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