"David Platt" wrote in message
...
In article ,
Maynard A. Philbrook Jr. wrote:

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

zenering the emitter will over time, kill Beta.

It might be worth considering the schematic, and seeing what the amp's
power-up and power-down behavior is like. Depending on how the rails
bounce around, and on what's plugged into the input, the input side of
the differential pair might end up being zenered in this way.

Adding some B-E clamp diodes might not be a bad idea.


OK. All good stuff. There appears to be neither clamp diodes nor current
limiter Rs in the bases. That said, it's quite an old design, and this amp
is quite an old example that's been working ok to date, and is working ok
again now, so I'm reluctant to start modifying it by adding R into the
circuit, but reverse diodes across the B-E junctions might be worth doing.

Cheers all for the insights and suggestions.

Arfa