[[email protected]]

Just a followup to my original question in case others have the same problem. I managed to localize an intermittent contact somewhere around the 680k resistors (R7, R9) by lightly tapping on every R with a stick while the Quad II was in operation. Replaced both resistors plus the 2.7k (R8) inbetween, and resoldered everything else on the tag board. I am not sure how this fault could produce an oscillating squeal, but now things are stable inside the amp.

Martin