Go grab the manual and print it out - it will come in handy !
https://www.ipdb.org/machine.cgi?id=1338
Some repair guides - Yours would be the first (System 3-7)
https://pinwiki.com/wiki/index.php?title=Williams_Repair_Guides
One of the most important initial checks is to check if the game is set for 240V - Do you know the history of the game ? Was is a locally operated game or an import ? Then check the fuses - every single one of them. If they are the incorrect value - change them. If they are the correct value, check them with a multimeter.
That should keep you busy