Little confused. Are you talking about negative five volts? This is usually only needed on some isa sound cards.

Changing to a psu that has it is the easiest way.

Other options are voltage blaster (everyone’s favorite auction site), or pico psu adapter (see sig)

I haven’t heard of the issue affecting pci cards though, I’m trying to remember if -5v is even on pci? I know -12v is.

Edit:

https://pinoutguide.com/Slots/PCI_pinout.shtml

Don’t see -5v unless I am blind.

Your problem may be lack of 3.3v

In which case, see the thread “adding 3.3v to pci”

No, +5v to PCI. There is only 4.12-4.19v, but on other board - 4.93-4.95v.

Change your psu. That voltage is probably directly from the psu.

See my edit.

Your woes are likely from incompatible pci version or lack of 3.3v on pci slots. Some bios settings can affect compatibility sometimes.

GoldenPentium wrote on 2023-07-05, 01:26:

it correctly works with other motherboards, too. Another one observation - 5v wires (left 3 if look on top) are hot - not critical, I guess, but I didn't notice the same on 486 board, with the same ATX-AT adapter.

Where should I look to uncover that mistery?

Thank you.

Are some of the Mainboard psu connector pins corroded by any chance ? That would easily explain the observed behaviour. voltage drop and heating up of wires.
Since you mention the psu working on another mainboard, I exclude the psu connector. Though heating up of the connection tends to be a bad thing. you might want to inspect the psu connector as well for any possible damage to the pins.

If the voltage is drooping and the plug is hot (= lots of current being pulled through it) something has probably shorted or very close to it. Disconnect the PSU, measure resistance between 5V and GND on the motherboard.

jmarsh wrote on 2023-07-05, 04:26:

If the voltage is drooping and the plug is hot (= lots of current being pulled through it) something has probably shorted or very close to it. Disconnect the PSU, measure resistance between 5V and GND on the motherboard.

Might be crappy psu with tiny guage wires?

The computer wouldn’t boot at all if there was a dead short on the 5v

I suspect the network card problem may be a separate issue. Probably pci version, or lack of 3.3v

The 5v behavior observed is probably from a lot of cards being installed and a cheap psu.

Time will tell, OP will let us know by thread’s end 😀

Thin wires could be the problem, but in the mentioned ATX to AT adapter instead of the PSU itself. I've even seen wires made of unknown high-resistance metal that was copper-colored all the way through (not CCA - the resistance was too high for even aluminum). That Pentium MMX will draw more power than a 486, and using a linear regulator for the VRM means that the 5V current will be higher than a socket 7 motherboard with a switching VRM.

It depends on where you're measuring 5V. If you probe the AT power connector (assuming you have thin enough multimeter probes to fit in the wire end of the connector) and get the correct 5V, then the problem is in the connector. If you're still only getting 4V, then it's the adapter.