Thank you very much. I guess somebody replaced it. At least, the wima caps in the PSU and the tantalums in place of A5 electrolytics seem to point to this scope being serviced after it's production.
Sorry if I only post questions and not any progress or effort on my part, I'm trying to understand at least superficially the scope via the service manual, but I have very little time right now (new job, lots of extra hours to learn).
Anyway, I've been reading parts of the service manual, and provided that:
1. The scope works well as long as the readout is off, so the normal signal path is OK.
2. The "intruder" waveform varies normally in the X axis as the timebase and sweep vary, so it doesn't seem to effect the horizontal stuff.
3. The depicted amplitude is, visually, always the same. The volts/division knob doesn't have any effect, I always see the same 1/2 division amplitude in the CRT, so the noise is being injected like that directly, either to the Vertical Channel Switch or the Vertical Amplifier.
4. Channels 3 and 4 don't depict the alien waveform whether the readout is on or not, however, the readout values (timebase and volts/div) are still on top of screen instead of bottom.
The problem should be either between the readout and the vertical channel switch (Readout Vertical Out), or somewhere betwerr the VCS and the VA. Maybe there are more options, but that looked like a good place to start. So I have been probing around the READOUT VERT OUT line (figure 6) and, voilà, R480 (78,7KOhms,) between the 15V rail and the Diff Amp U475B failed open. I made a quick patch with a 80KOhm resistor to check, and powered it up just enough time to see the readout and cursors, and it seems to work well enough: depicts frequency, no strange waveforms, and it doesn't shift up out of screen when I click the "auto" button.
So, before I power it up again, I will buy some 0.1% resistors of the appropriate values to substitute R480 and, maybe, R490, and also list and replace all components affected by the liquid damage, or whatever it was.
I guess I will have to remove the A1 board. Will be reading the manual about that but, does anybody have any tip, video or post about the process? I suppose it is not a very good idea to do the component swap from the top.