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

Henry Kolesnik wrote:

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've used an ESR meter to find shorts on circuit boards. The have
enough resolution below an ohm to tell when you're getting closer to the
short.
--
Andy Cuffe

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

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.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Change all those little tear drop bypass caps connected to the shorted line.
Won't hurt to replace them all, might save a future short. Ken

Henry Kolesnik wrote:

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

Right now it looks like 2 might be the problem marked AVX 103 and AVX 849.
I'm guessing that they're tantalums but not sure. The Wavetake was mfg in
1989. I need help on the values and voltages. There's too many to just
start changing.
tnx
hank
"kenneth l wright" wrote in message
...
Change all those little tear drop bypass caps connected to the shorted
line.
Won't hurt to replace them all, might save a future short. Ken

Henry Kolesnik wrote:

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

Henry Kolesnik wrote:

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

Use an adjustable DC power supply to feed the shorted power rail. Use
16 gauge, or larger wire to minimize the voltage drop. Make sure to
connect the meter negative to ground at the same point you connect the
adjustable power supply to the bad board. Set the adjustable DC power
supply to about a half amp, and use a DC voltmeter to read the voltage
drops across the traces. You will find a point where they level off.
Back up one part to the last linear voltage drop and you have found your
bad part. I prefer to use a 4½ digit voltmeter, or better to read minor
variations. Also, check the voltage on the ground buss if the board
isn't bolted to a chassis to find which part of the board has the
problem. I have fixed hundreds of shorted boards this way.

--
We now return you to our normally scheduled programming.

Take a look at this little cutie! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida

Power it up on a variac, spray the board with freeze spray. The shorted components will defrost first!

"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

Just change the ones across the B+ line, shouldn't be that many. Ken

Henry Kolesnik wrote:

Right now it looks like 2 might be the problem marked AVX 103 and AVX 849.
I'm guessing that they're tantalums but not sure. The Wavetake was mfg in
1989. I need help on the values and voltages. There's too many to just
start changing.
tnx
hank
"kenneth l wright" wrote in message
...
Change all those little tear drop bypass caps connected to the shorted
line.
Won't hurt to replace them all, might save a future short. Ken

Henry Kolesnik wrote:

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

On Thu, 15 Jan 2004 16:44:35 -0600, "Steve Nosko"
wrote:

Seems to me there's a way to use a moderate current and sense the
_magnetic_ field. Follow the field around the board along the runners.
However, I don't remember what was used to sense the field.

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