Just finished fixing a System 80B score display board in a Gottlieb "Chicago Cubs" machine. The lower score display had "garbage" (nonsense) characters all the way across the display glass. I found chip U2 (Rockwell # 10939) which controls the lower display to be bad. I replaced it with a new one and that cured the problem. I also installed a 40 pin socket for the new chip.
If you should have the same symptom, except that it is happening on the upper display, then chip U1 (also a 10939) is bad. U1 handles the upper score display.
I've added this information to the System 80 repair log at pinwiki since there is very little service information about the 80B displays.