How compatible are ATX12V v2.0 PSUs with older systems

[kesv]

I have a system as follows:
Athlon XP 1800+
QDI Kudoz 7E/333 (Via KT333 chipset)
ATI 9100 (passive)
SP80 harddrive
DVD-RW drive

This combination doesn't draw much power. I've measured the typical AC powerdraw to be 145W at full load, which should make around 95W of component powerdraw if we assume 65% efficiency for the old PSU.

Now I've got an Antec Smart Power 2.0 400W and the system will not post with this PSU. It does light the standby led on the board and when I press the power switch the case fans turn on and the DVD drive does its power on check. However that's all that happens. No post, not even an error beep. Switching back the old PSU makes everything work again.

This leaves me wondering what the cause is. I can think of a few possibilities:
- Faulty PSU unit. Although it does seem that atleast parts of the circuits are powered correctly since the 5VSB, casefans through the motherboard and DVD drive from molex power up and down normally.
- Some protection circuit kicking in although I fail to see which it could be. I'd also expect the PSU to power down if this would be the case, but it just sits there until I force a poweroff by holding the power switch down.
- The MB requires -5V rail. This could maybe be, but this MB doesn't have ISA slots anymore and the BIOS 'PC health' screen doesn't have an entry for -5V either.
- General incompatibility between MB and PSU. I suppose this is possible, but the observed behaviour doesn't quite seem to match what anyone else is experiencing.

Any comments or ideas ? I'll have to experiment some more.

Update 2005-11-21:

I took an even older system, an IBM Aptiva Pentium3 system with a 150W MicroATX PSU and plugged the Antec Smart Power into it. Worked splendidly. Interestingly the MicroATX PSU didn't have the -5V lead either so we know this system doesn't require it to work.

Obviously this proves that the PSU works with some quite old systems without any problems. This leads me to the hypothesis that I just happen to have a motherboard that needs the -5V lead to work correctly.

As a sidenote, there is an interesting behavior of the Antec Smart Power when in standby and 'power off' modes. With 'power off' I mean the mode where power is switched off from the switch in the back of the PSU. It seems the PSU draws 8W in 'power off' mode and 21W in standby mode. The old Enermax PSU draws 0W and 10W in the corresponding modes. The 21W vs. 10W can sort of be explained by differences in the power conversion circuits, but I wonder why the PSU draws 8W of power when supposedly 'switched off'.