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Isolating shorted PCB component ?

Discussion in 'Electronic Repair' started by Henry Kolesnik, Jan 15, 2004.

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  1. 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
     
  2. Andy Cuffe

    Andy Cuffe Guest


    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.
     
  3. Bob Shuman

    Bob Shuman Guest

    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
     
  4. Dave Platt

    Dave Platt Guest

    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.
     
  5. 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
     
  6. 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
     
  7. 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
     
  8. Steve Nosko

    Steve Nosko Guest

    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.
     
  9. Power it up on a variac, spray the board with freeze spray. The shorted components will defrost first!
     
  10. Just change the ones across the B+ line, shouldn't be that many. Ken
     
  11. budgie

    budgie Guest

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

    Uncle Peter Guest


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

    Pete
     
  13. Wild Bill

    Wild Bill Guest

    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
    ...................
     
  14. 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
     
  15. 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
     
  16. Franc Zabkar

    Franc Zabkar Guest

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


    - Franc Zabkar
     
  17. Steve Nosko

    Steve Nosko Guest

    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.
     
  18. Steve Nosko

    Steve Nosko Guest

    Some quick, off hand comments imbedded below:

    Steve N.

    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.

    I don't see a problem. If you get it set up so it detects at sufficient
    distance (.070 or so).
    Should still work, but you'll have to follow the target trace by "braile"
    (sp) since you might not be able to see it.
    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?
    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.
    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
     
  19. Eddie Haskel

    Eddie Haskel Guest

    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
     
  20. 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
     
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