Setting up free play on SS machines is usually an option listed in the
owners manual. Williams machines without an explicit free play option may
be set to free play by setting the maximum credits option to zero. Early
Bally SS machines had no free play option. Some suppliers sell an IC that
can be used on the MPU on these machines to allow free play.
The usual, and cheaper, option is to install a button, wired in parallel
with a coin switch on the front door. When pressed, this button adds
credits as if a coin were used. I discourage modifications to the front
door for mounting the switch. I recommend placing the switch on the
underside of the cabinet on the left front side, exactly opposite the on/off
switch. This makes it easy to find and does not affect the appearance of
the game. Sometimes people request that the free play setup be optional.
If so, install a switch in line with the button, so that the button can be