"Arfa Daily" wrote in message
...
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 ... :-)

Arfa



Behringer 24 channel M8000 Eurodesks used to regularly come in with one of
the 100's (if not 1000's) of 17v rail bypass capacitors shorted. I used to
stick 5 amps from a bench supply down the offending rail, and it would
disappear in a puff of smoke within 10 seconds.

Although not strictly a pro repair, I always told the customer what I was
going to do, how much it would cost, and how much it would cost if I had to
dismantle the whole desk instead and conduct a proper search.
Not one chose the dismantling route, funnily enough.



Gareth.