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I have a desktop that won't boot, and the monitor doesn't turn on.

The weird thing is as soon as you plug the computer, it turns on and then you can't turn it off.

There's no light on the board to see if the power supply is OK, I don't know if there's a mic on it to know if the post is good. There's not even a switch on the power supply.

All the lights on the computers are on, I can open the disk tray but there's no display, so it makes me think that there's a problem with the onboard graphic card. I restored the CMOS and afterwards the computer won't turn on itself but once it started, it does the same until I restored the CMOS again.

I also tried to unplug the disk tray, hard drive but I got no luck.

Do you think it's a problem with the motherboard or should I take the time to test the PSU?

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What did you do to the CMOS? Restore it? Standard operating proceedure is to remove all hardware (including RAM) and see if it POSTs or gives you any beeps. – terdon Aug 4 '13 at 18:06
I also tried with no ram, no hd, no cd but still no luck. I did restore the CMOS by moving the 2 pin to the left then back to the right. I had no beep, but I don't even know if that computer has a mic on the motherboard. I had no display also – Marc Aug 4 '13 at 18:11
As far as I know (may well be wrong) all motherboards have the ability to make beeps, hell even floppy disks do! Did you remove all the hardware at the same time or remove each and then try? Can you hear the disks spinning up? Does the CPU fan start? – terdon Aug 4 '13 at 18:19
Sorry, I forgot to mention this. Yes, I can confirm, the fan and the hard drive spin. I don't hear any beep, but it might be like on my own build. I forgot to plug the speaker on the motherboard so I don't have a beep either. That's why I don't know if it POSTs – Marc Aug 4 '13 at 18:46
Start trying to swap known good hardware in or swap out suspected bad hardware to another computer to diagnose if it is the motherboard. PS, RAM, and CPU are the place to start. RAM and PS are more likely than CPU. Also, try disconnecting you power switch from the header and see if it still powers on immediately. – Damon Aug 4 '13 at 23:31

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