What does your service manual contain ? Mine is in French, there are around 15 pages about operation, then 20 pages about how the scope works and all the rest are the schematics of various blocks on A3 sheets.
Low voltage PSU is good, everything is spot on and very low ripple, it's the first thing I checked.
I probed directly X and Y deflection signals and they were exactly what one would expect. I couldn't probe the wenhelt control voltage directly since it's riding on -1kV DC, so I probed just before the DC is injected (CR27-CR28) and it doesn't look good. On the attached oscillogram the X control voltage is shown in yellow, we can see a nice ramp followed by a blanking period. In blue it's supposed to be a 25 kHz carrier modulated by the blanking signal but this carrier always stays at the blanked amplitude.
To confirm that I have a blanking problem, I played with the cathode bias pot until I forcibly unblanked the CRT and the waveform finally appeared on the screen
It means the CRT and the HV circuit are working fine. I took out the tube for nothing
Now I only have to find why the Z circuit stays blanked.