Some possibilties for the very bright trace:

1. One pole of the beam finder (S450B on schematic <12>) is stuck

2. The grid bias R1375 might need adjustment, possibly someone tried to compensate for the previously weak display.

3. If the brightness control has no effect then maybe there is a fault in the Z axis amplifier.

Your measurements on the Y plates show there is a fault in the vertical amplifier, it may have gone into oscillation.

Roger