It's one of the two middle switches on the Lock relay.

With the machine on, press the contacts on each of the middle switches with a pencil or something til you find which one is for the back and clean/adjust after turning off the machine.

Diagnose this circuit with Alligator clip jumper wires.

Pinball (resized).png

Check 5A on the score motor. Also make sure you don't have "floating contacts" on those Lock relay switches by giving them a good crimp.

Don't test the switches one at a time. Instead, for example, jump from the Green/Yellow/White 6 volt wire on the transformer all the way to the Brown wire on one of the backbox display lights.

The two center switches on the Lock relay control lights: one does playfield and one does back box. I had an intermittent problem with the BB sw. when I had my Fun-Fest, and you really had to give the sw. a hard push to get them on. But with this Ball Index relay connection I think you're going to have to run thru all the switches that involve Ball Index and Lock. Do you have the manual? If not, http://mirror2.ipdb.org/files/1733/Williams_1973_OXO_Instruction_Manual.pdf

Start with the two M/B switches on the Outhole relay.

Hi jdapolito +
I join currieddog (post-10) - and I always like to use stripes of paper to sneak-in, in-between switchblades to keep THIS switch open --- jdapolito, use the ipdb-manual, page-25 - see the three switches on the Ball-Index-Relay. You claim that the lights in the head light-up when the relay actuates --- sneak-in a stripe of paper into the FIRST switch - do the lights still light up ? Do it on the 2nd - do it on the 3rd - so You can say "THIS switch on Ball-Index-Relay does the light-up". (The situation / fault actually is pretty weird) Greetings Rolf

That makes no sense unless something was rewired. According to the manual, that sw. is in series with sw. A on Extra Ball relay, which is the hold circuit for that relay.

Did you adjust it and did it do anything?

Hi jdapolito, currieddog +
if it is the "Switch A on Ball-Index-Relay" then we must say "some preowner made an "Hack / creative rewiring" --- original is "Switch A" handles 24 VAC - original is (together witch the switch on 1000-point-relay) - when an EXTRA-Ball is made: To turn-off the information-lamp (and the complete feature) "an Extraball has been made" (I see this in the schematics-area "C-23"). jdapolito - want to try "functionality of Extra-Ball" ? Greetings Rolf

Hi jdapolito, currieddog +
I didn't exercise care when reading the posts - I didn't exercise care when looking at the ipdb-manual --- (((see the words "Switch C" in the following writing)))
if it is the "Switch C on Ball-Index-Relay" then we must say "some preowner made an "Hack / creative rewiring" --- original is "Switch C" handles 24 VAC - original is (together witch the switch on 1000-point-relay) - when an EXTRA-Ball is made: To turn-off the information-lamp (and the complete feature) "an Extraball has been made" (I see this in the schematics-area "C-23"). jdapolito - want to try "functionality of Extra-Ball" ? Greetings Rolf

A great mystery --- please check the functionality on the feature "Extraball" - I see You have an "Space Mission" in Your collection - therefore You are familiar with the feature "Extraball" --- was the preowner doing his "hack" so good that he could do his hack - AND the Extraball-Feature ALSO works (?). Greetings Rolf

Hi jdapolito
the "preowner / hack" is an mystery --- want to try with an jumper-wire (permanent set) - I have taken HowardR 's JPG in post-5 --- see "my blue jumper" --- exercise care - on some or just on one of the many lights You will see "wire of color-brown in the pin" - wire of color-brown comes to one side of one or several light(s). IF (if, if) You happen to use "other side of the lamp(s): You then make an short and the fuse will blow.

IF (if, if) the lights work with the jumper: You may let the jumper in the pin and do some "documentation".
IF (if, if) You are interested in the mystery: You must follow the wires starting at "Switch C on the Ball-Index-Relay" - start at this switch-C - then follow to whereever they go - investigate on the wiring. Greetings Rolf

0OXO-pinside-picture-01-Work (resized).jpg

The part about those switches being involved because there's nothing to indicate that on the schematic or manual. And I meant did you adjust the sw. that you found.