"If I have time tomorrow, I'll just temporarily connect the new caps in
place and see what effect that has on the scope's performance and the -100V
That would be a good plan.
The caps of this kind can be shorted, but the tests and the fact that you
get -80V would indicate that they are not shorted or low resistance.
The capacitors can have low uF and/or high ESR, Placing a new caps in
parallel will solve both those problems.
As mentioned if the old caps have high leakage, the connecting just the new
cap will eliminate that issue.
At that point you can determine if the that fixes all the issues.
If there is still other issues I would locate those issues, and then make a
plan to fix all the issues, before "restoring" the caps to reduce the risk
of collateral damage or redoing work to get to other parts.
(I actually, in most cases on a cap like this I just solder the new cap to
the terminals on the bottom of old one and leave the old one in. But that is
just my personal choice)
Thanks for the update