Hum. How to diagnose (and repair)?

[misterwhizzy]

Re: Hum. How to diagnose (and repair)?

Probably not. It was a miserable failure regardless. I was thinking it was because of the depth pot, but that may be the trick. Glad you spotted it.

 

[GoldenVulture]

Re: Hum. How to diagnose (and repair)?

So my JCA50 has a pretty bad sixty-cycle hum. Other amps in the same socket don't have it nearly as bad. It seems to have to do with the power section, because preamp volume only affects it very, very slightly, and it happens even with V1, V2, or V3 pulled. Is it possible that the bias is too hot? Is it just house wiring or maybe less-than-perfect amp design? Any ideas on how to solve it? I don't mind getting in there if need be, but it's easier to replace preamp tubes, for example, than to remove the whole amp from the chassis and mess with bias.

In the process, I changed V1, V2, and V3 just to see if there was any noticeable difference with different tubes, and there was not.

Bad filter caps are quite often the source of AC hum. I'm not sure how old this amp is and the caps but as you have attempted other fixes without success, this might be a solution.

 

[misterwhizzy]

Re: Hum. How to diagnose (and repair)?

I swapped the red and white this afternoon and got back to about where things were before I started the surgery, except in putting a 500k pot in the overdrive channel, I made it completely unusable, so it'll have to go back to 1M. I also want to secure the depth pot more firmly and maybe add a 1uF cap to make sure there's no DC getting through there either. But for at least the next few days, I'll get on with the quite-usable crunch channel until i work up the desire to dig back in.