Check the GI connector on the power supply. I'll bet dollars to donuts it's burnt. I've recently repaired a system 9, 2 system 11's and a data east game all with the same issue. It should have 4 yellow wires, and 4 other colors, 9 pin connector total with one pin as the key.
http://pinwiki.com/wiki/index.php?title=Williams_System_9_-_11#General_Illumination_Issues