I know nothing about this scope, but looking at your pics there, there's some crusty solder joints that look like they need attention.
*** Safety warning for any newbies - Make sure it's all unplugged, and all the capacitors discharged, before working in side. ***
I'd clean and resolder a few joints there, especially R816 R815, C695 doesn't look good, and while cleaning it, it may be worth popping the cap out and checking it. Connection points 7 and 8 definitely need a resolder, The top joint of R786 is questionable, so i'd do both ends of that. C696 might be questionable, and worth redoing, and maybe testing while it's desoldered. C694, same deal. R788, doesn't look bad, but I think you should redo it's joints. I know I mentioned connections 7 and 8, but looking at it, 1-6 and 9 need redoing really. C700 needs checking and resoldered. D642 and D643, it would do no harm to resolder them. And C697 - C699, I'd redo them, testing them as well while they're desoldered. R812 needs a resolder, and I think D644 and D645 would benefit too.
It reminds me of the days of old CRT TVs, and their many dry joints.
I'd just go around that board, and anything that looks crusty, clean off the solder, and put some fresh stuff in it's place. I think it might make the world of difference.
It would do no harm to also do the same on the CRT neck PCB, but nothing really looks bad there. Again, make sure it's all discharged before working on it.
I'm not up on all the safe ways of working on these things, can someone else pitch in here? (just to save anyone reading from blowing themselves across the room).