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Is mobo shot?



 
 
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  #1  
Old December 26th 11, 11:30 AM posted to alt.comp.periphs.mainboard.asus
reqluq
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Posts: 61
Default Is mobo shot?

Hi. I plugged directly into the power without ups, when I got back the
computer was dead. though the green light is on the computer won't start.
The power does fluctuate in the room and two other computers seem to have
gone the same way. are these fixable?
the one in question asus p4p800.
tia
req


  #2  
Old December 26th 11, 01:53 PM posted to alt.comp.periphs.mainboard.asus
Paul
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Posts: 13,411
Default Is mobo shot?

reqluq wrote:
Hi. I plugged directly into the power without ups, when I got back the
computer was dead. though the green light is on the computer won't start.
The power does fluctuate in the room and two other computers seem to have
gone the same way. are these fixable?
the one in question asus p4p800.
tia
req



On that era of motherboard, there were some failures traceable to the
ICH5 Southbridge chip. It's the chip that doesn't have a heatsink on
top. You can get a burn mark on the chip like this.

http://onfinite.com/libraries/179057/2ea.jpg

Once the chip is burned like that, the motherboard won't start.

*******

You can test a power supply, by connecting PS_ON# to an adjacent COM pin.
Page 37 in the first doc, and page 30 in the second doc, show the position
and color of the PS_ON# wire.

http://www.formfactors.org/developer...public_br2.pdf (24 pin power)

http://www.formfactors.org/developer...X12V_1_3dg.pdf (20 pin power)

You disconnect the power supply from the system, and then test whether the power
supply fan comes on or not. When PS_ON# is at 5V level (open circuit), the
power supply is being requested to be "soft off". And when PS_ON# is connected
to COM (grounded, logic 0), that means the power supply is requested to be "soft on".
The power supply fan should spin if it is working. Strictly speaking, it's nice
to have a small load on the output of the supply while testing it.

The Southbridge and Super I/O chips are part of the power control path. They determine
when PS_ON# is driven. The front panel POWER button is a momentary contact switch,
which sends a "pulse" to the motherboard. The motherboard "saves" the pulse, or latches
is, and in turn, sends a steady logic 0 level on the PS_ON# wire to the supply. That's
how it's supposed to get turned on.

It could be motherboard or it could be power supply, and you'll need to test the
power supply a bit to get some idea what broke. But if the motherboard has that
burn mark on it, there's no doubt what broke.

Paul
 




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