Hi everyone. Stuck and looking for some help on where to explore next.

Have a '76 Williams Space Mission. Set to free play. Game plays through all balls and players and scores correctly. Issue is, if the Extra Ball relay is activated (even manually), The Game Over Relay Trips. Doesn't matter the player or ball, just goes straight to game over immediately.

In the picture below I've isolated the different ways the relay trips.

#1 - Lock relay when the game is turned on. I've verified that the Lock relay stays energized throughout play, even if Extra Ball relay tripped (not it)
#2 - Through the Coin Relay closing. I've verified this is not shorted and does not close when Extra Ball relay tripped (not it)
#3 - The Ball count unit... Has to be related to this one...

Next clue. When the Tilt relay is energized, playfield goes dead like it should, ball goes to outhole, Game Over relay trips...

Any thoughts on what to try?

Thank you.

Game Over (resized).PNG

After spending hours on this, I finally figured it out and the issue wasn’t anywhere I thought it would be. Turns out it was a short on a Jones plug in the backbox. I’d like to share how I figured this out in the hopes that it helps someone in the future figure out a similar problem.

Initial Problem: In a 3-ball game, getting an extra ball would immediately trip game over.

Per some great advice on Pinside (@HowardR), I put little pieces of cardstock in between switches on various relays to help isolate the problem. See Isolating Switches on relays picture. Circled here is the Extra Ball relay.
Isolating Switches on relays (resized).jpg

Start a 3-ball game. In game, I removed one isolation paper at a time, manually activating the extra ball relay and seeing what happened. If it did not go to game over, I replaced the paper and removed the next one. Discovery! The extra ball relay Hold circuit trips game over (wire Y-B-5)…

Looking at the schematics I noticed that the Game Over relay is affected by whether it’s a 3-ball game or 5-ball game based on the adjustment jack in the backbox. So, I set it a 5-ball game and tested again.

In a 5-ball game, the Extra Ball Relay Hold circuit did not trip game over. The game played as normal until I got to ball 4…
New Symptom/Problem: When advancing to ball 4 in a 5-ball game, the Extra Ball relay activates and never releases. Even after a ball drains, it’s not releasing and the game never ends. This problem is actually related and you’ll see how as we get to the finding.

Looking at the Ball Count Unit Disc in the manual we can see how it works (below). In a 5-ball game, the wipers advance after each ball and on ball-4 the configuration is the red line. Notice that the top wiper is touching the BLU-BR (this is -4 but not labeled) wire contact that trips Game Over Relay in a 3-ball game.
Ball Count Unit (resized).jpg

So next, I took a piece of paper and isolated the top wiper and advanced the game. See Isolating ball count stepper picture. At ball 4, the game plays as normal and the Extra Ball relay does not trip. Therefore, the BLU-BR-4 wire is causing the issue.
isolating ball count stepper (resized).jpg

Looking at the schematic I can see that the BLU-BR-4 wire comes out of the Ball Count Unit and goes to the 3/5 Ball Adjustment Jack in the back-box. That’s the only place it goes, so somehow it’s activating the extra ball relay. The jack is located in the back box, so I went to look there. See “3-5 ball adjustment jack” picture.
3-5 ball adjustment jack (resized).jpg

I took my voltmeter, set to continuity and started checking for shorts in various places. I found a short between the BLU-BR-4 and BLU-R-4 wires on the adjustment plug. Basically, even if the jack was set to 5-ball, it was somehow shorted to the 3-ball spot. Knowing that all of this is in the back-box but had to connect to the playfield and bottom relay board in the cabinet, they had to go through the Jones plugs. Using alligator clips I attached my voltmeter set to “tone” on the BLU-BR-4 and BLU-R-4 wires of the 3/5 ball adjustment jack. Then, while the tone was on, I started wiggling and adjusting the jones plugs until it stopped…

Bingo!!! See Jones Plug picture. When I pulled this plug or wiggled it the tone stopped. The short was there but where. Consulting the manual I found the Jones Plug Layout picture. Pins 3 and 4 on the right-hand side are BLU-BR-4 and Y-B-5 (remember this one from earlier?). There was the problem.
Jones Plug (resized).jpg
Jones Plug Layout (resized).jpg

In a 3-ball game activating extra ball put power through Y-B-5, onto the Jones Plug and shorted to the BLU-BR-4 Game over trip (see Extra Ball relay picture). On a 5-ball game, going to ball 4 wouldn’t end the game but rather send power through the BLU-BR-4 wire and short to the Y-B-5 Extra Ball wire (see Game Over Relay picture). As a side note, I wish the schematics noted where a connector was, like on modern schematics…

Extra Ball Relay (resized).jpg

Game Over Relay (resized).jpg
Now, I never could visually see where these two wires were shorted on the Jones plug but I cleaned up both the top and bottom of the plug, checking how the wires were soldered until all looked good. No short. To confirm this was in fact the problem all along, I took an alligator clip and shorted them together and yes, all the same symptoms re-appeared when I retested so that was the issue.

Hopefully this write-up was clear enough to help someone else in the future and maybe pick-up some troubleshooting tips along the way.

Good luck!