"Mark Zacharias" wrote in message
...
"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


I do this sometimes, but occasionally:

a.) The short will burn itself open, or

b.) The short is really zero point zero ohms and you start burning foil
runs before any component reveals itself by getting hot.

I might try the current limited method next time, but those very small
voltage differences may be a problem. I don't own such a DC supply though,
I'd have to use an actual resistor.

Might also be a good excuse to buy a non-contact thermometer ;-)

Mark Z.


I actually have a non-contact IR thermometer, and it's just about OK on
biggish items like chips, but nothing like tight enough on its sensing area,
to be able to detect a gnat's bollock cap getting hot !

Arfa