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Power related boot problem

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.

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1 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
you can try inspecting and reseating all cards and cables, but mobo or cpu are my first candidates.
oh yes, and test with the minimum devices connected; even without disk, only mobo, cpu, 1 ram stick, video card.
Like Callandor said
motherboard shorting on case, or dust accumulation

try taking out the board  might be a faulty case


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