Meat Plow wrote in message
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On Sun, 15 May 2011 08:34:09 +0100, N_Cook wrote:

Meat Plow wrote in message
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On Sat, 14 May 2011 15:47:18 +0100, N_Cook wrote:

At least these amps have separate LV DC supplies and the biasing part
can be probed out of casing , powered from +/-15V bench ps without
+/-92V around

TO92 60V, 0.6A , 9V on one end of the 47K base resistor and 8mV on
the other. Checks out DVM diode test and no obvious DVM resistance
between pins. Now removed can see ,under x30, slight build up of
corrossion crud where the legs enter the plastic, marginally more
than the layer of corrossion over the rest of the legs. I will let
the owner see the result under pocket magnifier , that and bill might
change his storage arrangements.

Some people just don't realize that electronics of this type really
need to be stored in a climate controlled storage unit.



--
Live Fast Die Young, Leave A Pretty Corpse


The other owner , with the Crest audio amps thought covering with a
plastic sheet , include a bag of silical gel, and leave in a Dutch barn
was sufficient. He never even recharged the silica gel - having no
knowledge of that process.

I wonder why TO92 and not TO220 or TO3 fail in these circumstances. I
suppose the leads corrode and swell and then like concrete spalling due
to rusting of rebar, the plastic splits enough for capilliary water
condensate to migrate up into the die. Something to do with small
diameter leads for TO92?
I will try to remember to split open this TO92 and see if it cleaves
along the corrossion plane.
Different maker to the Crest TO92 so not a batch problem.

I think the corrosion infiltrated the die. Like ice crystals in concrete
as you said. Repeated heat thaw cycles exacerbate the issue. Furthermore,
not having a fresh/sufficient desiccant furthers the process. Why the
TO92 suffers more would only be guessing. May be a batch issue. I know
there are plenty that don't suffer from extreme conditions. But they may
be saved from frequent use.



--
Live Fast Die Young, Leave A Pretty Corpse


Different manuafacturer and series of TO92 transistors rather than different
amp maker , so less likely a batch problem. In TO92, TO126 and TO220 the
encapsulation all goes down to the die IIRC. Perhaps corrossion on small
diameter leads has a better forcing/splitting effect into the plastic, then
capilliary. Continue the bad environment long enough and the other classes
of encapsulation would start failing also is my guess.
Further back in thread 80mV not 8mV on the base