Arfa Daily wrote:

Had a JTS radio mic receiver on the bench today. "No power", said the job
ticket. With 12 volts going in, the output from the four-legged LDO 8 volt
regulator, was almost nothing. A quick stab around with the ohm-meter
revealed about 1.7 ohms across the output. Nothing obviously short. Nothing
getting hot because the regulator was in a full foldback condition. Loads of
surface mount 4558's in there, as well as a good selection of more exotic
ICs, and the 1.7 ohms could be measured at any of them. I had a quick word
with the shop that it came from, and the guy there was of the opinion that
it would not be worth pursuing even with the manufacturer, as it was well
out of warranty. "He'll just have to buy a new one" he said. That made me
feel bad, as I felt that I had perhaps not pursued it far enough.

On the basis that the job wasn't going anywhere anyway, and time had already
been spent, I decided to get brutal with it, to see if I could make the
short show its face. I turned the power supply down to about 4 volts, and
linked across the regulator. I then turned the supply back on and settled
down to wait. As it turned out, it wasn't for very long ... A cloud of
smoke and sparks shot out of a tiny little surface mount solid tantalum 1uF
cap. There are hundreds of these - well, tens anyway! - all over the board.
It was but a few seconds work with the iron to whip this cap off the board.
The short disappeared with it, so I took my bridging link off the regulator,
and let it go back to working normally with a full 12 volt input. This time,
the output of the regulator was 7.96 volts, and the power LED lit. A quick
tune of the signal generator up to 863 MHz, with a bit of wire in the output
to act as an antenna, and the RF and AF LEDs lit. As a final check, I hooked
it into an amplifier, and got audio from the generator.

Sometimes it pays to persevere ... :-)


I used to use a current limited power supply and a 4.5 digit voltmeter
to track the voltage drop on power rails. You would see larger voltage
drops till you reached the short, and smaller ones after that. I did
this at the factor on boards that cost us $8000 in components to stuff,
so I had to use non destructive testing.


--
You can't have a sense of humor, if you have no sense.