Hello QuietEarp This should help you.

Manual >> http://mirror2.ipdb.org/files/2253/Williams_1976_Space_Mission_Instruction_Manual_Jan_1976_no_schematics.pdf

Schematic >> http://mirror2.ipdb.org/files/2253/Williams_1976_Space_Mission_Schematic_Diagram_continuous.pdf

1st. In the manual page (using the # 7 on the actual page)Check the coin switches gaps at the door and see if they are touching , they shouldnt be which will give that issue.

Response ?

2nd. How is the spring tension on the return spring ?
Do you see it trying to pull it to that 4th player and it fail to actuate ?
It may be the spring is too tight not allowing a step-up coil to overcome tension?

Response ?

Not much to it in the circuit.

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3rd. Ok that tells us the path between the 2 points is broken.
Check the wiring , solder connections and the jones plug also ?

No problem.

Its in the D- 11- 12 area (just about by the black dot between the#`s ) location of the drum units thats where you were looking for it.

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Heres more on the 3 + 4 reset re. info the one that triggers it.

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Thats the next thing to look at.
As you can see on the sch. located in the C - E / 6 - 7 area

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See how the 25 cent re. contacts when closed allow the credits to add. Which is what you are getting.

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Check the .25 re.contacts they should all be ( N.O. ) normally open gaps with the game off / un-energized state , adjust as needed....

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