Sean: from the symptoms, a possible scenario is that the EHT regulation has failed. I always dread going into that box!
I'd suggest you open up the back and find / remove F1437. You can then power the EHT section from an external variable PSU (current limit 2A). Power the scope and bring the PSU up from zero, whilst monitoring the -1960V.
If that can be set and the scope display is normal, this might confirm the loss of reguation. Note that if the +75V rail has gone high, this will cause the EHT to rise pro-rata, so check that first.
John
John Sykes