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Looks like I need tro replace the Power Supply on one of the machines, but I am confused with the plugs.

The MB (ASROCK 939N68PV-GLAN) has a 24 pin connector and a separate 4 pin (2x2) header.

The original PSU had a 24 pin header and the 4 pin. The 4 pin seems to be required (PC doesn't reach POST if not connected).

In what scenario do I need both the 24 plug and the 4 plug?

Some more info:

The board originally ran with a PCI Express card (ASUS EN8600GT SILENT). the 2x2 plug seems to be close to being fritzed (discolored, probably to much current). The problems observed were PC not reaching POST on boot, and PCI Express card not detected. Other than that, PC is rock stable.

The original PSU seems to boot ok when using only onboard graphics (I don't want to push my luck, though). I currently only have 20+4 replacement PSU's available.

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up vote 3 down vote accepted

The additional 2x2 standalone connector provides 12V DC power to the CPU voltage regulator and is required on all boards that have that connector. The extra 4 pins added to the 20 (also known as the 20+4) were added to support more power-hungry processors and were originally only needed if your processor needed it. Now it is always needed with any modern chip.

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So it's not the same four plugs in different places. – peterchen Oct 1 '09 at 15:27
That's correct. – MDMarra Oct 1 '09 at 22:34

You seem to be missing the greater question: Does replacement power supply have sufficient amps on each rail (3.3, 5v, 12v) to power this system?

Anyway, to answer your question:

In what scenario do I need both the 24 plug and the 4 plug?

You need them for motherboards that require both. Not all M/B need the 4 pin plug.

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I don't think the OP is missing anything. His question is specific enough. – MDMarra Sep 30 '09 at 1:51
The question is missing any data on the capabilities of either old or replacement P/S versus needs of the system. – kmarsh Sep 30 '09 at 12:30
Sorry, your reply is certainly correct, but not really helpful. – peterchen Oct 1 '09 at 15:26
It is intended to be helpful in that, when mix-matching old and new equipment, there is more to look at than plugs fitting. – kmarsh Oct 1 '09 at 17:26

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