Update
Updating database... Please wait.
Sunday 19 November 2017 at 21:33
How are you attempting to add credits? Through one of the coin-drop switches?
Does the credit unit mechanism advance and subtract crisply when you operate the respective solenoid plungers manually?
Does the double or nothing behavior relate to whether a high (10-15?) or low (1-5?) number is display...
Source: http://pinside.com/pinball/forum/topic/williams-blue-chip-credit-unit-malfunction#post-4081497