If you aren't getting the "Hisssss" that comes from loss of signal, yet it loses audio, then it sounds like the dreaded faulty Op-amp problem that is (apparently) common (?) on the factory ones. It's a straightforward replacement.

Hmm. Which op-amp? Don't know. Patrick does, I hope he chimes in on this one.
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One of the few remaining Mk1 owners... #00015