Having (by coincidence) had a look at the schematics recently for this, it might be worth checking what the display HT voltage value is. You can check this between earth (the case) and the test point marked TP1 on the photo. Select a scale that can reach 60V DC.

If the voltage is low ( < 50V) then you will have a dimmer than normal display, hence why it might not be readable even at 100% intensity.