hi folks,
my p4 is dead following storm.dead as in silent,not even fans. i think
it got hit via usb adsl modem (as i have no dial tone either now) and
exited into my home LAN via the network card as when i remove this
card and switch on the atx PSU fan turn on for 5 sec then shuts down.i
have other pcs on this lan (without modems) which wont turn on either
but i am not worried about them as they are obsolete.
does this shutdown after 5 sec indicate a short on the board?
how to i proceed to diagnose the short?would it be near the USB
port?anyone ever manage to revive his board?thanks
Sam
About all I can do for you is explain how an ATX PSU and motherboard
work together.
The ATX PSU has two parts - the +5VSB supply runs as long as the switch
on the back of the PSU is switched on. This should cause a green power
LED on the surface of your motherboard to light up. This LED indicates
your motherboard has standby power operating, and usually the power
supply can provide an amp or two on that output. That power can be used
to operate a keyboard, mouse, LAN card etc, or any other device that
provides a "wakeup" function for the computer.
The +5VSB is essential to supporting the "turn on sequence" for the
computer. Without +5VSB, the computer won't turn on. The +5VSB powers
chips on your motherboard, and one of those chips controls a signal
called PS_ON#. PS_ON# is pulled up with a resistor to logic 1, and
when the chip on your board grounds the PS_ON# signal, the power supply
then turns on the rest of the supply voltages +3.3V, +5V (main),
+12V, -5V, -12V.
A number of functions can be tied into the PS_ON# logic.
1) AGP burnout protection. Modern P4 Asus motherboards use 1.5V AGP
and have a small transistor circuit, that prevents the power supply
from starting, if a 3.3V AGP card is detected. Sometimes this
circuit gets confused, or if the video card is damaged, sometimes
this circuit stops the computer from operating. (So this won't
result in the board running for 4 seconds - it'll just be dead.)
2) Thermal protection. Some motherboards have protection against
overheat. That might be tied into killing the PS_ON# signal,
and may take a few seconds to trip.
3) Software controlled wakeup state. If I plug in my P4B based
computer, it operates for a couple of seconds - basically long
enough for the BIOS to figure out that the machine should not
be running, and then it shuts down again.
4) The power switch has a 4 second time constant. Pressing the
power switch for more than 4 seconds or less than 4 seconds is
detected by the motherboard chip, and can be used to commence
suspend or shutdown functions. A stuck power switch might
result in behaviors with a relatively short time constant.
5) The voltage regulator(s) on the motherboard contribute to a
signal called Power_Good. Generally, a processor will not run
unless Power_Good is valid in a relatively short time after
power up. What I don't know, is if this is tied into the PS_ON#
logic or not - it doesn't have to be - the motherboard could
safely sit there dead with no Power_Good, and with the power
supply still running happily.
To debug your problem, you need to simplify as much as possible.
You didn't say what model it is, but if the board has vocal POST
messages, you could strip everything off the motherboard, and
see if the vocal POST reports CPU missing or memory missing etc.
Then, add components back, noting whether the latest added component
stops it from starting properly.
If the motherboard and power supply by themselves don't seem to
be doing anything, you can test the power supply by connecting
PS_ON# to COM. Page 19 of this document shows the pinout of the
ATX 20 pin connector on the end of the PS cable harness.
http://www.formfactors.org/developer/specs/atx/atx2_1.pdf
Some people don't recommend running a power supply without a load.
The reason for this can be that the outputs can achieve higher than
normal voltages when no load is present. Check your power supply
label, to see if any output has a non-zero minimum load value
specified. I got some 5 watt resistors and pins that plug into
an ATX connector from my local electronics store, to draw anywhere
from 1/2 amp (for the +5VSB, -5V, -12V outputs) to 1 amp for the
main outputs. Loading all the outputs helps ensure the PS isn't
damaged when I connect PS_ON# to COM and test my supply. Then,
using a voltmeter, I can check that all the outputs are sane, and
leave the PS running for a while to see if it is stable and isn't
overheating due to some kind of internal problem. (Stay out of
the inside of the PS, because there are two big caps in there
with enough juice in them to stop your heart.)
Depending on your home insurance policy, you may want to just
junk the thing, and get the computer replaced at the same time
as any other appliances that got blown. I don't know how a
home insurance policy would handle the construction of a "DIY"
computer
Maybe have a local computer shop assemble one
from the parts list you specify - that way you get the parts
you want, and the insurance company gets a nice itemized bill
for what was done.
In terms of what damage the lightning can do, it can range from
damaging just the chip connected to an external cable (in this
case that could be the Southbridge, that hosts the USB interface),
all the way to blowing the tops off all the chips on the board.
A dead Southbridge would be fatal (which suggests that in future,
use a separate PCI LAN card to connect to the ADSL modem, on the
off chance that just the LAN card gets blown).
HTH,
Paul
