Open the coin door. Set the auto-up/manual-down to up. Use the step button to go to function 18 - maximum credits. Use the credit button to set the value to 0 Press the step button once to go to function 17 or 19-just get off function 18. Turn the game off and back on. You should be in free play.
The users manual should tell you how to step and set using the buttons on the inside of the coin door.
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