Does anyone have a copy of the schematic ?

I'd liek to know exactly what energizes that coil that is held

With Nino on this, do you have a copy of the electrical diagram Big Dog?  If so scan the part showing the coil for the credit wheel and post so we can see which relay/s it is activated by.  Obviously the machine is paying a credit from the 'Special' function of the centre target, so this special lite is lit?  Most likly a high resistance on a contact or backfeed from something else is stopping the credit unit from stepping fully.