There can be some variation depending on the era of machine and how far you want to go... But thinking of the essentials in my crash kit for housecalls and pinball shows, the key items are:
-schematics for the game
-auto-ranging DMM with continuity beep function
-good temp-controllable soldering iron
-switch adjustment tool... Seems kinda silly at first but the first time you use one, you'll "get it"
-burnishing tool and/or fiber (non glossy) business cards for cleaning switches
-ratcheting all-in-one screwdriver
-nut driver set... Or at least 1/4, 5/16, and 11/32
-fuse assortment... Or even better, circuit breakers
-small adjustable crescent wrench AND small channel lock pliers
-length(s) of insulated wire to use for test leads
-willingness to learn and a cautious troubleshooting approach
