new psu not working with older motherboard

Aug 17, 2018
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Hello, I have an issue with an older motherboard not working with a newer power supply.
The new supply is to replace an aging 20-pin atx power supply (generic brand hardware from 2002 I believe) with a modern 24-pin variety.

The old supply still works with the old computer, and the newer supply works with a more modern computer.

With the old board and new supply connected, when i hit power the cpu fan runs, some activity is present on the board, but the system doesn't POST.

Any suggestions?
 
Aug 17, 2018
2
0
10
I completely missed the forum reply notification in my inbox; sorry about that!
Thanks for the response, this computer has no discernible make or model, so I believe it was custom built.
The motherboard is an AOpen AX4B-533, and the original PSU is a Power man IP-S350CQ2-0 H
The new power supply I am trying to use is a Seasonic SS-660KM Active PFC F3

From what I can tell there isn't any obvious difference between the two PSU pinouts

(thanks again for replying, I'll watch my inbox more closely!)

--edit--

So I had another look and some behaviour differences are the Seasonic supply will turn the computer on as soon as the switch on the back of the supply is turned on, and the pc power switch doesn't appear to do anything.

The power switch works normally when the old powerman supply is connected.

The other difference I noticed is that the "power good" (gray wire) signal for the seasonic goes to 5V 256ms after the +5V rail goes to nominal voltage, where with the powerman supply it takes 290ms.