On Wednesday, April 2, 2014 6:41:42 AM UTC-4, Arfa Daily wrote:
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

Arfa
As Mr. Cook had asked earlier, did you happen to look at these two that you pulled out forward and reverse with an analog ohm meter such as a 260 on the R X100 or 1K range? I would be very interested to hear of the results of that little experiment if you do. BTW, these aren't germanium are they? Lenny