In the general illumination circuit there are only two switches in that 6 volt circuit. This would be at the lock relay switches. Check the following:

Lock relay switches (2) - yellow/brown wire and blue/red wire
6 volt side - 15 amp fuse and fuse block
Transformer lug connection - yellow wire

There could be an issue on the 50 volt side where something is causing the lock relay coil to de-energize. When you're closing one of the playfield switches, check to see if the lock relay is still being energized.

On the 50 volt side, three switches control power to the lock relay. Check the switch on the lock relay with the yellow wire and white/green wire. Make sure the switch is clean and adjusted properly.

Also, check the delay relay switch with the black/red wire and black wire...