Power related boot problem

Posted on 2006-05-03
Last Modified: 2012-06-27
We've got a Packard Bell in with a Gigabyte GA-8SIMLNF motherboard that refuses to boot or, indeed, POST (customer reports this happened suddenly - they had not opened the case and were not poking inside). When powered up, the MOBO DIMM LED flashes twice per second, as does the light on the on/off switch, the hard drive also spins on and off and the system fails to post. The machine cannot be powered-off using the on-off button (6 seconds), only by pulling the power cord.  We stripped all PCI adapters off the board and left a single RAM module in place - same result. We replaced the PSU with a known good PSU - same result.

I back-probed the known-good PSU and got the following:

+5VSB (Pin 20) @ 5.13V (Steady)
PWR_OK (Pin 8) @ 0-3.4V (Cycling)
+12V @ 9.7-11.7V (Cycling)
+3.3V (Pin 1) @ 2.8-3.3V (Cycling)

All the voltages cycling through the ranges listed above do so at ~once/second.

It appears that the PWR_OK pin is not holding the voltage above the required threshold for the correct period thereby forcing a reset and so it loops indefinitely, trying to reach a power good state.

We repeated the test with two other known-good PSUs and the results were the same, so we are pretty confident that its not a PSU issue. Are we looking at a MOBO fault and a replacement MOBO or is there something else that we can investigate? Comments/Suggestions appreciated.

Question by:bbrohan
    LVL 69

    Accepted Solution

    The motherboard is the most likely candidate in this case.  To be thorough, these other things can prevent POST:
    - motherboard shorting on case, or dust accumulation
    - bad connections
    - bad RAM, or wrong ones for your motherboard, or bad RAM slot (try in another slot)
    - faulty video card
    - bad cpu
    LVL 91

    Expert Comment

    you can try inspecting and reseating all cards and cables, but mobo or cpu are my first candidates.
    LVL 91

    Expert Comment

    oh yes, and test with the minimum devices connected; even without disk, only mobo, cpu, 1 ram stick, video card.
    LVL 13

    Expert Comment

    Like Callandor said
    motherboard shorting on case, or dust accumulation

    try taking out the board  might be a faulty case


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