On Wed, 28 Oct 2009 10:18:59 -0700 (PDT), Al put
finger to keyboard and composed:

On Oct 28, 8:55*am, UCLAN wrote:
Al wrote:
I have got a mobo here that is dead. I have done all the usual stuff
like trying new PSU, bench testing etc. Its dead - as in no fans, no
nothing when you hit PWR button - PSU doesnt even come on.

I connected the ATX molex from the PSU and shorted Green (PWR ON) with
GND on the molex and the PC came on. All seemed fine.

I think there is a IC on the motherboard between the PWR BTN headers
from the power on switch and the ATX header on the mobo that tells PSU
to start up.

The MB circuit only tells the enable circuit inside the PSU to let the
PSU turn on. To what voltage does the PWR_ON (green wire) fall when you
push the ON/OFF switch? The MB must pull this wire lower than 1.5v for
the PSU to turn on. I have seen enable logic failure modes where, for
example, 1.2v would not turn the PSU on, but 1.0v would. Check that
voltage level before replacing any ICs on the MB.

When I hit the pwr button that voltage level doesnt change.....(and
the switch is fine)

What are the standby voltages at the on/off pins on the motherboard
header? If there is no +3.3VSB or +5VSB, I would check the LD1117
linear regulator near the ATX Molex connector.

FWIW, my old socket 7 motherboard uses an SiS 5597/5598 all-in-one
chipset. The chip has an ONCTL# output pin which controls the PSU's
PS_ON input, and a PWRBT# input pin which appears to connect directly
to the on/off "power button". This is debounced internally by the
chipset by means of a 30ms delay. The 4 second off timer is also
handled internally by the chipset.

SiS5597, Silicon Integrated System, Pentium PCI/ISA Chipset:
http://www.datasheetarchive.com/pdf-...DSA-594853.pdf

- Franc Zabkar
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