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Friday 12 January 2018 at 02:58
CHEck the fingers on the bonus disc wipers. One is not making contact- the one thats in series to the score relay. Else if its both sides then something else.
A bit more detail would help to be sure but this is a 2 min check. So no loss if its not this. Check alignment as you index unit too.
Source: http://pinside.com/pinball/forum/topic/williams-grand-prix-problem-1#post-4171430