On Thu, 15 Jan 2004 16:54:48 -0500, kenneth l wright
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.

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

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" 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

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" 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

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" wrote in message
m...
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" 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

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
"Henry Kolesnik" wrote in message
...
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" wrote in message
m...
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" 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

"Eddie Haskel" wrote in message
.. .
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
"Henry Kolesnik" wrote in message
...
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

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.

23 voolts on output!
later
"Ralph Mowery" wrote in message
...

"Eddie Haskel" wrote in message
.. .
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
"Henry Kolesnik" wrote in message
...
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

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.

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" wrote in message
...

"Eddie Haskel" wrote in message
.. .
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
"Henry Kolesnik" wrote in message
...
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

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.