"Phil Allison" wrote in message
...

"Arfa Daily"

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 ... :-)


** This is known as the " tune for maximum smoke " method.

Often shows up short circuit bypass ceramics and regular pigtail tantalums
too.

How come you did not have the matching transmitter ??

I always insist on customers including them - more than once I have been
given a mic and receiver pair that do not work because they are on
different frequencies.


.... Phil


This work comes from a music shop. Usually, if it seems to be an RF issue,
they will send both bits to me. In this instance, as the receive unit was
obviously dead, that was all that the owner brought in. They pulled it apart
in the shop, in case there was a fuse to try, but once they saw that there
wasn't, they just shipped it out to me, knowing that I have a decent HP
generator that's good to 2.4 GHz. I do not see any that are a channel
mis-match issue, because that sort of thing is filtered in the shop. The guy
that owns the shop is fairly technical and can deal with testing and
replacing valves. He is able to replace HF drivers, and has been a bass unit
re-coner for years. But he knows his limitations, and anything beyond that,
is just passed out to me.

Oddly, a couple of months ago, I had two radio mic receivers in the space of
a couple of weeks, which both had the demodulator tank mis-tuned by a good
half turn. Neither had been 'got-at' as far as we know, and both remained
stable and correctly tuned over a soak test of several days. Neither has
come bouncing back either, so it's a bit of a mystery as to why they were
like it.

Arfa