Marshall JCM 900 4100 Volume spikes

[Frankrocks]

Have A 4100 JCM 900 that was re tubed. It works fine but after a little time the volume spikes.

Has anyone else experienced this problem. I am hoping I can get this up and running this week.
Thanks

[Jazz P Bass]

Did the problem surface after the retubing?

[Frankrocks]

I really not sure? I am trying to help a friend fix the amp. He said he tried to Bias the amp after re tubing as well. thinking it was a Bias issue.
If it was doing before the tubes what could the issue be?

[J M Fahey]

You may have a dirty Master volume pot, gain is active and a dirty/open pot throws that stage open loop.

Hit everything on the board and all pots with a plastic pen (thing BIC or similar) or a chopstick.

That is as far as shotgunning.

For a real troubleshooting feed it 100mV@1kHz at the input, scope the output and wait for the volume to spike, then repeat going backwards until you find the culprit stage.