How They Work - Double or Nothing - Probability/Random Unit
The probability unit (random unit in later games) is a motor-driven wiper/contact plate.
In some games, when the motor is turned on and the index is released, the wipers spin continuously. The index mechanism is used to grab a ratchet tooth when the game wants the wipers to stop. As can be seen in the picture, the ratchet is grabbed when the index coil is unpowered.
A different style of unit more akin to a printed circuit trace replay counter was used in other games like Ticker Tape. On this unit, there's no index coil. The wipers stop when the motor power shuts off, and gaps between trace patterns insure that wiper fingers won't bridge circuits.
The Probability Unit Circuits
When the game is reset, the shutter panel opens and the shutter cam #4 switch in the cyan circuit closes. This turns on the motor. Every time the machine is spin cycled, the start relay switch closes in the orange circuit - for a variable amount of time - which powers the index coil and the probability unit wipers spin around some random amount.
Eventually, the player will stop spin cycling the machine and shoot the first ball. This causes the shutter panel to close and the probability unit motor turns off.
Later machines varied this circuit somewhat. For example, in Dixieland the index coil is powered whenever the motor is on, and the wipers spin until the timer unit steps up once. The effect is the wipers spin starting when the shutter panel opens, and stop when all the balls have rolled down into the trough and the first ball is raised.
Apparently this wasn't enough randomization for bally and/or they wanted a backup circuit. I suppose the probability unit getting stuck at a position that always wins a double attempt is suboptimal (for the operator).
The motor will turn on whenever the search wipers aren't locked via the purple circuit, and the wipers will spin continuously using the blue circuit until a win is detected assuming the double play relay isn't already powered. When you push the double button doesn't really matter in this form of the circuit, as the search wiper indexing has caused the probability index coil to lose power and stop the probability wipers already.
On the printed circuit forms of the unit, motor power was usually removed when the double play relay switch opened, thus when the player pushes the double button determines if a double attempt is won.
The 4th ball relay switch and double play replay switch aren't in the circuit for the probability index coil power, though the current does pass through the switches. These two switches are there for the double delay relay coil power, which we'll get to soon.
Later games simplified this part of the circuit. They removed the 4th ball relay switch and moved the double play relay and double delay stuff elsewhere on the schematic.
