Yay, bought myself a nice 486DX80! It came untested but it works great!

It came on an untested morherboard. Simple to work with and fixed oscillator at 40MHz. Seems fine, except one part: the keyboard connector.

I connect my notsoworthy Logitech black ps/2 keyboard with a standard DIN converter. I van access the bios, adjust what i need and works fine.
When it boots past the post, the keyboard leds for caps and scroll light up and it stops responding.
Reconnect the plug to the converter resolved the problem and it works fine.

When I connect a nice yellowish Safeway ps/2 to the converter, the kB LEDs all start blinking about 4x/sec and the kB won't respond.

When I connect a true DIN keyboard (old without win keys) not a single response, no LEDs.

I suspect there's a problem with the power delivery.

The +5v line measures 5,15v on molex, motherboard connector and din port. The +12v measures 12,29v. Everything connected, cf for hard drive.

I did pour some ipa into the din slot (power disconnected ofc) and played 15 minutes with the din converter in/out (yay for online meetings working from home) so I expect corrosion and dirt to be gone.

Any ideas what else I can try?