I'm not sure if this gives any clues, but I also noticed that when configured per my original post, it would start a game and add a credit when the normally-open coin switch was closed. On the other 25c settings (2/3/4/5 credits), When the NO coin switch closes, it just energizes the 25c relay, and then the credits are added when the NC coin switch closes again.
Quoted from fredsmythson:
If you insert a coin and it starts a game, and also adds a credit, then there is something wrong with the options for coin/credit settings, or there is a switch/wiring issue. This era of machine would usually start a game when you inserted a coin ie 1 "play" / 25 cents. It wouldn't also add a credit. You could try to disconnect the 25 cents/1 "credit" option and then see if the machine would start a game by inserting a coin without adding a credit.
As rolf_martin_062 said, the circuitry in the machine wasn't designed to give 1 credit when a 25 cents was inserted. It was designed to start a game when 25 cents was inserted. This eliminated a few step when starting a game ie inserting a coin, having a credit awarded and then pushing the start button. As mentioned, you can always do some rewiring to get the results you want...
I know I've played games where the first coin just started the game, but I didn't know if that was every EM, or just most of them. This is my only Williams of the era. I'm fine with either approach, as long as the end result is one credit per quarter, effectively. When the first coin starts a game, the second coin should still just add a credit, correct? I believe it should not start a 2-player game immediately.
I do, see attached page for the pricing adjustments.
Quoted from fredsmythson:
The machine might be set up for 2 plays/25 cents. When the coin closes the coin switch, it will reset the game (play/credit #1), and then it will add a credit to the credit unit (play/credit #2)...
No, when set for 2 credits per 25 cents, it just increments the credit unit twice. It does not start a game. It was only in the 1 credit or 6 credit position where it would start a game.
Now that I've answered those, I'll say that I've accidentally solved the problem, but I don't understand why. When trying different things tonight, I tried these settings:
25c - 1 credit
10c - 1 credit (was - zero)
5c - zero
2 coins for 3 plays - on (was - off)
This gave me exactly what I wanted! It adds one credit per quarter, and does not start a game. I still can't tell if this makes sense per the schematic though. In particular, I'm surprised it worked when the 2 coin/3 play adjustment was set to on. It kind of feels like I may still have an error or misadjustment somewhere, even though I'm getting the desired result!