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ATX power supply -12V

Discussion in 'Electronic Repair' started by Chris, Oct 13, 2004.

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  1. Chris

    Chris Guest

    I have a Pentium III PC, it has an Abit SA6R mobo.
    The old power supply (250W) was bad, the PC wouldn't
    switch on instantly until after some uncertain warm up period.
    I bought this new power supply (300W) to replace
    the old one. Now it switches on instantly. But the PC
    still resets itself for no reason during cold boot.
    However, once it's warm up, it runs fine.
    Is the mobo or the CPU also bad?
    The new power supply -12V line value is from -12.85V to -13.05V.
    The Hardware Doctor software sets the lower limit for -12V line
    to -13.01V, it often sends warning that the -12V line
    is out of limit. Is the -12V line's value too high?
    Should I try another new power supply?

    Thanks in advance!
  2. Soren Kuula

    Soren Kuula Guest

    If the MB showed the same symptoms on two different PSUs then the fault
    is probably somewhere else.

    You may want to measure the -12v line with a decent voltmeter, just to
    make sure.

    If it were my machine, I would try to strip it down to the simplest
    possible configuration (sound cards, NICs, USB cards, CD drives, all but
    one hard disk and one RAM block .. out), then see if symptoms are still
    there. If still there, try a different RAM block. If still there,
    downclock (well you might as well begin with that, thinking of it..). If
    nothing helps, suspect MoBo or CPU.

  3. Does anything use -12V theses days? Even RS232 transceivers tend to
    have charge pumps and make their own power from 5V. The -12V output is
    probably not well regulated in the supply because it doesn't really have
    to be.

    BTW One cheap AT supply I turned into a bench power supply didn't even
    have a current limit on the -12V line. A short blew every semiconductor
    from the -12V output back to the line fuse.
  4. The -12 VDC is not likely to be the cause of problems.

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  5. Chris

    Chris Guest

    Thank you all for the helps!
    Here is some more info.
    1) SDRAMs (Crucial) were tested by Microsoft Windows Memory Diagnostic.
    They are good.
    2) Hard drive (30G Western Digital) was tested by tools from WD.
    It is good.
    3) Video card (Asus) was tested by swapping to another known good card.
    It is good. (same problem occurred on the good card.)
    4) The cold boot is ok when under the non-ACPI-aware OS like the DOS Prompt.
    5) The cold boot will reset itself under the Win98SE and Win2k.
    It does this several times, then runs fine after it warms up.
  6. Franc Zabkar

    Franc Zabkar Guest

    Lab cards?

    - Franc Zabkar
  7. Franc Zabkar

    Franc Zabkar Guest

    Check the electrolytic capacitors on the motherboard, particularly the
    ones in the Vcore power supply.

    - Franc Zabkar
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