My newly aquired Quad 303 power amp gets too hot. I have mesured the
bias current and the right channel is too high, about 100 mV across
R124 and it is slowly increasing. I have tried to adjust it with
RV201, but cant get it lower. The left channel is correct at 5 mV. The
strange thing is that the sound do not get distorted (as far as I can
tell). But I´m afraid the amp will go into thermal runaway.

I have tested the transistors and diodes onboard with my multimeter in
diode
mode, but can´t find any failure. I have also swapped the bias
transistor TR 107 between the channels, with no change in the bias
current.

Connected in series to a 60w light bulb I mesured voltage across most
of the components on the driver board and found 12 mV across R121 on
the faulty channel compared to 123 mV on the good channel.


Any help on what to do next in troubleshooting would be appreciated!

Thanks,