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January 21st 04, 03:45 AM

(Dave Platt) wrote in message ...
Sometime ago I think I recall someone posted or wrote an article on a neat
way to isolate a shorted component on a pcb using common test eqpt but I
can't recall the methodology. I'm trying to find a shorted component on a
Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms and
I'm not having much luck disconnecting componets. I don't have a schematic
and my eyes ain't what they used to be for tracing and I want to minimize
the unsoldering. Does anyone recall the article or have a good way?

Bob Pease gives a schematic for a short-circuit detector on page 21 of
his "Troubleshooting Analog Circuits". It uses an LM10 op amp and an
LM331 voltage-to-frequency converter, plus one transistor and some
passives. You feed some low-voltage, current-limited power into one
end of the shorted trace, slide the probe along the PCB trace starting
from the power injection point, and listen to the tone. When you go
along portions of the trace which aren't carrying the short-circuit
current to ground, the tone remains stable. When you go along
portions which _are_ carrying short-circuit current to ground, the
tone rises (lower voltage present on the trace) as you move towards
the short, and falls as you move away from it. When you pass the
shorted point, the tone rises to its highest frequency and stays
there.

Pease points out that you can use this same basic technique with
nothing more than a current-limited voltage supply and a sensitive
voltmeter (VTVM or FET-input DVM)... but that listening to tones is a
good deal easier.

At .4 ohms, if you feed in 100 milliamps you'll get around 40
millivolts at the injection point, falling to zero at the point of the
short. A good 3.5-digit voltmeter with a 2-volt scale ought to give
you enough resolution to get quite close to where the short circuit is.

Hi Dave
I find it works better if you don't run power through the short.
I run the supply along one of the traces. I then place one lead
of the DVM on the trace that it is shorted to. I run the other lead
along the trace with the power supply and look for a null. This method,
with a slight variation even works for power plane shorts. I go from
opposite corners and lay a piece of string along the null lines.
Where they cross is where the short is.
Later
Dwight