Maybe a very simple fix as it was with the C 37 I just got.
On the credit reel, find the switch the wiil be opened by the pin on the cog when there are no credits. Make sure the when there is a credit that switch is closed.
While you are there, you can set it to free play by adjusting this switch to be always closed even when the credit reel is on zero.
Go to pinrepair.com for a pic of the switch.
Dan