starting from rocker switch

mike091 wrote:
> Hi,
> I have a strange problem concerning a PC I have just put together.
> First of it starts as soon as the power cable is connected and rocker
> switch is on, front power button has no effect. The fans start and you
> can hear the hard drive but there is no output to the monitor.
>
> One thing that concerns me is the motherboard is a Asus P5VDC-X with a
> 24 pin ATX power on the board yet the supply has 20 pin connector with
> a separate 4 pin 12v connector that I've put in the other motherboard
> connection. However this maybe of no consequence.
>
> Could this be an earthing problem? The power was cutting out until I
> tightened the fixings on the board.
>
> Thanks for an advise,
>
> Michael.
>

Starting immediately is not normal. I've experienced it here, when
one of my IDE cables was only half installed. It could be that
the stress caused by whatever connections were being made, caused
the logic feeding the PS_ON# signal to be put in the ON state.
So something could be pulling down one of the Southbridge local
voltage rails, and causing the logic to malfunction.

You could try placing the motherboard on a table. Connect the
PSU to it. Connect a power button to the PANEL header. See if
the bare motherboard is able to properly switch the PSU on and
off that way.

Running a 24 pin motherboard with a 20 pin power supply is OK.
The issue, is whether the loading of the motherboard is too
much for the 20 pin connector. If you had an SLI video configuration
(two video cards), the answer would be yes, that would be too
much current. But if you have just one video card, most
video cards are designed to limit PCI Express slot current
to about 4A or so. The single 12v wire on the 20 pin power connector
is rated at 6A. So the 20 pin connector is sufficient if the only
PCI Express card is a video card, and there are no other extraordinary
loads. (Note that the motherboard fan headers also draw
current from +12V, which is another 0.5A or so.)

If testing the bare motherboard, still results in the power coming
on before the button is pressed, and the motherboard is sitting
on a nice insulating surface, then you should probably
return the motherboard for warranty repair.

Paul
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