Oh now this is getting a bit disappointing.
It was a bit too much to think the tri-state buffers had failed and changing them one by one made no difference. Hmm.
I think that the screen memory is OK because the noise on the screen shows each of the bits both set and cleared so no sticky bits either high or low.
I wrote a noddy program and blew ROM just to clear the video RAM and nothing else. It didn’t fix anything but, on reset it tends to look a bit different. I’m not sure what any of this means.
Sometimes I do wish I could get an old machine and it would just work.
Never mind. More next time.