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It seems there are two ways to get freeplay mode out of this sort of jukebox.
The first way is simplest: set the Selector logic board into test mode.
The second way is much more complicated, and gets the value '255' into the right place(s) in memory once the jukebox is placed in 'service mode' and memory values are viewable and editable.
Unfortunately this last method has various accounts given for it. One account, seemingly most reliable, states that the memory locations 05 through 09 are where the number of selections/plays per unit entered in cashbox, is set. Set that to the maximum, 255, and you get 255 plays for a quarter (or the like. Supposedly the 00 to 04 memory locations are 'levels of credit' e.g. 25 for 25 cents - a quarter. If that's true, set
memory location 00 to 5 (a nickel)
and set memory location 05 to 255 (255 plays for a nickel).
To enter service mode, where you can edit memory locations, you open the control console, and flip the switch at the upper left, into service mode, then enter your password,