Quoted from AlexF:
You need one of these to make it AAB.
ebay.com link » 1960s Bally Em Pinball Add A Ball Relay Scarce Item
DON"T DO THAT !!!!!
AAB are horrible , if you have any pinball skills you will hate add a ball games, they never end. Never convert the game from it original state, and no LED's Ever ! if it is a replay leave it as you found it.
AAB are great for the player with less skill but when / if you acquire true pinball skills you will rue the day you bought an AAB.