Working from memory here as a former SF owner, I believe the lettered relays A-E are energized/deenergized depending on the position of the arrow step unit in the back box. As the ball hits the green saucer or passive bumper the arrow moves left towards “A”, the purple saucer and passive bumper move it right towards “E”.

I no longer have the schematics (and reading them in not my forte, though I have managed to nurse four non-working ems back to life). Find the “D” relay on the schematic and work through the associated circuits, making sure each switch is as it should be. Also keep ah eye out for any foreign objects causing a short (stray screws, washers, drops of solder, bent and unprotected tabs on switches and coils, etc) along the way.