My thinking is the same Nino, with no ground, then there can be no ground issues right?
The plug it came with was a European schutco (?) style that had no ground pin.
You can see the staple marks where grounding strap was removed.
If the game EVER worked with this set up then it must have only come to life if the player had bare feet and one leg pushed against the door whilst wondering if pinball was meant to be painful...
