Trademark 60 Amp - Loses volume after a few min.

My "Tech 21" Trademark 60 will lose volume after playing it for a few minutes. Then the volume will come back after a few minutes and then fade out again. This is an analog, no tubes, amp. Could this be a transformer issue? If not, what could it be.

is there a repair guide for these units?

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Sounds like a capacitor issue. Either cold solder joints or overheating caps. Take a look at the capacitors on the logic board see if you find some obvious stuff.

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