With 5V CPUs all that can go wrong really is core specific features (for cyrix as that what the design is) won't be enabled, and POST screen might give you a strange output in the CPU line. ... if you plug them in at the intel dx2 5V settings or DX at right bus speed settings... with 3.x V CPUS and dx4 class, more can go wrong with multipliers and caching scheme needing to be set right, power features enabled ("S" CPUs) and so on.

If you haven't already you should remove the barrel battery asap.

@BitWrangler thanks for reply, not sure if I understood correctly. If I set jumpers to match intel 486dx2-66 bus and core speeds, it should be safe to stick st486-66 there?

@Doornkaat thanks, I already ordered button style battery kit, friend has soldering station and we plan to make it right. The barrel battery has leaked already and there is some corrosion in vicinity, but old guy still kicking

This is the motherboard in the end. My former identification was incorrect. Please have a look again.
But seems to me, that with this board configuration, it is even more straightforward.

Both CPUs are happy with 5V so you won't break anything as long as you insert the CPU correctly.
There is a chance of incompatibility with Cyrix' CPU designs though. (ST manufactured Cyrix' designs and put their own label on some of the CPUs, so your ST CPU is actually a Cyrix core.)
If that's the case you could try and see if you can just run your original DX2-50 @33MHz FSB (i.e. with the settings for a DX2-66). This won't break anything either. Worst case your system won't be stable.

Fingers crossed one method will work for you! Have fun!

Thank you!

Doornkaat wrote on 2021-12-13, 14:04:

If that's the case you could try and see if you can just run your original DX2-50 @33MHz FSB (i.e. with the settings for a DX2-66). This won't break anything either. Worst case your system won't be stable.

By the way, does this mean that it was just a matter of binning process even back then? That technically speaking it is the same chip, it's just not guaranteed to be stable at higher clocks?

Yup, and at one point they were binning for not needing heatsink with case airflow, some DX2-66 will do that, but it's preferable to have one on for stability, so DX2-50 at 66 probably just needs a heatsink to be fine.

Just to get back to you, I managed to run original dx2-50 with board jumpers set as if it was dx2-66, put a heat sink on it to make it more comfortable and it just works. Doom runs noticeably better. Thanks again!