Look at the sprite RAM on the video board, you either have 2 banks of 4 2147 each or 2 banks of 1 2148. Look at the activity on those IC pins and check for GND shorts. You may also check the associated ICs as shown in the schematics as well as RM5. The IC at 5a is not ls157, the schematics should read 4a. A logic probe will work to find faults but a logic comparator makes things easier.