Start by verifying the display HV is correct.
http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Testing_DMD_Controller_Power
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Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The new place for pinball repair info