Power related boot problem
Posted on 2006-05-03
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.