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I have an annoying problem with a computer.

The BIOS won't post after it has been turned off and started again unless I turn the switch off/on on the power supply. The fans, CD-ROM, HDD and everything starts running but the BIOS never beeps and the monitor stays black.

I tried: reinstalling Windows XP, flashed BIOS to latest, CMOS clear, new power supply, other HDD, new memory sticks, graphics card. I also removed the motherboard from the computer case and laid it on a cardboard box and only connected the PSU, RAM and CPU.

Other than this the computer works great but its annoying having to off/on the power supply switch before turning it on after it's been off.

My guess is that the motherboard is faulty, the motherboard looks fine tho no fried capacitors or anything but sure they can be destroyed anyways I would just like to hear what you guys think.

PS. This computer has never been overclocked or anything

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Are you overclocked at all? I have had this problem on machines where my overclock is pushing the stability envelope. – ubiquibacon Jan 15 '11 at 20:20
never overclocked it :/ – remy Jan 15 '11 at 20:33
I would send an email or support request to the motherboard manufacturer before replacing the motherboard. – Steve Jan 15 '11 at 23:21
I had the same problem with my current system 2 years ago. I can't remember what fixed it, but it was either a BIOS update, or using a different power cable (can't remember for certain, sorry). Try changing the cable for a quick test, maybe even in a different outlet just to cover all bases. – Gary Morris Sep 12 '13 at 15:09

You appear to have done quite a thorough job with process of elimination so I would second your thought that this is a motherboard issue.

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See my answer in another thread of how to analyze such problems.

Fans etc. starting up OK : eliminates the PSU.
No beeps : almost always points to the motherboard.

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aff :( i guess i just have to get a new motherboard such a shame because the computer works fine its just that i have to switch the power supply on and off before starting it else bios wont post – remy Jan 15 '11 at 20:35

I'd get someone to check to PSU. No beep at the start point to the power good signal.

Fans and stuff need 5V, 12V ( and aren't too fussy) The CPU needs the 3.3V supply.

Check the POWER GOOD pin on the power supply.

You'll need a voltmeter.

The Grey wire is Power Good, +5 VDC when all voltages has stabilized. measure against a black wire.

If it is not 5v, the PSU isn't okay ( and knows it!)

Orange wires should be 3.3V relative to the black wire.

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