It IS HV shutdown on brights or more accurate, current overdrawn through sum total of 3 CRTs. Triggers the SCR (Q903) There is network of 2 resistors and two caps, transistor and the divided voltage goes through a zener to the protection circuit. That circuit takes off the 6.3V heater and 240V cathode taps on the flyback.
Funny, when I narrowed down to green CRT, it didn't have diagonal lines even at max, just reduce the pot on that CRT stops the shutdown but the greyscale is bad. Focus is staying good. But the size of picture still increase if I turn up the contrast via user control.
As a check I did this reduction on a blue CRT and leave the pot stock on Green, it doesn't trigger as much compared to green CRT that needed much reduction.
This is bit diffcult because the sum of current through three CRT at defaults triggers but pull one of CRT board and won't trigger the shutdown. No coolant leak.
What now?
Cheers, Wizard