A7A266 Wont Power - Get Green Standby Light

Discussion in 'Asus' started by Kathy, Jan 6, 2004.

  1. Kathy

    Kathy Guest

    Hi All...

    I have an Asus A7A266 which was working fine earlier this week. Switched the
    MB to a different case, and apparently it doesn't like the new home, as it
    won't power up. I am thinking that the Mobo is fried somehow (as it was
    working earlier), and am not sure why the green stand by light is on when
    the PS is plugged in. I cant even get the fans on the PS to spin on start.
    It is completely DEAD.

    I checked the PS (550 Watt) which works fine with other sistems. I checked
    the Power switch by using both the Power SW and the Reset SW leads to the
    mobo connectors. They work fine on other system. I swapped the RAM, Removed
    the CPU, and still nothing. All I have in there is a video card.

    I also removed the MB from the case and tried running it on the cardboard
    box just to kill the possibility of it being shorted on a stand-off post in
    the case...

    Any ideas on why this wouldn't power up? I am seriously frustrated. This was
    working earlier this week...

    Kathy, Jan 6, 2004
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  2. Kathy

    Sick Willie Guest

    The most common cause of what you describe is that you have mis-identified
    the pins that act as the switch for the power. Asus uses the 1st two pins
    of the front panel header for reset, skips a pin, and then uses the next two
    as the power switch. This is from the orientation of standing in front of
    the board, looking at the header along the left hand side. The LED you
    describe is not a standby light per se, its purpose it to let you know that
    power, standby or otherwise, is applied to the board so that you do not
    install/remove boards without either unplugging the power source, or (a
    better alternative when available) flipping the power switch on the power
    supply to the off position.

    Sick Willie
    Sick Willie, Jan 6, 2004
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  3. Kathy

    Kathy Guest

    Yup. I had that correct. Does the LED being on indicate a good MB? Or, could
    that mean that there is power getting to it, but something else crapped out?

    Kathy, Jan 6, 2004
  4. Kathy

    Tyler Porter Guest

    Im having the exact same problem with my A7N8X motherboard. Tried
    everything you tried, plus resetting the bios. I have also checked
    and rechecked the power switch plug about 100 times. Its something
    else, and im pulling out my hair trying to figure it out.
    Tyler Porter, Jan 6, 2004
  5. Kathy

    Paul Guest

    Are you referring to the green LED on the motherboard, next to the
    three DIMM slots ? That LED is powered by +5VSB from the ATX power supply.

    Power supplies have two "sections". The +5VSB supply operates whenever
    the switch on the back of an ATX supply is in the ON position. The
    other outputs of the supply on the other hand, are controllable via a
    logic signal.

    When a power supply is connected to a motherboard, some of the motherboard
    circuits are powered off +5VSB. One of the circuits is the circuit that
    listens to the power switch on the front of the case. Momentarily closing
    the switch on the front of the case, results in a chip on the motherboard
    latching the signal, and pulling down the PS_ON# signal which is on
    the ATX 20 pin connector. This causes the rest of the PS outputs to be
    switched on. The PS runs as long as PS_ON# is grounded (connected to
    COM on the 20 pin connector). At shutdown, Windows tells the chip that
    listens to the switch, to release the PS_ON# signal, and then the machine
    is shut off.

    So, you have +5VSB present, but for some reason, either your momentary
    switch from the front panel is not working properly, the motherboard
    chip driving PS_ON# is malfunctioning, or the power supply is not
    listening to PS_ON# for some reason. That makes at least three possible
    pieces of broken hardware.

    Some people test by actuating PS_ON# manually, while the motherboard
    is connected, but I don't recommend that unless you know whether the
    chip driving the PS_ON# signal is safe to short to ground or not.

    A safer way to test, is to test the power supply by itself, by using
    the 20 pin connector and connecting PS_ON# to COM. Even if you don't
    own a voltmeter to check the voltages, you can at least see the PS
    fan spinning when the PS has successfully been switched on. Inside
    the supply, +5VSB is applied through a pullup resistor, to the PS_ON#
    signal, and only a small current should have to be shunted to ground
    by the chip on the motherboard - sometimes the PS puts too much
    current on this lead, and then the chip on the motherboard cannot
    fully shunt the current to ground to start the supply.

    Otherwise, if you've isolated the problem to the motherboard via
    swapping out other pieces of hardware, it could actually be a motherboard
    fault. I have a motherboard here, where the PS_ON# signal went nuts
    when one of the IDE cables was half seated, so make sure all of the
    cabling is correctly installed before blaming the motherboard.

    In terms of failure rates, the PS is tops on the list of dud
    hardware, no matter what brand is stamped on it. If you have another
    computer handy, use the supply from it to power your A7A266 and see
    if they get along together.

    Paul, Jan 6, 2004
  6. Kathy

    Tyler Porter Guest

    I tried checking the power supply by connecting it to another working board,
    and it started up fine. I then tried to plug an older power supply I know
    works to the asus board, and again nothing started. Seems like it must be
    the motherboard. Is it repairable?
    Tyler Porter, Jan 6, 2004
  7. Kathy

    Sick Willie Guest

    This is not really a very safe way to test a supply. True enough, it won't
    fry a board, but a power supply expects to find a load. Powering one up
    without some load is *not* good for the supply any more than powering an
    amplifier without a load is good for an amplifier.

    Sick Willie
    Sick Willie, Jan 7, 2004
  8. Kathy

    Sick Willie Guest

    All the LED signifies is that some voltage is being applied to the board -
    enough to power an LED. The LED is only there to remind you that power is
    present on the board. It does not indicate that the board is good.

    Sick Willie
    Sick Willie, Jan 7, 2004
  9. Kathy

    Sick Willie Guest

    Have you tried just shorting the two pins rather than using a switch?

    Sick Willie
    Sick Willie, Jan 7, 2004
  10. Kathy

    Paul Guest

    Another thing. I wonder if the CMOS battery is dead. The CR2032 is
    available at Radio Shack. When new, it reads at least 3V, and the
    Southbridge circuit it powers can work down to about 2V. Maybe that
    would be worth checking as well.

    Paul, Jan 7, 2004
  11. Kathy

    Kathy Guest

    That's what I ended up doing. I used the power supply to power up a separate
    system fine. I then used a different power supply with the "dead" system,
    and nothing.

    I have no drives plugged in, no cables in, all I have in is the memory, CPU,
    and Video Card (in the AGP slot). I should be able to get power one would
    think, unless the mb was dead... but then again, I am sitting here with a
    dead system...

    Kathy, Jan 7, 2004
  12. Kathy

    Paul Guest

    I have to admit Willie, operating a supply without a load is not a
    "best practice". I own a set of load resistors for the specific purpose
    of testing supplies, but they weren't exactly easy to find. The problem
    is, I cannot suggest to people that they go out and acquire a set of
    resistors and a connector to make their own dummy load, so the quick
    power up test is the best I can come up with. If I recommend they just
    go out and buy another power supply, that isn't much of an answer either.

    Any suggestions on how I should answer the question ?

    Power supplies come in a couple of architectures, and the old ones
    have a common primary feeding a set of secondary transformer outputs.
    I cannot honestly say how close the outputs stay to their nominal
    voltages when they are unloaded. There is some (light) feedback from
    each output back to the primary side, so they shouldn't really shoot
    to infinity.

    As a test, I just took a "Touch" brand 250W power supply manufactured
    in 1998, and shorted PS_ON# to COM while using no other loads. When
    I measure with my voltmeter, the +3.3V and +5V outputs are sane, while
    the other outputs like +12V, -12V, and -5V are all on the low side.
    This implies that the feedback in the supply is working so well as to
    shut down all but the lowest voltage outputs.

    While a sample size of one is insignificant, at least I don't see
    anything on this old architecture power supply to suggest the supply
    is producing voltages high enough to cause breakdown. AFAIK the supply
    simply won't meet its regulation spec, when it is unloaded.

    Here is a sample schematic of an old architecture switching power
    supply. See what you think. (The thing I like best about this
    design, is the two 470uF caps charged to line voltage. Talk about
    potential energy...)


    Paul, Jan 7, 2004
  13. Kathy

    Sick Willie Guest

    Have them either plug it into another board. If this is not available, most
    smaller computer repair stores, such as mine, will, if the customer's
    attitude is friendly, plug in a supply, while they wait. And, having a
    spare supply is actually an ideal situation.

    If your entire goal is to see if the fans spins, you can probably run it
    safely long enough to do so. However, a fan spinning, or a standby LED, are
    poor indicators of available power.
    The primary purpose of have a "power good" signal as feedback from the
    board, in both the old AT style and the newer ATX, et al, power supplies is
    to assure that there is a load present. Again, just as in the case of an
    amplifier, you can probably run it unloaded for very short periods of time,
    but all you can really ascertain is that the unit powers up. Even after
    having personally practiced something for a period of years, (in fact, I
    practice alot of things that I can't recommend <g>) I stil find myself
    loathe to suggest that others indulge in these same practices. <g>

    Sick Willie
    Sick Willie, Jan 7, 2004
  14. Kathy

    Sick Willie Guest

    The CMOS battery being dead should not keep the board from powering up.

    Sick Willie
    Sick Willie, Jan 7, 2004
  15. Kathy

    Sick Willie Guest

    I would say pull it out of the case and try, but you said you already did
    this. The #1 scenario is a standoff shorting the board.

    You might try removing the heatsink and processor. Reseat the processor in
    the socket and re-attach the heatsink, preferrably after re-applying some
    thermal compound. I know it sounds like voodoo - after all the processor is
    already in the socket - but a ZIF socket, just like any other socket,
    provides a metal on metal pressure contact connection and can, over a period
    of time, especially with a multitude of heat up/cool down cycles, oxidize to
    the point where a good connection is not being made. With the Athlon, there
    is the potential for somewhere around 462 bad connections.

    Sick Willie
    Sick Willie, Jan 7, 2004
  16. Kathy

    Kathy Guest

    Already Beat you to that idea. Attempted a boot with nothing attached with
    the exception of the power supply in order to try and force a boot error. No
    power. Fans wont even spin up on PSU.

    I would definitely think this was the PSU if I didn't try it on a couple of
    separate systems, and try good PSUs on this board which won't boot.

    Kathy, Jan 7, 2004
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