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   
      
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