[TenTec] RE: Corsair II failure

[dslosty]

Date: Mon, 17 Jul 2000 00:42:32 -0700
From: dslosty <dslosty@pipeline.com>
Subject: [TenTec] Corsair II failure

Just had the first failure in my Corsair II.
The digital readout either doesn't come on or comes
on after 5 minutes or so. When the rig is warm and if it is turned
off and then on, the readout will not reappear. All digits are blanked
out,
with the exception of the decimal point with is fed directly by Vcc.
Also the readout will occasionally freeze and not update.

I think I've traced the problem to the MC68705P3 on the logic board.
When the malfunction occurs, the processor's output enable 1 and enable
2
signals vanish. Also the keyer functions stop working. With no enable
signals being sent to the LED driver chips, they obviously stop working.
Thus the processor is probably at fault. All supply votages are normal.

Have others seen this problem? Is a failure of the MC68705P3 common?
Hopefully Ten-Tec still stocks these since the onboard firmware must
be custom.

73,
Doug/WA1TUT

----------

As a continuation of the above, here's what I found:

A call to Ten-Tec was very useful.
Their service manager mentioned that they'd had trouble with one of the
electrolytics near the processor chip. He wasn't sure which one, but the
hint was extremely helpful.

I got out my logic probe and found that the processor chip was keeping
its reset output line high. This reset high was being sent to the input
counter/divider chips and was stopping the enable pulses to the LED
driver chips.

Sure enough, the 2.2 mf electrolytic cap on the processor's reset line 
was bad. Since the only 2.2 mf cap I could find was a non-polarized type
typically for audio crossover networks, I used it. Hey, ya gotta make do
at times! Nevertheless it works fine and the trouble is solved. I may
replace that cap with a standard polarized one, but since the one I 
installed was rated at 50 volts, it shouldn't be a problem.

Thanks to all the folks who emailed me with suggestions.
Their response was appreciated.

Thanks again, Ten-Tec!

73,
Doug/WA1TUT

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