How do you find a microphonic component?

[pfrischmann]

I'm helping a friend with his trainwreck clone. It's a really well built amp but has recently developed an issue. The amp will start popping loudly, like a capacitor discharge and then start popping and swelling (in volume).
It takes about 20 minutes for this to start. I can also hit the chassis and make it do this.

I've done the chopstick test but cannot find anything conclusive. I've swapped out the tubes, exept for the power tubes (el-84's) I have tried pairing them up differently to see if that helps...nothing.

And Ideas as to how I should approach this.

I do not have a scope nearby to use...

thanks...
Paul

[Blue Strat]

Do any of the controls affect the level of the popping sounds? This could help isolate the problem.

[SatelliteAmps]

Check the power tubes. Popping like that is usually because of a part overheating (including tubes). That you can hit the chassis and make it happen makes it sound like a physical issue. It could be an arced tranny, or a cap or resistor that is arcing (bad part or bad connection).

[pfrischmann]

Sorry for the delay, I had to go out of town unexpectedly. It doesn't seem to be affected by any of the controls. It also takes about 20 min before it starts....

[Old Tele man]

ANSWER: use the "chop-stick" test...simply the process of 'probing' the circuit using a chop-stick to tap/nudge each component to discover the bad part or solder junction.

[pfrischmann]

I tried it...Sometimes it works but sometimes it doesn't. I'll poke and say "I think I have something"...then it will stop...and not do it again or the thing I think is the problem will not do it but somewhere else will......very strange....