When I worked as a circuit test engineer that produced fairly complex
multi-layer PCBs many years ago, there were three primary methods we used to
find shorts across the power rails.

1. Use a calibrated micro-ohm meter, fixing one lead to the PCB ground and
taking various resistance readings by moving the other lead from the board
edge connector along the traces to locate the point where the meter provided
the lowest resistance reading. You could then fix the lead to that point
and then take readings by moving the other (previously fixed) lead along the
grounds to again find the minimal reading. This usually led you to the area
of the board where the short or defective component was located. If you had
sensitive enough equipment and some good test leads, this procedure usually
worked pretty well when there was an actual hard short.

2. Apply a current limited, voltage limited (lower than the nominal design
voltage, for instance 5V DC) power source across the power rail at the PCB
edge connector. Start with a fairly low current limit and increase this as
needed, but keeping the current reasonable (you don't want a defective
component to explode - been there done that). We then used either a thermal
sensitive plastic sheet (material it contained was like the stuff used in
"mood rings" from the 1970's) or an infra-red camera to find the "hot spot"
on the PCB. This technique had the advantage of working for soft shorts,
such as were created by defective components (transistors, other
semiconductors, ICs, capacitors, etc.) or even resistive type shorts that
were created by contamination from foreign materials (conductive growths,
moisture, salt water contamination, etc.) We were even able to "see"
internal shorts on 8 layer circuit boards using the camera.

3. Visual observation and path tracing coupled with selective unsoldering of
legs of suspect components (assumes through hole mounting, not surface
mounting) and use of isolation rings (small plastic rings that slid over the
unsoldered leg of an IC isolating it from the multi-layer solder
pad/circuit. This technique was generally used as a last resort and usually
in combination with procedures 1 and 2 above, prior to scrapping
"difficult", but costly product that had been diagnosed with a power rail
short.

The above techniques are what I used about 20+ years back when I had
engineering responsibility in a large electronics factory. I'd imagine that
there are likely better approaches today due to improvements in technology
so would be interested to hear what others recommend.

Good luck!

Bob

"Henry Kolesnik" 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?
tnx
hank wd5jfr

I had good success with a variant of #2 on a very large digital board that had
a short between the +5V and ground -- both of which were planes due to the
high current and multiple loads.

I energized the +5V bus with a supply set for 1 V and current limited to 10 A.
Then went around the board measuring between adjacent +5 and ground points
[like across a bypass cap or an IC]. Where I found the lowest drop was the
short.

Dr. G.



In article , "Bob Shuman"
wrote:
When I worked as a circuit test engineer that produced fairly complex
multi-layer PCBs many years ago, there were three primary methods we used to
find shorts across the power rails.

1. Use a calibrated micro-ohm meter, fixing one lead to the PCB ground and
taking various resistance readings by moving the other lead from the board
edge connector along the traces to locate the point where the meter provided
the lowest resistance reading. You could then fix the lead to that point
and then take readings by moving the other (previously fixed) lead along the
grounds to again find the minimal reading. This usually led you to the area
of the board where the short or defective component was located. If you had
sensitive enough equipment and some good test leads, this procedure usually
worked pretty well when there was an actual hard short.

2. Apply a current limited, voltage limited (lower than the nominal design
voltage, for instance 5V DC) power source across the power rail at the PCB
edge connector. Start with a fairly low current limit and increase this as
needed, but keeping the current reasonable (you don't want a defective
component to explode - been there done that). We then used either a thermal
sensitive plastic sheet (material it contained was like the stuff used in
"mood rings" from the 1970's) or an infra-red camera to find the "hot spot"
on the PCB. This technique had the advantage of working for soft shorts,
such as were created by defective components (transistors, other
semiconductors, ICs, capacitors, etc.) or even resistive type shorts that
were created by contamination from foreign materials (conductive growths,
moisture, salt water contamination, etc.) We were even able to "see"
internal shorts on 8 layer circuit boards using the camera.

3. Visual observation and path tracing coupled with selective unsoldering of
legs of suspect components (assumes through hole mounting, not surface
mounting) and use of isolation rings (small plastic rings that slid over the
unsoldered leg of an IC isolating it from the multi-layer solder
pad/circuit. This technique was generally used as a last resort and usually
in combination with procedures 1 and 2 above, prior to scrapping
"difficult", but costly product that had been diagnosed with a power rail
short.

The above techniques are what I used about 20+ years back when I had
engineering responsibility in a large electronics factory. I'd imagine that
there are likely better approaches today due to improvements in technology
so would be interested to hear what others recommend.

Good luck!

Bob

"Henry Kolesnik" 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?
tnx
hank wd5jfr