Workaround needed for Mobo and ATX PSU .

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I am building a mediacenter computer with an Antec OvertureII case and Asrock 775Twins-HDTV mobo, but my surprise comes when i push the power on button, and nothing comes out. After a couple of tests, i realized that the problem is caused by the Antec psu,( with other psu the mobo starts ok, and this psu works ok with other mobo ,so.... I think that the motherboard checks the -5V signal, but the SP450 ATX v2.0 Antec PS does not have that voltage(there is only 19 pin in the 20 atx connector).

Any ideas?.

Thanks!.
 
Are you sure your motherboard needs that voltage? It's my understanding that - 5v is a nearly obsolete PC voltage and many newer power supplies no longer include it. That said, if yours does, I see no alternative but to replace the Antec with on that has - 5v.
 
To mailup.
Im not sure, but the point is that the mobo with that psu does not start (no video,beeps,nothing.Also if you press the power on button for 4 seconds or more, it does not stop.)
With an nonbranded 450W PSU, the mobo starts fine. I am going crazy with that. The problem now is that the OvertureII case, has an special issue with the psu, the ac plug is in the same side of the dc outputs, So changing the psu is not possible.
I was thinking to short the missing pin in the atx connector with gnd signal or -12v, but i need some advice before frying the mobo.
 
I don't know but shorting it doesn't sound like a good idea. Different motherboard perhaps?
 
The proper way to do it would be to get a voltage regulator and use that to convert -12V to -5V. You should be able to pick up a MAX664 chip or something similar for ~$10 in your local electronics shop.
 
Thanks everybody, but i give up. Still waiting answer from asrock (taiwan and european headquarters, but seems that's nobody home) so I am changing the mobo ( and the brand, of course!).
 
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