Marshall JCM800 Lead Series screeching with guitar connected

[nevetslab]

Having recently replaced an open Screen Resistor in this Marshall JCM800 Mk 2 100W Lead 2203 Master Vol. Noisy pots, which I sprayed/exercised and had a preamp tube that needed to have its tube socket pins tweaked to make proper contact again. All seemed working fine again. I hadn't plugged my 50 yr old Fender P-Bass into it, just using burst pink noise in checking it out. It came back from our Guitar Dept stating that at high volume levels, it's screeching.

I found the pots once again noisy and exercised them until they were quiet again. It wasn't until I plugged my P-Bass in that I heard the loud squealing with the vol and master turned up high, and only being maybe 10 feet at best from the 15" Hartke shop speaker. I could only stop the squealing by rotating my body and putting me between the bass and speaker. I didn't have another similar JCM800 amp like this to compare to and have requested one being sent over for comparison. I'm not a guitar player, and don't have guitar here in the shop (down the street/around the corner from my shop).

Have I missed something obvious?

[mozz]

D.C. on the pots?

[nevetslab]

D.C. on the pots?

Not that I measured when I was in the first time. Just age-old pots on an old amp, that contact cleaner didn't seem to clear. Exercising the controls tames it after first powering up. NOT related to the squealing problem. Still need to hear another identical amp over here.

[mozz]

I figured maybe there was voltage there and was slowly etching the pot track. I don't know how cap leakage would affect the next stage in terms of squeal. Maybe just a microphonic preamp tube. I guess output tubes go microphonic also but don't know if they squeal or howl.

[Tom Phillips]

Maybe a bad ground in the system or a filter cap that is not doing the "Decoupling" part of it's job.