• Are your machines set to "freeplay"?
• Do you miss the coin-up sound?
• You don't want to insert real coins?
• You don't want to do any soldering or permanent modifications?

Here is an easy solution:

Remove the coin checking mechanism behind one chute, unscrew the microswitch and stick it to the back of the return button's return spring with strong double-sided tape.After some adjustment to the wire you only need to press the return button to "insert" a coin.

"C'mon in", says Rudy now.