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Mainboard repairs

Discussion in 'Electronic Repair' started by Wayne, Mar 25, 2006.

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  1. Wayne

    Wayne Guest

    Digi-Key has the caps you might need

  2. I have several mainboards of the old slot 1/ socket 370 combo
    type that still take an old style keyboard but can handle both AT
    and ATX power supplies.
    None of these boot up, some act dead, others will cause the PSU
    to jump in overload protection when powered up.
    None give a post screen.
    As i have several older pc's in from a school i would like to find
    a way to revive them.
    No bulged capacitors but considering their age is it possible the
    caps have high esr anyway, causing the board to fail?
    I have no esr meter at hand, any test i can do?
    Where does one buy computer capacitors anyway?

    Bart Bervoets
  3. Art

    Art Guest

    Check the voltage regulator ICs on the M/B you are checking out.Seen a few
    of these just flat short out.
  4. Guest

    Digikey, Mouser, B&D Enterprises are sources I've used. Watch
    diameters since many boards won't accept capacitors even slightly wider
    than the originals. Those companies also have a good assortments of
    schottky diodes and MOSFETs, and check those parts for shorts because
    bad caps can make them overheat.

    If you have a scope, you can test for ESR by connecting the capacitor
    to its square wave calibration output with a resistor Rin in series
    (try about 10K). Ideally the waveform will look like a ramp that
    starts at 0V, but ESR will make it start at a higher voltage.
    Calculate the ESR just as you would calculate a voltage divider. A
    search for "99 cent ESR" will give details.
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