Peavey Impulse 200 powered speaker issues

[tonepumpamps]

Hello,
I recently got a Peavey Impulse 200 powered speaker in the shop to repair.
Everything works fine except when the mic input is turned on and the volume turned up (no inputs plugged in at all) it starts to develop an almost feedback like howl (lower frequency). I can tap my finger by the inputs and it picks it up and makes it howl worse. It acts like a tube amp with a microphonic preamp tube.
Any ideas on what the problem could be? I'm in the process of going over the preamp board now.

Thanks!!

[soundguruman]

Call the PV techs at 601 483 5365 and they probably already know what the issue is. They will usually help you, no problem.

[J M Fahey]

You already made half the work; now tap lightly with a wooden or plastic chopstick everything connected or close to that input,you may have a microphonic capacitor (most likely) or some other part.