Valcom 760 Transceiver Troubleshooting

I'm trying to solve a problem with a Valcom 760 that appears
to be related to voltage regulation. The symptoms are that the
radio seems to power up normally, then the display digits fade
away after several seconds. It will neither transmit nor receive
in this state. The outputs of several internal voltage regulator
chips jump up from about 4.5V to about 5.15V at that time.

Having no service manual, I can't troubleshoot the unit any further.
Can anyone out there help? Thanks

David Johnson

David Johnson wrote:

I'm trying to solve a problem with a Valcom 760 that appears
to be related to voltage regulation. The symptoms are that the
radio seems to power up normally, then the display digits fade
away after several seconds. It will neither transmit nor receive
in this state. The outputs of several internal voltage regulator
chips jump up from about 4.5V to about 5.15V at that time.

Having no service manual, I can't troubleshoot the unit any further.
Can anyone out there help? Thanks

David Johnson

Open it up on the bench and power it up. Don't forget the antenna!

Have on hand a can of freon with a tiny spray tube, a heat gun
with the smallest nozzel you can find, and some poster board.

Fold up small pieces of poster board to fit around the area your
are examining, and start the testing.

Spray with freon to chill an area, noting when you find an area
that responds (things start working again). Warm (NOT heat) it
back up so it flakes out again.

Your intent is to narrow the test down to to a single component
that responds to the chill/warm process.

You will then have very likely found the culprit.

Its rude, crude, and Jim will probably flame my shorts for suggesting
it, but the technique is effective.

Having said all that, before going this far, do a through visual
inspection of the circuitry. I'd suspect you may find a leaky
electrolytic capacitor by eye alone.

Best of luck,

Richard

Hi:

I'm aware of the hot/cold technique, and will try it if I can find
some freon in a can (Don't think you can buy it anymore).

I don't think it is a heating problem, as it happens immediately
even when the radio is cold.

No joy on the physical inspection, either - this one looks brand
new inside.

I do suspect a bad cap - but which one? Wish I had a schematic.

BTW - I have been building & fixing radios for 40 years.

Thanks for the suggestions.

Dave Johnson

"David Johnson" wrote in message
om...
Hi:

I'm aware of the hot/cold technique, and will try it if I can find
some freon in a can (Don't think you can buy it anymore).


Dave Johnson

A few possibilities. Air in a can, for cleaning computers, and stuff gets
pretty cold. Janitorial supplies carry stuff that is used for cleaning gum
off carpet, and butane (for refilling lighters) gets damn cold.

For those last two suggestions, having the power off while spraying, may be
a good idea. ;-)
--
Jim in NC


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David Johnson wrote:
Hi:

I'm aware of the hot/cold technique, and will try it if I can find
some freon in a can (Don't think you can buy it anymore).

I don't think it is a heating problem, as it happens immediately
even when the radio is cold.

No joy on the physical inspection, either - this one looks brand
new inside.

I do suspect a bad cap - but which one? Wish I had a schematic.

BTW - I have been building & fixing radios for 40 years.

Thanks for the suggestions.

Dave Johnson

So have you thought about calling Val?

Jerry (Two Valcoms in my RV-6) Springer

On 27 Jun 2004 17:45:04 -0700, (David Johnson) wrote:

I'm trying to solve a problem with a Valcom 760 that appears
to be related to voltage regulation. The symptoms are that the
radio seems to power up normally, then the display digits fade
away after several seconds. It will neither transmit nor receive
in this state. The outputs of several internal voltage regulator
chips jump up from about 4.5V to about 5.15V at that time.

Having no service manual, I can't troubleshoot the unit any further.
Can anyone out there help? Thanks

David Johnson

The regulators are going off load.

Brian W

Update: Found the problem - high resistance contact in the on/off
switch. The radio was trying to run on about half voltage. Not
having documentation made it hard to find.

This type of problem can cause all kinds of weird behavior -
especially in equipment with microprocessors.

David Johnson