No boot - mobo or CPU toast?

I have been trying to help my son to get his PC fixed.

He got a new mobo (Asus A8N-SLI Deluxe) because we thought the original (Asus A8N-SLI) was toast as it wouldn’t boot. He also upgraded to a pair of graphic cards (GF 7800 GTX). Fitted the original four sticks of Kingston 512MB PC3200 memory. The PSU is a Seasonic SS-550HT.

Got everything together, but like the old configuration it won’t boot. The small light is lit on the mobo so it is getting power. The fans start to spin but only do a half a dozen or so rotations, just like you gave the blades a flick round with your finger. Nothing appears on the monitor.

To check the PSU I swapped it into my own PC. The PSU works perfectly there.

Took out one of the graphics cards. No go.

Took out parts one by one and kept trying but still no go. Eventually all that was left in the case was the mobo, CPU, memory, graphics card and the PSU. Still connected to the monitor and keyboard. Still no go. Took the battery out of the mobo to clear the CMOS and replaced it after a couple of minutes. Still no go. Took the mobo out of the case as there may have been a short with a screw. Still no go.

Disconnected the monitor, graphics card, keyboard and took out the memory and tried after each removal. Still no go. Now it is just the PSU, mobo and CPU. Still no go, as always just those few rotations of the fans.

Removed the CPU so the bare mobo connected to the PSU. Still no go, but didn’t really expect much without the PSU (Edit - Should have written "without the CPU" - not "without the PSU" )

What is the verdict - mobo or CPU. Or am I missing something else completely.

Thanks for the reply adrianmk2.

Isn't it more likely to be the CPU when the same no go is from two mobo's?

Unfortunately my own mobo is a socket A and don't know of anyone who has a socket 939 who would be willing to test my son's CPU. Perhaps I shall have to take it to a repair shop.

Probably cant RMA the mobo as it was purchased via ebay in May/June.

The CPU is AMD Athlon 64 X2 4400+ Dual Core Processor - 2.2GHz, 2MB, (Socket 939)

I'd stay away from a computer repair shop. For the same money, you can still get a brand new 939 board. Etech4sale has a foxconn 939 board for $33 plus shipping with pci-e interface, and possibly onboard video. They also have an asus model for $50 with agp slot. Before spending any more money, try reseating the memory. Use just one stick and see if it posts. I always have better luck with the board on a phonebook or a motherboard box.

Thanks for the reply guys. They are very much appreciated.

It is my sons PC that the problem is with. I swapped his PSU with the one in my PC. My sons is in mine and working ok. Mine in his - Still no go. So the PSU is not the problem.

Current thinking in my family agrees that the problem is most likely with the CPU as two mobo's have been tried. Thinking to buy a cheap socket 939 CPU on ebay. If it works that will prove where the problem lies.

Most likely it is one of the 24 pin wires not making good contact with the asus port. I have reinstalled the 24 pin atx plug before to get a board to post. Usually, the fans will spin as soon as the ps is on, or some boards have an indicator light that comes on as soon as power is received. My compaq has the light on the power supply, but no switch. I had one old FIC board that just wouldn't go; I found a loose wire on the reset switch of the case bezel. Once I snapped the switch back in place on the back of the plastic bezel, the board worked fine.

Interesting problem.

I think you might want to test the stability of the current supplied by the PSU. If there's too much fluctuation with the voltage, that sometimes can cause some weird problems.