On Tue, 19 May 2009 00:50:13 +0100, "Dave Plowman (News)"
wrote:

Got an 'interesting' fault on an ancient Linsey Hood amp. Switch on a
light etc, and sometimes *one* channel bursts into loud mains hum - but
carries on working. And the output transistors get very hot. It's not
normally susceptible to mains born interference - so any guesses before I
rip it apart?

Both channels seem to match ok before this happens.
Can't remember what the biasing arrangement is, but if the quiescent
current is set with a preset variable resistor, in some circuit
arrangements if the preset goes O.C. the quiescent current goes up
resulting in very hot O.P. transistors and a lot of ripple on the
P.S.rails.

Bill