A close-up inspection revealed the culprit – a tantalum capacitor on a small voltage regulator board (front left and black).
For me, tantalum capacitors are a complete pain in the ass. They always seem to fail by going short circuit and produce the most horrible smell. They can also cause a LOT of damage to the surrounding PCB if you don’t catch them in time.
Anyway, PCB removed, both capacitors exchanged for standard electrolytics and everything now works a treat.
If you have ever wondered what happens if you don't cut the power in time, the pictures below are from the battery backup board in my Nixdorf 8870 mini computer. "ONE" tantalum failing caused all this damage.
I never did manage to get rid of the smell completely.
Personally, I don’t know why tantalum capacitors don’t have an in-built piece of fuse wire !!!