Update.

The board boots up if I remove the transistor and feed 3.3V to the CPU directly.

So definitely something wrong with the CPU voltage supply.

A typical NPN Power transistor, pdf here https://datasheet.octopart.com/2SC3420-Inchan … et-15978886.pdf

I'd do the diode test with the multimeter to test the transistor, remove it if You have to. If the base is ever lower than the emitter then the
transistor isn't even being turned on. Nothing funny on the other side of the board?
The fact that the Power rail on the collector doesn't change indicates no problems from the collector side, maybe look at the base to emitter
side of the transistor and get a manual for the board from TH99 and make sure those jumpers are in the right place.

Thanks imperius

I had the transistor out already to inject an external 3.3V anyways. I tested it and indeed I measure a voltage drop of 1.9V between collector and emitter - so I ordered a new one!

I'll keep you posted!

The new transistor did the trick! It was curious that when I went to test it again the following day it was NOT showing that 1.8/1.9V across E and C.

I have made a repair video which is now on youtube: https://youtu.be/jwChQct2ghc

(I am not a professional youtuber and I do not make any money out of it!)

Thanks again for your help!

Awesome news! I always like to see successful repairs. I'll check out the youtube video tomorrow.

Watched youtube video good job, I didn't get bored while watching so don't criticize yourself.
Here is full manual i believe:(Your hard copy has missing pages.)Also attached to post.
https://www.elhvb.com/webhq/models/486pci/m921.htm
I also liked your CPU testing SIS 486G board that you repaired before.What was it repair story?
I really like old 486-386 motherboard repairs.

hey thanks a lot for your comment! @whitepawn

Good question about the SIS 486G board - I do remember I had to add a 3.3V regulator as the board was not featuring one. I posted some questions here - and the community was very helpful as usual.
Adding 3.3V regulator to a cost reduced 486 motherboard

I have the feeling the issue was just some corroded chip sockets - but I cannot remember for sure, that was before my (very slow and amateurial!) youtube career!

I am currently repairing another 386 board - which you guys are helping me with here Keyboard crashes on old 386 motherboard - which should become a video at some point. I will surely mention Vogons!

Thanks again - and also for the manual!