Your best friend for all things Gottlieb is Steve Young at www.pbresource.com
But, I see that he is out of stock for System 3 MPUs... http://www.pbresource.com/boards.html#gtbsys3
If you don't have significant experience repairing a System 3 MPU, send it to a pro. The traces on those boards are very, very fragile. Without the right experience and technique, the board will be damaged.
I can repair the board if you'd like, assuming you are CONUS. Drop me a line. See: http://ChrisHiblerPinball.com/contact
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Chris Hibler - CARGPB #31
http://ChrisHiblerPinball.com/contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info