So if you jump the F socket to another lit socket, the bulb does light.
Did you check one of the other Relays (C,D,E) with a volt meter
to see which terminal is the 6V, then checked the terminal in the
same position on the F Relay to see if it's pushing out 6V. If it is,
then a little jumping should narrow things down. If it's not, then something
in the box lost connection and you'll want to jump a wire to that F terminal's
circuit so it pushes out 6V.

here's more info.

what happens if you jumper the Y-BLU wire on the lamp socket directly to the green wire (on the transformer or one side of the playfield GI lights)?

if jumpering turns on the F light, what happens if you close the E relay?

if jumpering doesn't turn on the F light but the playfield GI lights dim, remove the F lamp and try again. If the playfield GI is still affected, then one of:

1] the F lamp socket has a short
2] the E or F relay switches with the Y-BLU wire have a short
3] the Y-BLU wire itself has a problem ... like it got squashed under a unit when you screwed stuff back on the playfield and the insulation got sliced thru ... however, if that happened the F and/or double bonus when lit lights would be on or burned out. If the E/F relays aren't on the playfield, check Y-BLU wire on the plugs.
f-lite (resized).jpg