Hi
I have a 6M and it started behaving badly - the MDI screen started to flicker and then to go to garbage.
The problem appears digital (not the CRT)

Power supply is OK - 5V is stable (added some extra caps just in case) 24V, +-15 also OK.
Replaced the A16B-1200-0310 monitor board - no change
Replaced the main board A16B-1000-0030 - also no change
The system does not have a character generator board A20B-0008-0430 but does have a PC model B A20B-0008-0440 board - does this perform the same task - this has no cable attached but presumably sends the CRT pattern over the address/data lines ??
The service manual I have is not quite the one for the machine - the CRT interface should either be a A16B-1200-0170 or the A20B-0008-0430