[Coco] Died from old age?

[Chad H]

I looked up the part number for that Motorola transistor and  found another CoCo2 user was trying to replace it.  He found that the 2N3055 was a great replacement.  It's even carried at the shack... http://tinyurl.com/nf3xuuy

I will do the cap replacement first I think and let it run from there.  If I have this problem again I will try that transistor replacement.   Thanks!
Sent from my Galaxy S4

-------- Original Message --------
From: Gene Heskett <gheskett at wdtv.com>
Sent: Tuesday, December 23, 2014 05:51 AM
To: coco at maltedmedia.com
Subject: Re: [Coco] Died from old age?

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
--
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>
US V Castleman, SCOTUS, Mar 2014 is grounds for Impeaching SCOTUS

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