I really liked to work on Capcoms.  They really were ahead of all the others with the onboard diagnostics and their manuals were soooo much better on the tech stuff.  Not only did they put the schematics in the manuals they did a circuit analysis as well.  For the EM's, it was Gottliebs.  For the early SS's (1970's-80s') it was Bally (before the Midway disaster).