Hartke AH500 on the bench this morning. It struggled to come out of protect
from cold, but if it did make it, then it worked ok, and mostly stayed on
producing clean power of the right order, although the owner had complained
that it had cut out during a gig on more than one occasion.

As the thing powered up there was a small(ish) DC offset of a little under
negative 2 volts at the output stage midpoint, and it was this that the
protect circuit was rightly objecting to. With + / - 90 volts across the
output stage, it doesn't take too much imbalance to produce a 2 volt offset
....

As it warmed up, the amount of offset varied a bit, so I went in with a can
of freezer to see if I could see anything that was particularly sensitive.
That led me back right to the front end where one transistor in a
long-tailed pair had a significant effect when sprayed, the offset rising to
around negative 4 volts. So I pulled both out and checked them on my cheapo
component tester. Both correctly registered as NPN transistors, and the BCE
pinning agreed. Both had 645 mV B-E voltage, but one had a beta of 215, and
the other, just 35.

A new pair of matched transistors had the offset down to a few mV, and the
protect circuit was happy with that. But it got me to wondering what could
be the failure mechanism that resulted in a transistor still being a
recognisable transistor in that the tester still saw it as one, and it still
basically worked in the amp, but had a very low gain ?

Arfa