Power problem on MB

[Rongee]

Good morning,
I am having a problem with an Asus motherboard, an A8S-X. With all power cables connected and hitting the power button, the system lights up momentarily and then goes dark. Strangely, if I disconnect the 4 pin 12V plug to the board, the CPU fan will run, but nothing beyond that. I've checked the plug, and it is providing 12VDC.

So, it seems to me that this is a problem with the Mboard. Does that sound about right? I've tried resetting the CMOS, but it makes no difference. Anything else to try, or is it time for a new motherboard?

Thanks for any assistance.

Ron

 

[shortstuff_mt]

I would go through this checklist to try to narrow the problem down:

http://www.tomshardware.com/forum/261145-31-build-post-checklist

 

[jsc]

There's a good chance the problem is the power supply. Disconnecting the 4 pin plug removes the CPU power. That's why nothing works.

After the checklist, try this:

Try to verify (well as you can) that the PSU works. If you have a multimeter, you can do a rough checkout of a PSU using the "paper clip trick". You plug the bare PSU into the wall. Insert a paper clip into the green wire pin and one of the black wire pins beside it. That's how the case power switch works. It applies a ground to the green wire. Turn on the PSU and the fan should spin up. If it doesn't, the PSU is dead. If you have a multimeter, you can check all the outputs. Yellow wires should be 12 volts, red 5 volts, orange 3.3 volts, blue wire -12 volts, purple wire is the 5 volt standby (on all the time). The tolerances should be +/- 5%. If not, the PSU is bad.

The gray wire is really important. It sends a control signal called something like "PowerOK" from the PSU to the motherboard. It should go from 0 volts to about 5 volts within a half second of pressing the case power switch. If you do not have this signal, your computer will not boot.
Unfortunately (yes, there's a "gotcha"), passing all the above does not mean that the PSU is good. It's not being tested under any kind of load. But if the fan doesn't turn on, the PSU is dead.

Connect the CPU power plugs. Monitor the gray wire and see if it goes to 5 volts after pressing the case power switch.