Isolating shorted PCB component ?

[Henry Kolesnik]

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

 

[Andy Cuffe]

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
baltimora@psu.edu

 

[Bob Shuman]

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" <wd5jfr@oklahoma.net> wrote in message
news:100dsk7hvh9hi0f@corp.supernews.com...

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

 

[Dave Platt]

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 <dplatt@radagast.org> 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!

 

[kenneth l wright]

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]

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" <wklw@cox.net> wrote in message
news:40070C28.FF6C97B3@cox.net...

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

 

[Michael A. Terrell]

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 4˝ 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

 

[Steve Nosko]

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.



I'd probably try an audio sig gen to get an AC current and an old tape
recorder magnetic head...and an audio amp...or let me see. What else is in
the basement that can pick up a mag field. Hey! I have a stash of Hall
sensors... Probably just about any high micro-henry or mili-henry inductor
will do. I'll bet even a toroid will be a good "close-in" sensor. If it is
laid right on the runner with the current, it'll probably pick it up. Seems
like it would have a short range therefore accurate locating ability.

I was recently playing with a Simpson 60Hz AC current probe (5-100 amps)
(made for a 260) and compared it with a small toroid I had lying about. To
have something to sense other than 60Hz. I took the output leads of a 600
ohm AF generator and ran one through the toroid then just clipped them
together (shorted output generator).and was able to sense it with the toroid
(about 1/2 inch dia. about 40 turns on it). Using an o-scope. The higher
freqs were better.



However, this has the current going through the toroid center. the best
case. Have to see what I can induce with he wire right up against the
outside.



--
Steve N, K,9;d, c. i My email has no u's.


"Henry Kolesnik" <wd5jfr@oklahoma.net> wrote in message
news:100dsk7hvh9hi0f@corp.supernews.com...

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

 

[Robert Mozeleski]

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

"Henry Kolesnik" <wd5jfr@oklahoma.net> wrote in message news:100dsk7hvh9hi0f@corp.supernews.com...

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

 

[kenneth l wright]

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" <wklw@cox.net> wrote in message
news:40070C28.FF6C97B3@cox.net...
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

 

[budgie]

On Thu, 15 Jan 2004 16:44:35 -0600, "Steve Nosko"
<suteuve.nosukowicuz@moutouroula.com> 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.

 

[Uncle Peter]

"Henry Kolesnik" <wd5jfr@oklahoma.net> wrote in message
news:100e3puk17cm2fe@corp.supernews.com...

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

The tantalums in WaveTeks were the weak links. I was continually chasing
shorted tantalum bypasses when I owned two WaveTek 3000 signal
generators.

Pete

 

[Wild Bill]

I haven't tried one, but floppy disk drive heads are sensitive. I was
reminded of a circuit in a magazine (many years ago) that used a floppy head
for a pickup.

WB
...................

"Steve Nosko" <suteuve.nosukowicuz@moutouroula.com> wrote in message
news:bu757r$gmo$1@newshost.mot.com...

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.



I'd probably try an audio sig gen to get an AC current and an old tape
recorder magnetic head...and an audio amp...or let me see. What else is
in
the basement that can pick up a mag field. Hey! I have a stash of Hall
sensors... Probably just about any high micro-henry or mili-henry
inductor
will do. I'll bet even a toroid will be a good "close-in" sensor. If it
is
laid right on the runner with the current, it'll probably pick it up.
Seems
like it would have a short range therefore accurate locating ability.

I was recently playing with a Simpson 60Hz AC current probe (5-100
amps)
(made for a 260) and compared it with a small toroid I had lying about.
To
have something to sense other than 60Hz. I took the output leads of a 600
ohm AF generator and ran one through the toroid then just clipped them
together (shorted output generator).and was able to sense it with the
toroid
(about 1/2 inch dia. about 40 turns on it). Using an o-scope. The higher
freqs were better.



However, this has the current going through the toroid center. the
best
case. Have to see what I can induce with he wire right up against the
outside.



--
Steve N, K,9;d, c. i My email has no u's.


"Henry Kolesnik" <wd5jfr@oklahoma.net> wrote in message
news:100dsk7hvh9hi0f@corp.supernews.com...
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

 

[Michael A. Terrell]

Wild Bill wrote:

I haven't tried one, but floppy disk drive heads are sensitive. I was
reminded of a circuit in a magazine (many years ago) that used a floppy head
for a pickup.

Some questions. How are you going to position it against the trace
accurately? What about double sided, or multilayer boards?

The problems with using AC to find shorts is that you get false peaks
and dips from the inductance of the traces, and the characteristics of
the components. Another problem is that some parts self destruct with
only a small reverse voltage so you can damage a lot of parts while
troubleshooting the board.

I used the DC voltage drop & sensitive digital meter method on boards
that people couldn't fix with AC, then had to find the parts they
damaged. The whole idea is to find and fix a problem quickly, and
reliably.

--
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

 

[Henry Kolesnik]

I found the bad tant cap, it was a 22 ufd, 20 volt, not any AVX but a
yellowish tan blob with a big L. I replaced it with an electrolytic. I
found it using my Fluke 87 that measures to the nearest tenth of an ohm and
only the suspect cap would flicker between 0.3 and 0.2 ohms, all the rest
were 0.4 or 0.3. Now I'm kicking myself for not buying an HP meter that
could read 1/100ths maybe 1/1000ths because I could see no use for it. Now
I can see a use and one is on my list but nevertheless my Fluke saved a lot
of desoldering. The Wavetek still doesn't work as something else is
keeping the voltage low and I think it might be a regulator. Now I wish I
could find a schematic for the Wavetek 188-S-1257, as it would keep me from
wasting so much time.
Thanks for all the tips. guys.
73
hank wd5jfr
"Henry Kolesnik" <wd5jfr@oklahoma.net> wrote in message
news:100dsk7hvh9hi0f@corp.supernews.com...

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

 

[Franc Zabkar]

On Thu, 15 Jan 2004 16:54:48 -0500, kenneth l wright <wklw@cox.net>
put finger to keyboard and composed:

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

I agree. I saw a *lot* of shorted tantalums when I was troubleshooting
multilayer minicomputer PCBs during the 80s.

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

- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

 

[Steve Nosko]

Well, I tried the toroid and absolutely zip when trying to tightly couple to
the outside. They really are well contained fields. It did sense 60KHz
well when through the center (It appeared to be resonant since it also
picked up capacitively).

Hafta go looking for that mag tape head. I'll bet any ole' multi-turn
non-toroid coil could work. Like those used in tube rigs ot older solid
state. No shield. 1/8 or 1/2 diameter.


--
Steve N, K,9;d, c. i My email has no u's.


"Steve Nosko" <suteuve.nosukowicuz@moutouroula.com> wrote in message
news:bu757r$gmo$1@newshost.mot.com...

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.



I'd probably try an audio sig gen to get an AC current and an old tape
recorder magnetic head...and an audio amp...or let me see. What else is
in
the basement that can pick up a mag field. Hey! I have a stash of Hall
sensors... Probably just about any high micro-henry or mili-henry
inductor
will do. I'll bet even a toroid will be a good "close-in" sensor. If it
is
laid right on the runner with the current, it'll probably pick it up.
Seems
like it would have a short range therefore accurate locating ability.

I was recently playing with a Simpson 60Hz AC current probe (5-100
amps)
(made for a 260) and compared it with a small toroid I had lying about.
To
have something to sense other than 60Hz. I took the output leads of a 600
ohm AF generator and ran one through the toroid then just clipped them
together (shorted output generator).and was able to sense it with the
toroid
(about 1/2 inch dia. about 40 turns on it). Using an o-scope. The higher
freqs were better.



However, this has the current going through the toroid center. the
best
case. Have to see what I can induce with he wire right up against the
outside.



--
Steve N, K,9;d, c. i My email has no u's.


"Henry Kolesnik" <wd5jfr@oklahoma.net> wrote in message
news:100dsk7hvh9hi0f@corp.supernews.com...
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

 

[Steve Nosko]

Some quick, off hand comments imbedded below:

Steve N.

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:4007F428.136C22A5@earthlink.net...

Some questions. How are you going to position it against the trace
accurately?

First you do some controlled experiments where you put the test signal
through known runners to characterize the given pickup. How to orientate it
& where it picks up from.

What about double sided,

I don't see a problem. If you get it set up so it detects at sufficient
distance (.070 or so).

or multilayer boards?

Should still work, but you'll have to follow the target trace by "braile"
(sp) since you might not be able to see it.

The problems with using AC to find shorts is that you get false peaks
and dips from the inductance of the traces, and the characteristics of
the components.

At audio this can't be a problem. I'm convinced it will just be current
defined by the generator.

With a runner short you won't have much current in anything but the
runners anyway, no?

Another problem is that some parts self destruct with
only a small reverse voltage so you can damage a lot of parts while
troubleshooting the board.

Also, there won't be much voltage (remember the DC method?). If you do
have this, you're in risk of burning up runners. That's too much current.

I used the DC voltage drop & sensitive digital meter method on boards
that people couldn't fix with AC, then had to find the parts they
damaged. The whole idea is to find and fix a problem quickly, and
reliably.

Hard to believe a 600 ohm audio gen will blow up anything, but it certainly
is possible... of course you just don't go in there blazing away with
power..

I don't mean to say the DC method is bad, or that AC is better. AC is
just another option, especially if you don't have a good enough DVM. Seems
to me I saw a commercial system which did use AC.

My Fluke has problems under an ohm and at fractions of a volt even though it
is a 5 digit. Should be good down there. May try it to see how DC works
just for info.

73

--
Steve N, K,9;d, c. i My email has no u's.

 

[Eddie Haskel]

Henry, dont laugh at this method..it works....read it thru before nixing it.
I have had this happen before too. simply put about 5 volts at 500 mils on
the b+ line, regulated at the 500 mils. Let it sit for a few minutes and
then go looking for the part to be running warm. The part will be
dissipating 5*.5= 2.5 watts of heat. sooner or later the bad part WILL get
warm. It will NOT lift traces unless they are VERY small.
If this approach fails, the next thing I do is go in with a new(sharp)
razorblade and start as far away from the power supply and cut B+ traces one
at a time until the short goes away. This isolates the short to a smaller
area.
Suspect Tantalum caps as they usually fail in this mode of low ohms
shorts...let us know when you find it...Eddie

"Henry Kolesnik" <wd5jfr@oklahoma.net> wrote in message
news:100dsk7hvh9hi0f@corp.supernews.com...

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]

See my previous post about finding the culprit but I had another problem and
it was the 7815 3 term reg which I replaced with NOS and the Wavetek came to
life but not great. I measured the 15 volt terminal and it was 23 volts so
I may not have much left. I have question on your method. For a shorted
component with very low resistance, 0.2 I can't see much heat created since
P=I squared R. A short of 0.2 ohms wouldn't dissapate much power and all
you'll do is heat the traces, other components and perhaps blow short if
you're lucky!
73
hank wd5jfr
"Eddie Haskel" <wiseguy@clever.com> wrote in message
news:7h%Nb.2084$eJ7.1586@newssvr29.news.prodigy.com...

Henry, dont laugh at this method..it works....read it thru before nixing
it.
I have had this happen before too. simply put about 5 volts at 500 mils on
the b+ line, regulated at the 500 mils. Let it sit for a few minutes and
then go looking for the part to be running warm. The part will be
dissipating 5*.5= 2.5 watts of heat. sooner or later the bad part WILL get
warm. It will NOT lift traces unless they are VERY small.
If this approach fails, the next thing I do is go in with a new(sharp)
razorblade and start as far away from the power supply and cut B+ traces
one
at a time until the short goes away. This isolates the short to a smaller
area.
Suspect Tantalum caps as they usually fail in this mode of low ohms
shorts...let us know when you find it...Eddie

"Henry Kolesnik" <wd5jfr@oklahoma.net> wrote in message
news:100dsk7hvh9hi0f@corp.supernews.com...
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