I'm new to this forum so be nice.

I have a ASUS A7N8X board with a Allied AL-A400ATX power supply. Starting this weekend, with no notice, my system will not boot up. Instead, it seems to be stuck in a On/Off loop. I can't get into bios, safe mode or use my bootup floppy because the cycle doesn't last long enough to reach that part of the startup routine. The front On/Off switch does not work. I have to power down by using the main power switch on the Power Supply. I think I've narrowed it down to my power supply or my motherboard. I tried a new hard drive . . . no change. After disconnecting all my various drives including hard drive(2), CD, DVD, and Floppy; I am still getting the On/Off loop. The power supply seems to be automatically turning itself on/off. Even when I disconnected the On/Off switch cable from the motherboard it continues. I looked at the capacitors on my motherboard and there appears to be no sign of bulging or discharge. Do you think I should try to replace my power supply? or motherboard? or something else? I'm getting in a little over my head, but don't think I'm ready to buy a whole new system. Please advise.

I would definitely try a new psu first.

Regards Howard :grinthumb

You were right

It was indeed the power supply. Replace it with a COOLMAX CMAX 400WCX-400B BK RT replacing the defective Allied AL-A400ATX 400W. Everything powered up just fine after the repair. Thanks you your help.

what if replacing PSU doesn't work

i stripped down my system to Soyo Dragon 2 PLUS mobo, Intel PENT 4 CPU, and Ultra V series 500watt ATX PSU. the PSU is the new one, but i still get the 2-3 second boot up, then shut down. only time it continues to run the PSU and the fans is if i disconnect the 12V 4 pin CPU plug-in and only have the 20 pin connection plugged in. then it "runs", but w/ no display on the monitor.

i've had this system for over 4 years and it's never given me any trouble till a couple days ago.

thanks in advance

DO NOT hijack threads. Start a new one.