[Coco] Died from old age?

[Chad H]

After further 'burn in', it started dying again. At first I could turn it off for a few minutes and it would then power back up and be OK for a few minutes. Finally it wouldn't do anything at all. The most run time I was able to get was about 15-20 minutes. I decided as a temporary fix to put the 63v 2200uf cap in place of the questionable 6.3v 4400uf. During that process I noticed a loose solder joint in my ground lead off the transformer!! After refluxing and repairing that it is running again...original caps. To test if this might have been the problem all along, I intentionally left that wire disconnected and powered on the CoCo and got the the power failure symptom s I was previously experiencing: fan speed ramped up but no relay click or other activity. After reconnecting and letting it 'burn-in', its now going over an hour without issues.

The question remains though... Should I go ahead and replace the cap that showed the crazy ESR reading?

On a side noite... in reasearching the power supply components it looks like everything could be found easily enough. Even the SALT chip (77527P) which seems to be a Tandy-only combo chip providing both power regulation and some control logic for serial and cassette motor control was found in stock... http://tinyurl.com/o8l63nh

Sent from my Transformer Infinity

-------- Original Message --------
From: Chad H <chadbh74 at hotmail.com>
Sent: Tuesday, December 23, 2014 11:19 AM
To: coco at maltedmedia.com
Subject: Re: [Coco] Died from old age?

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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