[ralferoo]

The more likely risk with a 6845 is creating a way out of spec vertical sync that can cause the monitor's frame flyback transformer to fail. Although how you describe the PET monitor's beam being "parked" suggests it's actually the same effect.

[greenbit]

I recall seeing it just stop dead center, quite bright. Immediately just turned the machine off, but that did seem to account for why a couple of the other machines in the room had small round-ish phosphors burns right in that spot