My best guess would be something between the point marked 'SCREEN' and the CRT itself -or- a fault around 'B+200V'. If the picture size stays fairly constant it is unlikely to be a fault to the right side of D411. Unfortunately, we can't see what B+200V feeds but it is probably the video output transistors. Depending on the output stage configuration, it is possible a reduction in the 200V could make the CRT conduct higher current and increase the brightness, probably causing 'crushing' of the contrast (white out) as it does so.
Brian.