Super! Once again, nice going. That is really interesting. Particularly the sort of repeating pattern for 2mS and 5mS. I will re-watch this a few times.
So, we've isolated down to the block level for sure... and at least a subset of circuits at least in my view --- I still think the trace is telling us that the bottom 'half' of the vertical amp is wonky.
Ideally we'd want to try and isolate the various circuits within but not sure if that is going to be fruitful. Life is a bit harder since the svc manual doesn't show proper waveforms at these points.
Btw, do you have access to another scope??
So anyway I think it'd be worth testing a couple of key resistors and diodes; and also the transistors in the bottom half of the vertical amp (I am assuming, btw, that Y- corresponds to the lower half of the screen...)
Please be safe!! Ideally, the power supply caps should be discharged and you should try to work with one hand and the other behind your back (at least that is what they taught us in EE lab 15 yrs ago
)
So, the two resistors are the ones on the right of the ckt diagram that are inline with the Y plates and in parallel with a couple of inductors. They are 680Ω each. They are 3W -- fairly large and easy to find. IIRC you will find them and the two TO-220 power transistors on the left side of the board, midway between front and back panel. To help orient yourself -- the power supply caps are back left. The high voltage section encased in aluminum cage is back right. You can check the service manual for help on board layout, too. And the component IDs (e.g., Q14) are labeled on the board, too.
Beyond that I would check diodes next. It's easy so why not test them all. I'm assuming you have a diode tester on your DMM and that you know how to test diodes... if not lemme know.
D2,3,4,5 (the circuit diagram shows D14 where I'd expect to see D4 -- this is probably a typo).
D6,7,8,9.
D10,11,14.
Am I missing any?
After that I'd do a diode test on the transistors (do you know how to do that for NPN versus PNP?)
Q6,8,10,12,14,16,18,20
Not sure all of the diodes and transistors will test right in circuit, though. If not, make a note of them and we'll think about removing them and testing out of circuit.
Good luck!
Michael