Do all the coils and flashers fire correctly in test mode?
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Check to see if the knocker is energizing at all as well, that might help narrow it down.
If you look in the manual (http://mirror2.ipdb.org/files/859/Williams_1987_Fire_Instruction_Manual.pdf, PDF page 64) it's got a diagram of all the coils. Each flasher and coil has a diode between it and their common drive transistor, so if one of those is letting current go backwards then lots of strange stuff can happen. Not sure where the isolation diodes are located in the actual machine though...
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