How to find a short on the motherboard

Discussion in 'General Electronics Chat' started by sony85, Aug 26, 2012.

  1. sony85

    Thread Starter Member

    Aug 26, 2012
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    Hi there, I trying to find a short on my motherboard,I dont know which component causes a short, I use a multimeter but it always shows zero since there is a short all over basically when I connect a multimiter to the postivie and the negative end it always gives us zero ohms resistance since it is shorting, but where, that I can't figure out. Any help would be appreciated on what equipment to buy, Thank you Tom
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Your description is somewhat vague and leaves me to make some assumptions.

    By "motherboard" I assume you mean a PC system board.
    By "multimeter" I assume you mean you are measuring resistance.
    By "short all over" I assume you mean measuring across VCC and GND in both directions.

    Those are all a lot of assumptions to make but any how, here goes.

    Finding a dead short across VCC and GND on a complex system board is one of the most difficult repair jobs to perform.

    Generally speaking, it can be a short caused by any one of the many ICs but quite often it is a shorting electrolytic decoupling capacitor. It can also be a short between supply lines. I have seen a short caused by excess solder blob underneath an IC chip that crept under heat and gravity after 25 years of operation.

    To locate a short between supply lines you will need a current tracer. I am aware of only one that was ever made by HP, the HP 547A (and I am not going to let you have mine).

    Without such a current tracer, you have to use the binary search technique where you cut traces on the board and figure out which path the current is taking.

    It might be more expedient to get a new motherboard.
     
  3. sony85

    Thread Starter Member

    Aug 26, 2012
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    The problem comes from my power jack .It is broke ,and when i touch the cable my laptop shut down and i kept doing this, and now it doesn't want to turn on any more .
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The problem may be in the power supply brick. Have you checked its voltage output?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    For a short on a board, the method I have used is to apply a current of perhaps an amp or two through the shorted traces and use a sensitive voltmeter to measure the voltage drop as you move the probe along the traces. The voltage drop will reduce as you approach the point of the short.
     
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  6. sony85

    Thread Starter Member

    Aug 26, 2012
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    I don't have sensitive voltmeter ? What can substitute it ?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    What? :confused:

    If you don't have a sensitive voltmeter you can make one using a simple op amp amplifier to amplify the voltage to the voltmeter. A gain of 100, for example, would increase the 200mV low range full-scale sensitivity on a typical multimeter to 2mV.
     
  8. sony85

    Thread Starter Member

    Aug 26, 2012
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    I am very beginner of this .
    I need more information . Can you send me your e-mail .
     
  9. lightingman

    Senior Member

    Apr 19, 2007
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    You will need a current limited PSU (about an amp or so).

    Spray the PCB with freezer until it has a thin film of ice on it. Switch on and the ice will melt along the tracks to the short.

    Dan.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    That sounds like an interesting idea.

    If you don't have freezer spray you might try placing the board in a freezer at about 0°F for a half hour or so. When you take it out, a layer of frost should form on the board. But you need to work fast before the board warms up. ;)
     
  11. ramancini8

    Member

    Jul 18, 2012
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    Use a current limited supply of about 1A. Either use the freeze technique, or without freezing use a temp probe. With a dead short like you seem to have your finger may be a sensitive enough probe.
     
  12. JMW

    Member

    Nov 21, 2011
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    Follow crutchows advice. Limit the current half the fuse value. Use your finger, the shorted component will get warm. You can always use freeze spray, if you're concerned with ESD
     
  13. dataman19

    Member

    Dec 26, 2009
    136
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    Or...
    You could wire a 14Watt lamp with a 10 ohm 2Watt resistor (the resistor and lamp in series with each other) in parallel with your power supply and another in series with the power supply. This will give a two glowing lamps (one bright and one slightly dimmer) when there is a short, and only one glowing lamp when you isolate the short. The 14Watt bulb and 10-Ohm 2Watt resistor are used to shunt source 1 amps of current (the lamp will glow). When You locate and eliminate the short, then the bulb will go out and the bulb in-line will glow brighter.
    ...
    This will also help you protect your power supply while you troubleshoot.
    ...
    By the way - I would speculate that your laptop power jack is the problem (usually is 90 out of 100 times). Just replace the power jack.
    *** This is assuming that your power supply connector hasn't been crushed or damaged by say a rolling chair wheel? (another source of laptop power issues).
    ..
    Just my 2.5 millicents worth...
    ..
    Dave
    Phoenix, AZ
     
  14. MrHam

    New Member

    Jul 20, 2012
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    4 wire ohm measurement can help you trace resistances differences to the mili-ohm level. you can see the difference between ends of a pc board trace.
     
  15. sony85

    Thread Starter Member

    Aug 26, 2012
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    How can i apply voltage to the laptop motherboard . I got DC power supply ?
     
  16. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Since u dunno how to apply voltage I doubt u can fix it buddy.
    Why don't u find some one who knows the stuff at least a little bit so tht we can help u.

    In order to fix the board, u got to have basic knowledge on trouble shooting such devices.

    A mobo is not for a n00b!
     
  17. sony85

    Thread Starter Member

    Aug 26, 2012
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    Can you teach me ?
     
  18. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Couldn´t that be just a simple broken contact issue?
     
  19. JMW

    Member

    Nov 21, 2011
    88
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    In addition to the troubleshooting, the desoldering/soldering is an art. It takes a lot of practice. This board is a good place to start.
    Surgeons spend years working on cadavers before they work on a patient. While a MoBo is not brain surgery, you will find you need similar tools and skill sets.
    Troubleshooting is detective work, you must know what the equipment does, how it does it, and what it is not currently doing. Electronic/Electrical theory is a must. Test equipment is necessary, and an intimate knowledge of its capabilities is required.
    Start with de soldering components with this board and then try and reattach them without destroying the traces and creating solder bridges.
    The below quote is attributed to an Air Force Master Sergeant from the '60's. "The quality of the technician in the field is directly proportional to the amount of equipment he smoked in A School." As for me, I think it took a decade or so until I broke even. Started out with tubes,.... Even so, reading this and a couple of other similar sites, I'm still learning. Amateur Radio is a good place to start. Get a Technicians License, then build some of the equipment listed in the ARRL handbook.
     
  20. sony85

    Thread Starter Member

    Aug 26, 2012
    43
    0
    I know how to solder very well . I need to learn how to find short on motherboard .
    Can you help me ?

     
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