Did you check wether the keyboard gets stable +5V?

I would recommend to clear CMOS (I assume you added battery to the board). That capacitor is also a possible culprit however I would unsolder it only after ruling out other causes. I would try to hard reset it a few times when it hangs. Is CPU voltage set correctly? I'm afraid if you want a vintage computer you need to have a spare of everything to troubleshoot these issues. I don't think keyboard controller can die in a computer in storage for one year. If all other causes are ruled out and it still doesn't boot then a voltage spike from an old PSU may have killed it.

Doornkaat wrote on 2021-10-21, 07:12:

Did you check wether the keyboard gets stable +5V?

What do you know, I just tested with another power supply and it worked. So I will check the voltages on the original supply and report back

Are you using only one dimm ? dont you nedd two ? a pair ?

The 486's 32-bit bus only needs a single 72-pin SIMM. Pentium required them in pairs.

I measured the voltages from the 4 pin moltex connectors and they were within range.

I went and tried the original power supply again and lo and behold... keyboard works

So maybe the fault is with the motherboard cap?

Could just have been an iffy power connection.

With 12 months of non-use, connections could have become slightly corroded and not making good contact. Simply reconnecting it (with the same or a different PSU) may have been all it neede.

I’ve noticed when entering the bios setup instead of showing the setup screen in one go it draws the interface line by line (although it only takes a few seconds).