Switch maintenance by type
Microswitches need no cleaning; adjust them by loosening the two screws and turning the switch, never by bending the arm, and do not overtighten. Leaf switches: draw a business card through the gold contacts, then reset the gap. Optos: clean both lenses with a cotton swab dipped in glass cleaner.
Systems
Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Three, Clay Harrell