In general a modern PSU should be able to power on with an old ATX board attached. Just a single HDD should draw enough on +12V to provide needed load. You may encounter issues with old cards because ATX 2.x PSUs don't supply -5V anymore, but that will generally just cause trouble to some ISA cards and won't prevent the system from booting.

So if that's not it, what then? First thing to check is if it really is an ATX board. Some OEMs (Dell, Compaq...) used non-standard power supply pinouts with connectors that resembled or were even identical to regular ATX 20-pin connectors. What motherboard do you have exactly? And what is the brand+model number of the old power supply?

If it's not some kind of OEM board, I have a Dual Pentium II 440LX Tyan board which sounds like it had similar problems. It is ATX 1.0, and it would only power on with very specific power supplies; very inconsistently at that, too. Further, the power on switch would never work, but if I toggle the power supply on/off physical switch at the rear of the case about 10 - 25 times, it might...sometimes... eventually power up... like rubbing two sticks together, heh.

This is beyond my technical skills, so here's a quote from the guy who fixed my board:

"...basically ATX 1.0 allows Power Supply 'on' to only need to be pulled to 1.5v. This board {My Tyan} pulls it to 1.1v. Most PSUs for ATX 2 and newer don't like that and want .8v or lower."

He built a microcontroller for my board so that it could convert the voltage for Power Supply 'on' to the correct amount for compatibility with modern power supplies.

Thanks for the responses 😀

To answer the questions, my motherboard is an ASUS P2B - it is a standard ATX board, as far as I am aware.

I don't have any ISA cards in my machine so I am not so worried about the lack of a -5V supply. I have a HDD in the system so you would think that would draw the necessary current on the +12V line to switch the supply on. I can try to add a couple more drives (power only) to see if that is enough to push it over the edge - worth a try.

Interesting note about the "on" voltage. I am assuming he means pulling the voltage down to as close to ground as possible? I can try with a meter to see what my motherboard does. I hadn't heard about this before.

jaskamakkara wrote on 2023-08-18, 09:36:

Interesting note about the "on" voltage. I am assuming he means pulling the voltage down to as close to ground as possible?

I use ATX Power supplies as bench pwer supplies without any load at all... I just ground Pin 14 (Green, PS_ON) in the ATX plug and it starts to output power.

jaskamakkara wrote on 2023-08-18, 09:36:

Thanks for the responses :-) […]
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Thanks for the responses 😀

To answer the questions, my motherboard is an ASUS P2B - it is a standard ATX board, as far as I am aware.

I don't have any ISA cards in my machine so I am not so worried about the lack of a -5V supply. I have a HDD in the system so you would think that would draw the necessary current on the +12V line to switch the supply on. I can try to add a couple more drives (power only) to see if that is enough to push it over the edge - worth a try.

Interesting note about the "on" voltage. I am assuming he means pulling the voltage down to as close to ground as possible? I can try with a meter to see what my motherboard does. I hadn't heard about this before.

I have the SCSI version of this board (P2B-S), and I have had no problems using a modern Antec 380w PSU. Sounds to me there is something wrong with the board itself.