barrowhill said:
Having switched of in normal fashion, son's PC will now not start. No
activity what so ever.
Motherboard is Foxconn 945G7MD/945P7MD with 3GB RAM, 160GBHDD, ATI Radeon
4850 Graphics Card and twin DVD RW.
When case is opened the ATX power LED situated in front of 20+4 pin molex
connector is flashing.
The PSU has been tested and is OK (pins 14 & 15 on 20pin molex shorted).
I've removed grahics card and disconnected DVD's etc. still same.
Currently I've got connected 1GB RAM, 8pin ATX 12v power connecter and 4pin
(3/5/12v), all fans (Case & CPU) but have disconnected 20pin ATX power and
shorted pins 14 & 15. PSU runs when connected to supply
CPU fan and case fan running and mobo Power LED on constant.
If I connect 20 pin to motherboard, no activity, LED flashing.
Can't see any signs of burning etc on mobo.
Is there anything else I should try or look at before concluding mobo faulty
?????
I'm most interested in:
"the ATX power LED situated in front of 20+4 pin molex connector is flashing"
On an Asus brand motherboard, the single LED on the motherboard is tied
directly to the +5VSB rail (it's not switched with a transistor or
anything). And as such, it monitors the voltage coming from the power supply.
Such a LED is not allowed to blink. It should light up, as soon as the switch
on the back of the ATX supply is switched on. The +5VSB supply, is used for things
like powering RAM contents, while the computer sleeps. On an Asus board,
that LED basically tells you not to pull any RAM sticks, because
the system is still powered. Once the supply is powered off, the
LED should die about 30 seconds later, indicating it is safe to
add or remove components from the computer. It's a safety LED, to
help avoid damage to components.
I'll assume the Foxconn implementation is similar, although I don't
remember too many other brands doing that. Asus is pretty consistent
in putting that LED on their boards, and I think it's worth an
extra 10 cents, to have it there.
That +5VSB rail on the ATX supply, has limited current output. Depending on
supply, it delivers between 2 and 3 amps, on typical supplies. The
motherboard might need about 1 amp, so there should be some margin.
The ATX power supply fan does not spin while the computer is sleeping.
A little bit of heat is dissipated by the supply, while it is making
the +5VSB, but not enough heat to require the fan.
Now, on modern motherboards, the clever designers have decided it would be
fun, to power all the USB ports off +5VSB. That allows a USB keyboard,
to be used to wake the computer. But a downside of doing that, is any
"dumb" electrical loads on USB, can overpower the supply. In marginal
cases, the +5VSB overheats and shuts off. When it cools, it can come
back on. And that can lead to a "blinking" monitor LED.
An alternative theory, is the +5VSB supply inside the ATX supply, is
weak. The motherboard loading of 1 ampere, isn't excessive, but if
the supply is weak, you can get the "blinking LED" problem.
For this fine theory to be true, you'd have to know whether that
is the function of the Foxconn LED you're looking at. I can't find mention
of a LED in the manual. Can you provide a URL to the manual
you're using, and the page that shows the LED. While I have a
(crappy) picture of the motherboard, the resolution isn't fine
enough to locate the LED you describe.
In any case, you can try disconnecting all the USB loads on the
computer, and see if the blinking stops. Or, try another
power supply, and see if the +5VSB holds up better on the
alternate supply. Yes, the motherboard could easily overload
that rail. It wouldn't take much of a fault to do it.
My bet is, this is a weak supply problem.
Paul
