Quoted from KingNine:Oh. There is not a free play mod
You could wire one of the coin drop switches (below the coin mech) to the start button. That should give you a credit everytime you start a game.
Though, this would only work if the RAM at least held memory when you turned off the game.
Can you use your voltmeter and make sure you have 3.6v on the red/black batteru wires where they are soldered into the MPU board?