A friend asked me to fix his keyboard amp. 50 watt solid state combo, 4 inputs. Loud crackling noise, kind of random. I'm suspecting a failing semiconductor. All the noise is coming from Channel 1. Long story short, Channel 1 has a phantom power option, and with the phantom power switch in the off position the switch is leaking voltage into the front of Channel 1. A shot of contact cleaner and all is well. Had a similar problem with a Peavey PA head. With the channel volumes at zero the channels still output sound. Dirty level pots that had significant resistance even when on zero. More contact cleaner and now all the channels go to zero and cut off the input. I've had loss of signal in pedal chains and rack systems that cleared when the input and output jacks were cleaned. Moral of the story is keep a good quality contact cleaner on hand! It's not just for crackling pots anymore And some problems are simpler than they appear.