The ever-vigilant researchers at Williams Electronics Games, Inc. have been able to dig up a copy of the Bally document entitled Modification Procedure for AS-2518-17. You may have seen references to F.O. 597 in other Bally pinball documentation for older games.
This document, in scanned format, is provided here for your reference. We apologize for the somewhat poor quality of the scan, but it's the best we could do with the 7th generation photocopy of the original document that we located.
Please note: We do not in anyway recommend performing the described board modifications to your AS-2518-17 CPU board. The modifications should only be attempted by a technically competent person. If you wreck your circuit board attempting these modifications, do not contact us to complain. While we will feel sorry for you in your moment of pain, there is nothing we can do to help you with your problem.
Here are the five pages of F.O. 597:
F.O. 597 page 1 F.O. 597 page 2 F.O. 597 page 3 F.O. 597 page 4 F.O. 597 page 5