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Generic new 500W PSU refuses to work with an old HP DC7900 computer

HP Compaq dc7900 Convertible Minitower PC
Microsoft Windows 7 (32-bit)

Hello all,

Came here to try and find a fix to a perplexing problem I've been encountering for the last few days:
Bought a graphics card with a 6 pin connector so needed to switch between two computer's PSUs as just one of them has it.
Anyways, when I tried pairing a one and half years old PSU, the generic Sigma P4-500EW with an old HP Compaq dc7900 Convertible Minitower PC computer, I hit a snag.
For some reason that computer that works fine with the other PSU wouldn't start. All I get are continuous 4 short beeps and the PSU seems to be dead in the water with no fan activity.
I returned that PSU to the original system, everything's fine so the PSU is fine.
So I've got a working HP system with anothr PSU and a working Sigma PSU but just not with the HP system.
I did try taking out and repositioning the HP's 3 memory sticks and resetting the CMOS, to no avail.
Tried it with everything else in the system disconnected (no external GPU). Didn't help.
The motherboard in question is HP 3032H (xu1 processor) with Intel Core 2 QUAD Q9400 CPU.

The BIOS:
Brand Hewlett-Packard
Version 786G1 v01.23
Date 11/12/2009

Saw this paragraph about possible compatibility issues between new PSUs and older MBs:

"You can also have overloading problems sticking a new PSU into an older computer. Most ATX12V 1.3 and earlier PSUs provide enough 3.3 or 5 volt power to run an older motherboard but some newer ATX12V 2.0 and newer supplies have reduced the available power on 3.3 and 5. If you're using a new supply with an older motherboard then it's best to check that it has enough wattage on 3.3 and 5. You may also run into a different kind of trouble related to the -5 volt rail. -5 volt support has been optional in PSUs since ATX12V 1.3 because it's rarely used anymore. It's rarely included in new PSUs. But some old motherboards or ISA expansion cards require -5. So even if motherboards and PSUs from different eras have compatible connectors, you may have problems using them together. New and old ATX PSUs may look the same but what's going on inside is very different."

Could that have anything to do with the problem?

I was thinking of trying to connect an old 230W PSU I extracted from another system to the HP but am weary that it might do some damage to the motherboard.
Any guide to what may be happening here would be most welcomed.

 

 

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