's actually a really good idea! haha I'll try that! Thanks

[Bassist001]

I forgot to say also, That if you're playing the amp and it starts humming. If you hit a chord hard, it stops.

[Tom Phillips]

I forgot to say also, That if you're playing the amp and it starts humming. If you hit a chord hard, it stops.

That's consistent with the chop-sticking you showed in the video clip. The sound vibration is affecting the fault area. A fist thump to the cabinet would probably have the same effect.

[Bassist001]

Yeah that's what I was thinking too. Thanks