Arfa Daily wrote:

...one transistor in a
long-tailed pair had a significant effect when sprayed,
[...]
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 ?

Were there any diodes to protect the BE junctions against reverse
breakdown?

In a long tail pair operating from relatively high voltage supplies, it
is quite possible for more than -5vto appear across the BE junctions if
the designer hasn't built in some protection. Sometimes this can occur
during start-up, or under signal overload conditions, or if there is any
tendency to oscillate with certain loads. A 'grid stopper' resistor
will limit the breakdown current but will not prevent the junction from
being gradually destroyed by repeated reverse breakdown.



--
~ Adrian Tuddenham ~
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