Bally S5500 high topper "Ring The Bell" board problem

[badbaud]

The customer has two S5500 slot games. The first is a "Roll The Dice" game with a high topper that rolls dice. We got that one fixed. Took the boards to the shop and plugged in our test slot programs, cleared the boards with clear chips then reinstalled the game chips and cleared the boards again. Pesky 54 error code problem was some "Jack" shoving a piece of wire into the two spade terminals that went to the BV drop box. The wire had fallen out. Cut the spade terminals off and wire nut them together.

The "Ring The Bell" game boards are a problem. The main reel board cleared with our test chips and and with the game chips but the topper board would clear with our chips but not the original game chips. Got a flashing reset LED. Tried the same test procedure with a known good shop board and got the same result with the original game chips. Assume U3 or U4 or U18 or U20 is bad.

U3 is a 27C512 #F5G10BR00000-03 L99-0676
U4 is a 27C512 #F5G20BR00000-03 L99-0676
U18 is a 27C256 #F5P1BR000000-00 L98-2086
U20 is a 27C256 #F5P2BR000000-00 L98-2086

just in case:
U12 is a 27C512 F5M100000000-02 L99-0675
U15 is a 27C512 F5M400000000-02 L99-0675

Has anyone had experience with this kind of problem and can point me to a solution... such as substitute chip numbers that will work or chips?