Problem with Heathkit IO-102 oscilloscope - vertical deflection is same regardless of voltage connected

I have a Heathkit IO-102 oscilloscope - a modern marvel of electronic engineering! However my particular scope has a problem. It may have been caused by me applying a voltage that was too high for the settings (don’t know whether they’re susceptible to that) whilst I was figuring out how to use it (I had trouble finding the trace initially and didn’t know about the 1V-PP output) or it may be something else entirely.

The problem is that regardless of the voltage applied, the deflection is 1.5 of the (smallest) subdivisions, in the wrong direction (red to positive gives a downward deflection). I get the same deflection with 3.8 volts or 9 volts with the vertical set to 1x DC and maximum gain.

I opened it up (had been unplugged for over a month) and I see no burned or burst components which is about the extent of my electronics troubleshooting skills. I paid particular attention to the vertical gain board.

I took the extended cover off the back of the CRT and inspected the wires there. Perhaps it was always like this and is of no importance, but I noticed that one of the vertical amplifier wires connected to the tube was a bit bare (purple demarcation). I used my multimeter to determine that the resistance between the two wires was 11K ohm (right green demarcation). The resistance between the two wires from the horizontal amplifier is >1M ohm (left green demarcation). Maybe this is of no relevance whatsoever?

I have the schematic here: - has anyone any idea of what I could try checking?