It’s normal for there to be a red light on the output optical port.
The AES/EBU ports use the same circuitry as the optical ports so I would think that if there is a fault, it will apply to all.
As for the digital part not working… I don’t have any spares for those, or any circuit information on that particular digital sub-board. But things have moved on since then! I would suggest using an outboard digital system…… even the very inexpensive ones are as good as the ones we were using in 97/98 when the work on the SC3 was done. I have been using some digital converters sold by ‘Lindy’ recently and they are remarkably good.
I was checking out an SC3 last week and that’s a very nice compressor!