Clean the contacts of all the CPUs and see if the Slot 1 slots have any dirt in them. It might also be another common component or even the bench you used to test your stuff on. Also clean the RAM contacts and/or try different RAM (in different memory slots if the board allows this). Or try without graphics card or switch from AGP to PCI or the reverse.
Or try without CPU and/or without any RAM.

Attach a PC speaker.

Try another wall outlet, different power cable, anything that was similar when testing your different stuffthingies.

Already all done... only the PSU is the same. And all three different mobos have completely the same problem. Start but do nothing. All have different CPUs.

Pics?

Could you post your exact hardware? (especially the motherboards)

Have you removed any of the coin cell batteries for extended amount of time?

Are they jumpered correctly? Were they stored together?

Have you tried that PSU with any other hardware since you tried out your Slot 1 systems?

kixs wrote:

All have different CPUs.

Have you tried swapping CPUs around?
Just to be clear, are all boards removed from their cases?

And you might want to try a different PSU anyway.

Might be a bios problem, are the bios chips intact?

Today I'll try with another ATX PSU.

Changed the PSU and Soyo SY-6BA+ is finally posting 😀 - but then again it also POSTs with previous one... no idea what's going on. Capacitors look good.

ASUS P2B is like dead 🙁 Fan is spinning and that's all - CPU is good as it works in Soyo board. It did work the last time I tested it around 2 years ago. And even looks like new...

kixs wrote:

Changed the PSU and Soyo SY-6BA+ is finally posting 😀 - but then again it also POSTs with previous one... no idea what's going on. Capacitors look good.

ASUS P2B is like dead 🙁 Fan is spinning and that's all - CPU is good as it works in Soyo board. It did work the last time I tested it around 2 years ago. And even looks like new...

Good news! 😀

Is the CPU you tested with the P2B a Coppermine? Because not all P2Bs actually support that (cut-off point is around version 1.10, of which some support Coppermine and some don't).
Try it with a Deschutes or a Klamath just to be sure it supports it and clean the contacts of the CPU before installing it, it may need more than a single reseat and sometimes it's difficult to notice a Slot 1 CPU is actually not fully seated.

The CPU is Celeron 333/66. I'll try again later...