I have a 1980's GK 400RB that has become microphonic. I can tap on the knobs and hear myself knocking through the speaker. If I'm not careful, this causes really nasty feedback at a loud volume, even when the master is all the way down as well as the boost.

I need the following:

1)Possible diagnosis of what might be causing this

2)a proper schematic for this amp (1980's version as mentioned above)

It's a solid-state amp and I'm handy with a soldering iron, so if I can do the work myself, I will.

Any help would be appreciated.
Washburn Dime 333, Ibanez RG7620, LTD MH250NT w/ EMG 89x's
Modded Boss Metal Zone, Boss CE-5, DD-3, Intellifex
Jet City JCA2112RC, ADA MP-1 Preamp,
GK 400RB Bass head
Marshall and Ampeg 4x12 cabs