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Dave Platt[_2_] Dave Platt[_2_] is offline
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Default fewer through-hole transistors available

In article ,
Jon Elson wrote:

Analyzing the bias supply for the base, I can't really SEE how it could get
a reverse voltage spike. I certainly thought about this on power-up.
OHHH, wait! Maybe it could be on power DOWN! There is a bias network on
the base with a capacitor to ground, and the supply to the emitters is
connected to a reference supply that might dump VERY quickly when power is
cut off. That could maybe put about -6 V bias on the base when the emitter
supply drains back toward zero. I could probably stick a diode there to
limit the reverse bias.


Probably a good idea, and (assuming that the diode's capacitance when
biased-off isn't a problem) it should be good cheap protection.

Thinking about it... I wonder whether those poor old transistors might
not just have been Plumb Tuckered Out? A friend of mine has told
stories about the early generations of Fairchild transistors - the
mushroom-shaped ones with a plastic case - and said that a lot of them
he'd dealt with had failed over the years as a result of problems with
the encapsulations. Apparently the plastic didn't seal to the leads
very well, allowed oxygen to infiltrate, and the internal wire-bonds
or metallization would oxidize and fail over time.

Possibly this is something which might have affected those old parts
of yours, with the (modest) amount of heat in the "on at idle"
transistors having the effect of accelerating the aging process.