I think a schematic is probably necessary to do any serious work work on that pin ball machine ... I know I'd put a lot of effort into finding one and I'm an electronics engineer.
One of the first things I did was look online for a schematic. - No luck so far but I'm sure I'll track one down eventually. All of the relays seem to function as they should. The mechanical side gets more complicated. I've been checking out the game credit indicator and it appears that someone has already messed with it. There are some pretty funky gears, brackets, and levers that may not be assembled correctly. Without a diagram I'm not sure.
While I was on eBay yesterday I came across a nifty Dart tee-shirt. - Just the ticket for some of you Dart guys. I'd love to get a Dart myself but it'll probably never happen.