Hi

I'm new to this forum and need some help solving an issue with a new PC that I'm building.

After I assembled all of the components the computer powers up for less than a second and then turns off. I've plugged the power supply into another computer and it's working there. I've disconnected all of peripherals from the motherboard and the problem still occurrs. Does this mean I've got a defective motherboard that's causing the power supply to shut itself off.

Any troubleshooting advice?

Have you checked the motherboard for a short?
It is unlikely to be add-ons- eg RAM,graphics .....
as this would take longer to detect.

Overheat? The CPU does have thermal paste, heatsink and fan right?

As mentioned by sparkax, a very swift shut down of the PC can be attributed to a short. This causes the PSU to detect an over amperage and it immediately shuts off.

Other causes might be the main board connector isn't well plugged in; or another power connector is in a reversed or off-set position.

I would suggest you reseat the CPU.

Also there may be more than 1 connector from the PSU that you need to connect to the mobo.

If the PC shutdown in 1 second, I believed not a overheat problem.

I wonder if you have paid attention when putting the mobo to the PC case.
Sometimes, touching the bottom part of the mobo to the PC metal case when installing the mobo to the PC may short the mobo .

Try remove the mobo battery to reset the BIOs and try again.
Hope this helps!
Bill