Of all the ways I know of to make a game operate on free play, including cpu controlled, I think the best one is by using the 2 player start button which will work on a good 95% of games. Open your coin door & ID one of your coin switch wires that goes back to the pcb... not the ground wire that is looped to all switches... and then locate the wire leaving the player 2 switch & going back to the pcb. All you need to do is join these 2 wires together in some fashion. You can use splice connectors and a piece of 20ga wire to connect the two together. Getting fancy with this version, you could add an on/off switch on the inside of your coin door in-line with this circuit enabling you to flip the free play on & off when coin play was desired. You could go to the edge connector end of the harness to simply splice both lines together as they leave the pcb eliminating the need for a piece of wire. For all you shade tree techs... yes you can do it on-board, but you know I hate hacking up classics :-(
Once this is done, pushing the 2 player button will put up 1 credit to play a single game and pushing it a second time will start a 2 player game if desired. The reason that I prefer this method over the others, including dip switch free play, is that the games attract mode is left intact adding many years of life to the CRT's phosphor, i.e., less burn in on the screen.