If the processor is processing, then it interprets the power button
straight away. If the processor's not processing, the motherboard
circuitry will interpret the power button after it's been held down
for several seconds. So your mileage tells us the processor is not
processing, and because the fans stay spinning, it's not a simple
short circuit (which would normally cause the PSU to cut power).
First of all strip the machine right down to the bare essentials.
Remove all repeat all accessory cards except the video card.
Unplug both the power and data connections to all of the disk drives
Disconnect all external devices connected to USB. Firewire, Serial, or
Parallel ports other than the keyboard and mouse.
If you do this (as I would), then KEEP THE HARE DRIVE UNPLUGGED FROM
THE SYSTEM!! There are several reasons for this:
a) Protect hard drive against physical damage, e.g. excess volts
b) Protect hard drive from data corruption due to bad RAM etc.
c) Avoid OS detecting changed hardware and "PnP storms"
d) Avoid OS detecting changed hardware and WPA DoS payload
e) Avoid file system corruption from bad exits after partial boot-ups
I start with just:
1) Motherboard, processor, PC speaker, power supply
2) Add RAM
3) Add SVGA, monitor
4) Add keyboard
5) Add 1.44M drive
On (1), you want to hear "bad RAM" beeeeeeeps.
On (2), you want to hear those beeeeeeeeps go away
On (3), you want to see the BIOS POST screen coming up
On (4), you want to check temp and voltage in CMOS setup
On (5), you want to boot MemTest86 and leave running for hours
At step (5), you are doing two tests; the obvious one of the RAM, and
the ability for the PC to stay powered up and running. So take the
boot diskette out of the drive before you walk away, so that if the
system's not running MemTest when you return, you know the second test
failed, irrespective of whether the RAM was OK or not.
Until everything passes with no errors, don't let the sick system even
sniff the HD's power socket ;-)
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IRC is just multiplayer notepad.