I have a mid-80's Marshal 5210.  Its one of Marshall's first attempts at a Solid State Amp. It has two channels, clean and hot which can be combined and a pretty nice Accutronic reverb.  I am having an issue with the "Hot" channel.  When ever I turn the "Gain" (VR-4) up  to 8 or 9 (out of 10), it will begin to squeal.  It is more prone to this when a guitar is plugged in, but it still will squeal without any signal input.  The pitch of the squeal will change with the "Volume" knob and the "EQ" knobs for that channel.
This is a completly Solid State Amp, No-Tubes.

I believe that it may be a Capacitor going bad, but I really don't want to change out a bunch of caps out just guessing.

Any ideas on which Cap and/or what you think it might be?

The Clean Channel is working just fine and the Hot Channel sounds pretty good if I back the "Gain" down just a little (to 7 or less).


Attached is a copy of the 5210 Schematic.


Thanks!
GD