So I got this 486 board from an IBM Aptiva PC a while ago. This week, I got a ISA POST card i ordered on eBay to further debug this. I'm using an ATX power supply with an ATX-to-AT adapter. When starting up, the POST card shows +5V, +12V and -12V fine (the +3.3V LED stays dark), CLK and FRAME are lit up, RESET is on shortly and then is off, IRDY blinks randomly and then stays off. The number shown is 00AA (I haven't been able to figure out what that would mean, but the missing +3.3V is a starting point?). Also, no beeps from the PC speaker. Removing and reinserting the RAM doesn't really help. I've re-seated the CPU too just to be sure. FWIW, there's no drives (floppy, hard drive) connected, but I assume that it should at least get to VGA output without those?

The board has "65G4187" written on it (see photos). I haven't been able to figure out what the 00AA code means, googling for "IBM BIOS post code AA" didn't reveal anything.

Interestingly, sometimes if I insert the coin cell battery or disconnect the VGA or keyboard connector, the "RESET" LED keeps on (I haven't found a pattern there yet, so maybe that's just a red herring).

Any hints where I should start looking to fix the issue? Is the missing +3.3V the first problem I should solve? Where would I start measuring voltages? Should I just try replacing the capacitors? All of them? Only some (which ones)?