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It's the power supply...or is it? >:)

Discussion in 'PC Hardware' started by Norm Loman, Feb 17, 2009.

  1. Norm Loman

    Norm Loman Guest

    I had this computer that won't turn on. No POST. No fans. Everything
    was plugged in. I pushed the power button and its totally dead. The
    only sign of life was the green light on the motherboard still lit. I
    took a multimeter and did a continuity check on the wire going from
    the on switch to the motherboard, and everything looked fine. So I
    narrowed it down to a problem with the motherboard or a problem with
    the power supply.

    Luckily I had another power supply hanging around. I did a quick swap,
    and to my amazement, the computer turned on fine. I thought I had
    fixed it.

    Well an hour later I come back to the computer room. The computer had
    shut itself off while I was away. Now I have the exact same problem as
    I did before. I push the power button and nothing happens. No POST. No
    fans. The green light on the motherboard is still lit.

    I'm just a little unclear on what this means. If a new power supply
    didn't fix the problem, I'd assume the mother board was just bad. But
    how come replacing the power supply got the computer to come back to
    life (if only temporary). What is your opinion? Is this a power supply
    problem, or a motherboard problem?

    Also, does anyone really know what that little green light on the
    motherboard is for? I've always wondered.

    Thank you in advance to anyone who answers.
    Norm Loman, Feb 17, 2009
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  2. Norm Loman

    Arno Wagner Guest

    Well, it may still be both. If you know the replacement PSU is good,
    then it is the mainboard. However if the replacement is an ElCheapo
    thing (that you should not use anyways), a DOA or near-DOA PSU is
    a real possibility.
    Typically showing that +5Vsb (that standby voltage) is present to
    keep you from changiong components and the like, because you
    may kill something that way. It basically says "power present, keep

    Arno Wagner, Feb 17, 2009
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  3. I had a similar problem myself, where the reason was the insufficient
    contact between the CPU and the heat sink, due to the broken heat sink
    holder on the CPU socket. Some (newer) CPUs shut down themselves, as
    soon as their internal temparature reaches some limit. This can happen
    before even the BIOS has a chance to output anything on the screen, or
    at a later time.

    Hans-Peter Diettrich, Feb 17, 2009
  4. Norm Loman

    Arno Wagner Guest

    This is a possibility and I agree that it can happen fast enough
    to cause the symptoms described. However for the successfull
    start observed, adequate cooling would be necessary. Basically
    the only way to check this is to remove the mainboard and have a
    very close look at the cooler. Maybe even remove it and remount
    with new thermal compound.

    Arno Wagner, Feb 17, 2009
  5. Norm Loman

    w_tom Guest

    Nobody has answered because, well, in less than two minutes, numbers
    from a 3.5 digit multimeter would answer everything the first time.

    First, a power supply is only one component of the power supply
    system. Is some part of that system defective? Again, disconnect
    nothing. Take numbers. Have an answer in minutes.

    Second, normal is a defective supply powering a computer. Too many
    swap supplies, see a computer work, then assume the problem is
    solved. You have simply demonstrated why part swapping is
    problematic. A defective supply that boots a computer can only be
    identified using the meter.

    Critical are voltage numbers from the green, gray, and purple wire
    (from power supply to motherboard) both before and when a power switch
    is pressed. Those numbers obtained in 30 seconds would have answered
    what others suggested in 'check this or check that'. 30 seconds
    answers that and much more as long as parts are not swapped.

    Purple wire (also called +5VSB) must be greater than 4.87 volts.
    Green wire must be above 2.0 volts before switch press. Then fall
    immediately to less than 0.7 volts when switch is pressed. Gray wire
    must rise above 2.4 volts within two seconds of pressing the power
    switch - and stay there. What do you have – an answer that must list
    each number.

    Those numbers say far more than you can imagine. Without numbers,
    those who actually know this stuff will stay silent.

    Some sometimes fear. It is only a 3.5 digit multimeter. That
    sounds complex! Well, if you cannot use a multimeter, then you have
    no business using a cell phone or Ipod. Both are far more complex and
    expensive. A meter is sold even where hammers are sold for about the
    same price. Even sold in Kmart and Wal-mart. Get the meter. Post
    numbers. Have a useful answer (no more 'it could be this or check
    that') in the very next reply.
    w_tom, Feb 18, 2009
  6. Norm Loman

    Arno Wagner Guest

    My Advice: Ignore this incompetent.

    Arno Wagner, Feb 18, 2009
  7. Norm Loman

    Norm Loman Guest

    THank you everyone. I did check my replacement powersupply with a
    multimeter, and everything seems to be fine. I even tried another
    power supply. but I cant seem to get it to turn on. I'm going to
    narrow it down a bit and say it's the motherboard, and see if a
    replacement works.

    Thank you all so much!
    Norm Loman, Feb 24, 2009
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