The Z axis amplifier is kinda an op amp in the sense that it is a global feedback amplifier. It does work correctly, both with manually provided inputs and when connected to the Z axis logic.
The intensity pot isn't hooked up backwards.
In the mean time I also checked whether all connections between the tube socket and the pin header in the HV unit are correct, but, unfortunately, they all are correct.
Another possibility is that someone inadvertently changed the circuit duringThis seems most likely at this point, however there are very few spots in the scope that have been reworked before. Some are on the HV unit (the transformer and the thick film network where exchanged or reworked), but not in that area. I also can't see any changes in parts, except that some European parts have been used in non-critical parts of the circuit (e.g. BZX series Zeners instead of whatever Tek Beaverton specified, I guess this is normal for a Heerenveen-built scope).
I should try to probe the grid and cathode voltages again and see if this is really an issue with the HV unit. Maybe it works correctly after all and there is some strange fault in the tube itself? --- I'd prefer a fault in the HV unit, though.