titani said:
Hello,
I built the PC myself, yet it does not boot. When I press the Power
On switch, the fans turn on for about a second, then they turn off
automatically. I checked the standoffs. I remove the motherboard
from the case. I disconnected all drives, and the video card. I
removed the CPU then put it back. Yet the behavior is still the
same. What could it be?
Thanks,
T.I
Reasons for a PC to turn off
1) If the CPU has no heatsink and fan placed on top of it,
it will overheat in one or two seconds. Both AMD and Intel
based motherboards, will turn off the PC power supply if
this happens. (Even a lack of thermal paste or thermal pad,
between the top of the CPU and the bottom of the heatsink
assembly, may be enough to cause an eventual overheat and
shutdown.)
2) The CPU Vcore circuit has overcurrent protection. It would
turn off the power, if the CPU draws too much power. You
may have to switch off the power supply at the back of the
computer, in order to try again.
3) If the computer switches off after 4 or 5 seconds, it could
be that the CPU fan is not connected to the CPU fan header.
The BIOS monitors the CPU fan header, for the presence of an
RPM signal. If no signal is received, the BIOS assumes the
fan is jammed and not spinning. A CPU fan should have 3 or 4
wires, and connect to a 3 or 4 pin header near the CPU
socket. If the CPU fan has only 2 wires, then there is no RPM
signal on it, and the BIOS will be most upset.
4) The power supply has internal protection against overload.
For example, some power supplies have a thermal sensor
connected to the internal heatsinks inside the power supply.
After some number of seconds, the power supply may shut down.
Power supplies also detect overcurrent, but that can trip in
as little time as 35 milliseconds. If a computer fan only
"twitches" and will not rotate even once, then you know you've
got a 35 millisecond type of overload or overcurrent issue.
You could test the power supply by itself, by connecting PS_ON#
to COM with a paper clip. I like to have some loading connected
to the supply while testing it, and have a home made load box
that I use for the purpose.
PDF page 37 here, shows the location of PS_ON# (pin 16) and an
adjacent COM pin (pin 15 or 17). They're on the "tab side" of
the power supply connector. PS_ON# is a green colored wire, so
you know you've got the right one. The power supply should
remain running, for as long as pin 16 makes contact with pin 15
or pin 17.
http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf
It is also possible to force the computer to remain running, by
connecting pin 16 to pin 15, while the computer is completely
assembled. Doing so, overrides the thermal protection feature
of the CPU, so there is some risk of hardware damage if you
do that. But a few people in the hardware groups have tried
that as part of their hardware testing. Since I have my
load box with power resistors, for testing a power supply,
I don't have to endanger any other hardware, to test a
suspect power supply.
For what a load box looks like electrically, scroll near the
bottom of this posting.
http://groups.google.ca/group/alt.comp.periphs.mainboard.asus/msg/19647caf2c65504b?dmode=source
HTH,
Paul
