[Coco] Died from old age?

[Gene Heskett]

On Tuesday 23 December 2014 00:54:51 Chad H did opine
And Gene did reply:
> Gene, are you saying that the voltage increase during the power failure
> means I likely have a failing power transistor?  The big one with the
> heat sink I presume.  I never touched the solder joints for that and
> they look really solid underneath the board.  Should I just look up
> the number on it and find another that meets the same voltage , etc?

Matching or exceeding it. There is a very near zero problem in using a 200 
volt, 25 amp rated bug with 100x the gain-bandwidth product in such a 
circuit. Very occasionally, the base trace may need to be cut as close to 
the solder island as possible, and a short wire bridging the cut installed 
that has a ferrite bead on the wire. Used to stop any 300 mhz (and up) 
oscillations the faster bug might do in a poorly laid out circuit. In 66 
years of chasing electrons & making them behave, I only had to do that 
once.  The OEM bug is probably a much slower device, more suitable for the 
output stage of a transistorized car radio.  More speed would be a plus in 
terms of mid frequency bus noise reduction that heavy bypassing is now 
used to absorb.

FWIW, I have encountered several power transistors of that vintage with 
intermittent internal emitter connections since it is a small gauge gold 
wire that may be only ultra-sonically bonded to the chips connection pad 
on top of the transistor die.  You can measure the usual junction voltage 
from base to collector, something that s/b the .55 volt range if your 
meter has a diode checker function.  Leaving the probe on the base 
connection alone move the other probe from the collector (case) to the 
emitter, and you should see a quite similar reading for the base/emitter 
diode.  This is not, due to the external circuitry, as easily checked due 
to paralleling circuit components in the base/emitter circuit unless the 
meter can supply several 10's of milliamps of test current.  Few meters 
will meet that requirement because it represents a huge drain on their 
battery power supply.

This is one of those cases where the de-soldering heat to remove it may 
make it good again, or conversely make it permanently bad, so I would clip 
onto the connections once removed, and rap the case with a spinning 
screwdriver handle, watching for meter needle movement.  Digital VOM's 
will miss that tell tale noise.  The $15 analog meter needle will wiggle 
and tell you instantly.

I was in a shop doing repair and re-certification work for the nuclear 
energy folks last week, and was amazed at the use of 6 to 9 digit, very 
pricy meters on the test benches, meters that take up to 10 seconds to 
settle to a valid reading, and not a single $15 analog meter in the 
considerably large facility was to be found.

Piss-poor accuracy with those, but THEY DON"T MISS such noise. I pointed 
out to the shops owner that such a device had every reason to exist on the 
cubicle shelf, to be used exactly for such noise testing.  I was dismissed 
out of hand.  Everything that comes out of that shop is subjected to a 100 
hour burn-in, with the ambient temps cycled back and forth over the full 
mill-spec temp range at least 5 times, all recorded on a strip chart that 
becomes part of the certification record.  I assume they bill by the 
hour... ;-0

[...]

Cheers, Gene Heskett
-- 
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Genes Web page <http://geneslinuxbox.net:6309/gene>
US V Castleman, SCOTUS, Mar 2014 is grounds for Impeaching SCOTUS