"Arfa Daily" wrote in
:

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



Ive done the same thing on TEK scopes.
sometimes,I paralleled the current limit resistor on a supply with another
R to increase the current output and see what smokes or pops. those glass
axial ceramic caps would pop,the dipped tantalums would smoke.
Sometimes,the scope would begin working,because the current limit was right
at the hairy edge.
I found a series pass XSTR with a bad B-E junction that way.It affected the
current limit point.

--
Jim Yanik
jyanik
at
localnet
dot com