Quoted from baldtwit:

another variation - http://bingo.cdyn.com/oddities/jackpot/
so far, nobody has one of those machines and been willing/able to come up with how the jackpot is wired into the game. In the 5-in-lit-color version of the jackpot, there's max 10 wires going into the upper compartment. Assuming 3 of them are wires 30, 70 and a 6V power wire, that leaves up to seven wires to tie into the original game.
I'd guess there's a continuous stepper in the jackpot unit which controls which color (if any) is a jackpot and it could also add credits to the jackpot every N steps, but I just made that up with no evidence to support it ... hmmm ... do I have political aspirations?