I have an old Telequipment D54 oscilloscope. I rarely use it nowadays, but it's a big help when I actually need it.
When I turned it on yesterday, I couldn't see a trace. All I got was a very slight fog if I turned up brightness all the way. Fortunately, I have a manual, with circuit diagrams and nice detailed descriptions inside.
I saw the CRT was blanked, and the sweep generator wasn't running. Even "EXT X" could not be used. The sweep-gating bistable was the cause. When I unplugged the transistors to test them they were fine, and when I put them back, everything worked. I guess it was just a bad connection in a transistor socket. So, it's not a very exciting fix, but here are some interesting photos of the insides: