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Thread: IBM PC XT 5160 -12v circuit board shorted to ground

  1. Default IBM PC XT 5160 -12v circuit board shorted to ground

    IBM PC XT 5160 -12v circuit board shorted to ground
    Power supply works but stops when plugged into system board. Using voltage meter figured out -12v circuit board shorted to ground.
    Anybody know a solution?
    Thanks,
    organizethisorganizethat
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  2. #2
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    Dead tantalum cap on the board. Very common issue. Find the dead one and replace it, though I'd suggest just replacing them all.

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  3. #3

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    You need a bench power supply that can put out 10 to 15 amps, regulated.
    You need a meter that can show micro volts.
    You need some clip point to connect to the board.
    You need a couple pieces of string and maybe a little blue tape.
    The supply should be set so that the voltage limit ( no load ) is less then 0.5v.
    Find a ground point on each of the 4 corners of the board.
    I like to solder a small piece of wire to these points.
    Connect the supply across two diagonal corners so that the current flows through the ground plane.
    That is right, not between ground and -12v line.
    Put one lead of the meter on the -12v line. With the other lead, measure points across between the diagonal points
    that the supply is connected to. You are looking for a measurement of 0 volts. It will usually form a curved arc.
    Lay the string along this line of 0 volts.
    Now move the supply leads to the other diagonal corners.
    Again probe for this 0 volts.
    Where this 0 volt line is, is the point that the two are shorted.
    I've used a scope instead of a meter but it is harder with a ground plane than a trace because the voltage
    drop across the power plain is quite small.
    If you can locate the -12v line, you can force the current along it instead of the ground plane.
    This is likely a trace and may branch. If the short is on the branch you may need to move one force lead
    to the end of the branch. You might wand to reduce the current as well, to about 5 amperes.
    What ever you use to measure with, make sure you can measure some voltage across from attachment point
    to attachment points. This is why you need a sensitive meter. Some galvanometers might work as well.
    An alternate way is to force the current through the short and look for something hot or smoking.
    This has some disadvantages. You may do more damage or blow the sort out without locating
    the failing component. Often it is a tantalum capacitor but if an IC, locating a failed IC is often harder
    than locating a short. This is why I recommend the voltage drop method first. It is a little harder but
    doesn't depend on forcing current through the shorted device, with its higher risk of damage.
    Dwight

  4. #4

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    Way overkill...

    Try this:

    http://minuszerodegrees.net/5150_516...iag_config.htm

    But you pretty much already know it's the tant on the -12V line ( C58 ).
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