Both the backbox flashlamps and the "reveal" flashlamps are driven by a "chase lamp" board (2 boards). You can swap chase lamp boards to see if the problem follows. Those boards will drive the lamps during attract mode periodically.
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Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info