Computer shuts down 3 seconds into boot.

  HighTower 09:04 01 Dec 2009

What do you make of this?

Computer is unplugged from mains, when you plug it in it instantly begins to start up, fans spin and light goes on CD drive. After between two or three seconds it turns itself off. It just stops dead as though the power has been pulled. The hard disks don't have time to start up, there isn't time for anything to appear on the screen, there is no POST, nothing at all.

The green LED on the back of the unit stays lit as long as the power is connected, and while this is the case the computer cannot be switched on. When the power cable is removed it takes 7 or 8 seconds for that LED to go out, reconnecting the power cable now will repeat the process.

I've had the front cover off and the on / off switch seems fine, it is a rocker switch rather than a traditional on or off switch. It sounds to me like a PSU issue, any opinions?

  howard64 09:29 01 Dec 2009

I would agree it sounds like the psu is not supplying power to some point on the mobo. They are relatively cheap and easy to replace so that would be my first step.

  Technotiger 09:31 01 Dec 2009

I agree with your own assessment, PSU!

However, if this is a Desktop PC, then surely it should not turn on instantly from Mains connection - what about the on/off switch on back of pc, don't you switch off here before disconnecting from the mains?

  Eric10 09:51 01 Dec 2009

The symptoms sound like a stuck on/off switch. If you disconnect the power switch from the motherboard does the PC still turn itself on when you plug it into the mains? Your description of the switch as a "rocker switch" worries me because the on/off switch has to be a momentary type. If the connection through the switch is constant then you get exactly what you have described. If your case has a reset switch you can connect that in place of the power switch as a test because that is also a momentary action switch.

  HighTower 10:07 01 Dec 2009

Hi folks, thanks for the replies.

I have to admit my first thoughts were that it was a jammed switch, and that the PC started up, but a switch was jammed into the 'on' position which in effect shut it down. That's what happens when you hold a switch in for three or four seconds so it would make sense.

The switch doesn't really have an on / off position. It's kind of like a spring loaded button which doesn't click into place (and never has). You just press it in to turn the PC on, then press it in again if you want it to sleep or shutdown, though usually this is done from the shutdown menu. It's on the very top of the base unit facing upwards. I'll try and find an image of it to show you.

I'm guessing that to disconnect this I just follow the wiring from the switch and remove where it plugs in to the board? In theory with this removed the computer should not turn itself on when connected to power? Unfortunately the PC isn't mine so I don't have it here, although I can pop down and try any suggestions out tonight.

  howard64 10:15 01 Dec 2009

note the polarity of the connections as the coloured wire goes onto the + connection on the mobo. I know it should make no difference being just a push button but it does seem to on certain mobos.

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