PPS: when you run the suggested tests above ...
Now that we're really diving in, whenever you film the trace behaving a particular way, leave the position alone and film the trace for say 10-20 seconds. That will help me see exactly what the trace looks like without any additional effects from the position knob.
To set up the scope---
1. Make sure you have the time/div knob set to where you can clearly see the square wave -- it's period should be taking up about 2 of the major divisions on the screen.
2. Set the position and volts/div so that the trace is taking up 1/3 to 1/2 of the screen and showing a normal square wave.
3. Set it to trigger off of INT (internal, ie, trigger off the waveform being displayed) and using the AUTO trigger to make sure the square wave looks 'still' on the screen.
Sorry if you know how to do all this already. I forgot to ask how much you know about using 'scopes.
When you film, see if you can reposition the trace and film 20 seconds EACH of:
- the trace in the top half where it looks like a normal square wave
- the trace on the lower part of the screen where it is all wonky
- a position on the screen where the top half of the square wave is ok and the bottom half is wonky (adjust volts/div to have the trace take up the whole screen if you want)
I'm hoping this will further verify that the problem is in the vertical amp Y- plate driver circuitry. That'll narrow us down from block to circuit (following the methodology of that troubleshooting manual)
After you do these tests I have another test I want us to do, to see if we can get a clearer picture of what the trace looks like and maybe get a sense of what components might be failing. Since the position of the beam on the screen is telling you precisely what the voltages are on the Y+/Y-, X+/X- plates we will take advantage of that to see if we can get any sense of what component(s) might be failing.
Michael