Rock and Load
Creating a machine that could accept coins, allow customers to select specific songs and then play them loud and clear enough to fill an entire building was in itself an amazing engineering feat. That's especially true when you consider that this was an era devoid of microchips, robots and in many places, electricity.
Developing dependable coin activation wasn't easy. A machine had to work when a customer dropped a valid coin into its slot, but it had to reject cleverly designed fakes, or slugs. They also had to resist vandalism and dirty environments. All of this was just for starters.
Customers pressed buttons that corresponded to the songs they wanted to hear. Then jukebox had to play those tunes in the right sequence.
Records were stacked inside the machine, suspended in individual rings called carriers. When a customer made a selection, a select bar rose along the stack until arriving at the right record; then the appropriate carrier swung out from the stack. Finally, the turntable would rise up to the record, begin spinning it and lower the needle to begin playback.
The real trick was making a machine that remembered which songs to play and when to play them, and this was a matter of clever mechanical engineering. Many jukeboxes had gear-like components (called cams) organized onto a memory drum. Just like the record stack, this drum was a cylinder and stacked with cams that corresponded to each record's carrier.
Selecting a record caused the cam for that album to rotate. As the select bar moved up and down the record stack, it stopped when it reached a cam that had been rotated. Once the song played, that cam returned to its original position and the select bar moved on to the next album. Fancier jukeboxes had additional mechanisms that flipped each record, allowing the turntable to play both sides, ultimately doubling the number of songs that the machine could play.
As compact discs replaced records in the 1990's, the mechanisms for changing discs actually remained somewhat similar to those used in previous decades. But the CD format offered far more songs and more consistent playback.