So...you must have a System 7 MPU connected since it has the 7 segment display.
When you press the lower diagnostic button, does the 7 segment display go blank, and then cycle 1, 2, 3, indicating that all three memories are working?
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info