Just developed is a free play board that that can be used on early Bally machines and will be in stock soon.
This board plugs in where the coin door connector is, just inside of the machine. Unplug the harness, plug it into this board, and you are done.
Make sure that coin slot 1 is set for 1 coin/1 play. When you press the replay button, this board will take care of the credit process and start the game for you. The end result is that each time you press the replay button, a player is added.
There are jumpers on the back of the board that can be cut and soldered if you do not want to use coin slot 1. You can select 2 or 3 instead. Either way, you want to set that slot up for 1 coin/1 play.