I collect and repair Williams System 3 through System 7 pinball games.
I personally refer to the following web sites for help and suggestions constantly:
Mark's page is regularly updated and has an extremely practical bent to diagnosing and repairing problems. Mark also offers EPROM burning services at very reasonable prices. A test EPROM is essential for debugging CPU or driver board problems.
Marvin's page is not updated as frequently but goes into much more detail on esoteric situations that don't occur often -- but when they do, you'll be glad for the information. Marvin's page also contains a lot of good information for the novice. Finally, Marvin takes a proactive approach -- fix things before they break -- which can be appropriate for a machine which has been abused or has intermittant faults.
Here are some parts lists and suggestions for upgrades:
Page last updated December 14 2003.