the double pay is interesting.
the payout counter is mechanically stepped, meaning a coin exiting the hopper lifts a roller pivot arm that pushes down a link bar to move the step-up arm on the payout counter.
the wipers on the payout counter actually move when the link arm goes back up.
on the 808, there's a step-up coil on the payout counter, but it's job is not to electrically move the step-up arm. Instead, it holds the step-up arm in the down position every other coin, so the payout counter only moves the wipers every two coins ejected.
the doubling circuits controlling the payout counter step-up coil are around N6 and F6 on the 808-B schematic. The basic idea is:
- when 2 coins are played, the 2nd coin relay is powered
- coins ejecting from the hopper toggle the hopper microswitch at F8
- that pulses the double unit step up coil which moves the double unit wipers
- every other wiper position of the double unit (F3) holds the payout counter step-up coil powered
so make sure the double unit wipers are mechanically stepping ok, the contact plate is clean, then you need to see if the double unit is stepping every time a coin ejects.