Marshall fault - please help!

Hi everyone.

I've just bought a JCM900 second hand. For some reason, after I've played through it for a while, the volume starts to fluxuate like someone turning the knob up and down repeatedly. I turn the amp off, switch it back on and its ok again.

Also, whenever I use the channel switch pedal, it sometimes makes the controls from channel B work on channel A (never happens if I just press the button on the amp) and it completely cuts out the reverb in both channels.

Any ideas? I've got a gig coming up and I'm a bit stuck! :(

The on thing I will say is of you're unsure, take it to an amp tech. Unless you're an expert don't try to fix it yourself, you could break it worse or electrocute yourself. If it has to be fixed, make sure you take it to a guy who knows what he's doing. It's worth it.

The one thing I will say is of you're unsure, take it to an amp tech. Unless you're an expert don't try to fix it yourself, you could break it worse or electrocute yourself. If it has to be fixed, make sure you take it to a guy who knows what he's doing. It's worth it.

if the tubes are old they may need replacing, this might explain the volume fluxtuations, but i dont know about the channel switching problem

I used to have a JCM 900 and had both of those things happen to mine. I can't remember now what the problem was that caused the volume thing, it was like 8 or 9 years ago, but I do remember having it re-tubed at that time. Can't remember if that's what solved the problem though...

And for the footswitch, it got a short in the wire, so I had to get the wire replaced that runs from the pedal to the amp.