I bought a new psu yesterday because the ones i had lying arround were
old ones that did not have a 4pin connector for the cpu. Anyway i
placed the new psu but the problem remains.
So the CPU fan doesn't stop at all ?
I thought the description matched an oscillating power supply. But
I missed the part where you said the fan was still spinning.
You've already checked the CPU, and if the CPU was overheating,
that could cause the power to go off. But a good design would
make the machine stay off (at least, until you flipped the power
switch on the back, off and then on again).
The next thing I'd inspect is the reset and power switches.
Are the switches in good shape, or are they binding ? Both the
reset and the power switches are momentary contact types. They
only conduct when the button is pushed. They should stop conducting
when you release them. In an emergency (assuming the header has the same
pin spacing), you can substitute the reset switch in place of the power
switch. (Then you use the reset button to turn the computer on and off.)
I have also used a slot head screwdriver tip, between the power
switch terminals on the PANEL header, to give a momentary contact to
start a computer. But I generally do that while the motherboard is out
of the case, as then it is easier to see what I'm doing, and touch
only the two pins where the power switch would normally go. If the
motherboard is inside the case, it is pretty hard to see what you're
doing, so the screwdriver idea wouldn't be wise then.
Other than that, if the thing seems to be oscillating in reset-POST
cycles, it could be some kind of motherboard problem. But at least
have a look at the switches first.
You say the green LED stays on, and you might also want to observe the
green LED during one of the blank-out cycles. See if the green LED winks
out, even for a moment. If it did, I'd get out my clamp-on DC ammeter, and
check how much current is being drawn from +5VSB. But if +5VSB was being
overloaded, the thing should shut off and stay off. Which really
leaves a power switch or a reset switch problem, and barring that,
a motherboard problem.
The five second number, sort of sounds like the amount of time you have
to hold in a power switch, to make a machine shut off. Like the power
switch was still making contact.
Does any text appear on the monitor, during the first five seconds ?
Does the reset happen just as the text reaches a certain point ?
Also, another test you can try. Power off and unplug the machine.
Remove the RAM and store it in an antistatic bag. Turn on the machine.
The machine should beep a "missing RAM" pattern. With the RAM
removed, will the machine stay running ? You can do a similar test
with the video card (assuming the motherboard is not using integrated
graphics). If the responses of the system change, with components
unplugged, that might also hint at the nature of the problem.
Paul
