Isolating shorted PCB component ?

[Jim Adney]

HP used to make a hall effect (IIRC) probe for current tracing. One of their
Bench Briefs technotes described the probe and the process.

It was the HP 547A Logic Current Tracer combined with the HP 546A
Logic Pulser. These worked only with TTL or CMOS circuits. The pulser
would drive current into any point you wanted and the tracer could be
used to follow the current path.

You would set the pulser to put out a continuous pulse train, and then
adjust the sensitivity of the tracer so that it would just trigger on
the current pulses sent out by the pulser. It would thus ignore other
current that was flowing in the same trace as long as that current was
either DC or consisted of pulses which were smaller than those put out
by the logic pulser. I think the logic pulser could put out an amp or
so, but they were short enough that they didn't damage anything.

They work well, but the current tracers are rather hard to come by now
and they are rather expensive when you find them.

-

 

[Eddie Haskel]

The 7815 needs only 3 volts of headroom, so 23 volts is plenty. Any more
than that will be dissapated as heat. Your right that the traces will also
get warm but it's still safe. The other post that advised the use of freeze
spray was a good one too. I have used both...Eddie

 

[Roy Lewallen]

There have been quite a few good suggestions, but there's a little I can
add.

I've successfully found shorts between inner layers of multiple-layer PC
boards due to misaligned layers. (When layers are misaligned,
plated-through holes -- vias -- can contact layers they're supposed to
miss, shorting them.) The method I used was to connect a high-amplitude
signal generator (one that puts out a few volts into 50 ohms) of about
50 kHz between the shorted layers. Then, I used a small coil as a
detector, connected to a scope. The coil was a few hundred turns on a
ferrite bobbin (essentially a ferrite rod), about 1/4" diameter and 1"
long. With it, I was able to trace the current from the "hot" signal
generator terminal to the shorted via -- the signal drops off pretty
quickly beyond the short. The via was drilled out to clear the short,
and connections to the intended layers made manually.

If the frequency is too low, the AC current spreads too much on a
plane-type layer. If it's too high, it won't penetrate through layers
which are over the conducting one. If you're not dealing with multiple,
plane-type layers, you could use a higher frequency and it should be
pretty simple.

A toroid, as someone mentioned earlier, is a poor choice for a detector,
unless you grind a slot (gap) in it -- half of a split toroid would work
fine. I tried a floppy drive head, too, but found that it had very poor
sensitivity. They're apparently intended to be driven by high current,
and at quite a bit higher frequency. The resolution would be good,
though, if you put an amplifier between it and the scope. I found the
bobbin coil to be adequate.

I used this method quite a number of times. Knowing how to find
inner-layer shorts does have disadvantages, though. The very first
prototype PC boards for the Tek 11400 series 'scope mainframes arrived
at about 2:00 a.m. on the day they absolutely had to be built, and all
had inner layer shorts. Guess who got called in to fix them. As I
recall, I managed to fix something like two out of the four or five --
just enough for that day's build. The rest had too many shorts to salvage.

The same technique can be used to find shorted components, although some
of the other methods suggested might be easier depending on the
circumstances and type of boards.

Roy Lewallen, W7EL

 

[Ralph Mowery]

The 7815 needs only 3 volts of headroom, so 23 volts is plenty. Any more
than that will be dissapated as heat. Your right that the traces will also
get warm but it's still safe. The other post that advised the use of freeze
spray was a good one too. I have used both...Eddie
came

shorted

I think he was referring to a 15 volt rgeulator that has 23 volts on the
output indicating it was bad and the overvoltage caused other components to
go bad.

 

[budgie]

It was the HP 547A Logic Current Tracer combined with the HP 546A
Logic Pulser. These worked only with TTL or CMOS circuits. The pulser
would drive current into any point you wanted and the tracer could be
used to follow the current path.

That may have been the sensor, but the article I saw related to tracing shorts.
To that extent, it was environment-independent.

(snip)

They work well, but the current tracers are rather hard to come by now
and they are rather expensive when you find them.

Most HP stuff I've seen works well. Being HP, I'm sure they were expensive way
back then.

 

[Henry Kolesnik]

23 voolts on output!
later

 

[Henry Kolesnik]

I found a good 7815 and installed it and except for the frequency dial being
off the thing seems to work. If I find a manual I should be able to cal it
or find out if the cal is off because of a failed componet because of being
overstressed to 23 volts when it was designed to operate at 15 volts. The
output remains flat from 0 to 4 MHz but the when the dial reads 4 MHz the
counter shows 3.5 MHz. At 1MHz on the dial the counter reads 0.570MHz and
similar readings on at audio frequencies.
hank wd5jfr

 

[Ralph Mowery]

23 voolts on output!
later

The way I read it , the 15 volt regulator had gone bad and was passing a
much higher voltage as it was not regulating . That was giving 23 volts on
the output instead of the 15 volts it was suspose to.

 

[Jim Adney]

That may have been the sensor, but the article I saw related to tracing shorts.
To that extent, it was environment-independent.

I'm sure you're right that it would work anywhere that you could use a
+5 to +15V pulse.

Most HP stuff I've seen works well. Being HP, I'm sure they were expensive way
back then.

The Logic probes and pulsers in this series are fairly easy to find
today, but I had to look a long time before I found one of the tracers
at a reasonable price. Yes, they cost more when they were new, but the
tracers were only about 25% more than the probes. Used, they usually
cost at least twice as much as a probe.

-

 

[Henry Kolesnik]

When I got the 188 the 7815 output was nearly zero and I assume the shorted
tantalum caused the low voltage. After I found the bad tantalum and
replaced it the 7815 voltage was still very and the output pin to ground
mesured 12 ohms. I had a NOS Radio Shack 7815 still sealed in its package
so Iused it and that's when pin 3 went to 23 volts and that nearly made a
grown man cry because I thought I fried addtional components. The PCB was
running too hot. After getting a known good 7815 the B+ stayed at 15 volts
and the unit runs cooler and seems to function except that the dial
calibration is off. I'm trying to find a manual.
73
hank wd5jfr

 

[Mike Burch]

Thanks to all of you for posting to this thread. Man did I learn a lot!

Mike Burch K8MB
Apache Junction AZ

 

[dwight elvey]

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