Free play on System 80 by adding a credit switch behind the start button

Steps

System 80 DIP switches have no free play setting. Set at least one coin chute to 1 coin/1 credit and the credit limit to 25.
Take the start leaf switch stack off the inside of the coin door (two screws).
Restack it with an extra two-blade leaf switch, removing spacers as needed and fitting insulating fish paper between the two switches. Longer screws will be needed.
Adjust so the new switch closes before the start switch.
Wire the new switch in parallel with the coin switch you set to 1/1.
Pressing start now adds a credit and then uses it. If credits pile up from replays and matches, turn those off with the DIP switches.
The coin door slam switch can be reused as the extra switch: join its two wires together permanently, remove it with its fish paper and cut the weight off its long blade.

Source: Repairing & Upgrading Gottlieb System 80 Pinballs from 1980 to 1989, Part Two, Clay Harrell (pinrepair.com)