Get the game's schematic before starting
EM makers did not publish repair manuals; most games only have a schematic, and 1970s games often also came with an operation booklet covering resets, relays and game-specific parts. A systematic clean-up fixes most games without reading the schematic, but order a copy anyway.
Systems
Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part One, Clay Harrell (pinrepair.com)