[Boater Sam]

Had some time on this board yesterday.

Coupled a 20v transformer and sorted out the PG1992 diode, stole a good one from a scrap box.

Sadly I found that though the 12v rail is OK at 12.2v and the diode bridge is all good, the 5v rail is at 25v !
There is no regulation at all. All I can now presume is that either or both chips are duff as all the discrete components test out OK.
Hence with an O/C TIP 31 it explains why the 100 ohm resistor would fry when any load was put on.
My previous measurement of the 5v rail was obviously incorrect, perhaps I was on the -5v rail.

Or am I missing something?

I know nothing of the UA723C or the TL331 chips other than they are hard to source in dil package and that they seem so expensive that I may well be wasting my time trying to go any further with the repair, which is sad as TI99/4As are now a bit thin on the ground.

Unless I can find some chips at a reasonable price I may have to admit defeat.

Sam.