Hi all,
I recently picked up a Willaims Grand Prix EM 1976. Overall it's in great condition and I only had to do minimal cleanup to get things working properly. The game was previously setup for free play so the prior owner didn't use coins. The one issue I've not been able to resolve is to get the coin/credits working properly. I currently have the game set to award two credits per quarter.

The schematic can be viewed here: https://www.ipdb.org/files/1072/Williams_1976_Grand_Prix_Schematic_Diagram_large_size_continuous_includes_overlay_correction_To_L_R_Kickers.pdf

When a quarter is dropped in the chute the system will award 0-2 credits. What I've been able to observe is that in the times 0 credits are awarded the 25c relay engages and releases but the score motor does not run. If I change the plug to N (1-6 )credits per coin I will get anywhere from N, to N-1, to zero credits awarded. The inconsistency is driving me crazy.

I've adjusted the switch contacts for the 25c relay, 2E, 3E, 4E, and 5B switches to no avail. I've also played with the adjustment of the 25c switch and cam. In testing, rotating the 25c coin acceptor cam on the door reproduces the 0-2 credit award. But manually pressing the 25c coin switch (by pressing the contacts so both sides of the switch contact) seems to consistently award credits - sometimes multiple depending on how long that is held.

I've also observed that if I manually engage the 25c relay it will consistently award credits. Again the number of credits awarded depends on how long I hold the 25C relay.

I really enjoy using coins for these games. Any help or guidance is appreciated.

pinballdaveh - Thanks for that option. I don't think I'm ready to give up on the 25c relay just yet but will keep that as an option.

HowardR - I would say the 25c relay does not stay on as long when 0 credits are awarded. But more importantly, if I manually rotate the coin cam I can sometimes get it to energize the 25c relay and hold for as long as I hold the cam and no credits are awarded. But if I press the coin switch (not using the cam but pressing the leaf contacts) it aways awards credits. It the later case I'm effectively not letting the NC contact to open.

I have two coin slots on this door and both coin drops / cams / switches give the same results. So while its acting like the coin switches need to be make/break I can't see that they are both adjusted incorrectly. I also have a Big Deal right next to it that works correctly with the same contact gaps.

It feels like one of the contacts on the 25c relay is suspect but I've not been able to identify which after cleaning and re-adjusting all the contact gaps.

Lastly, I'll add that if I physically engage the 25c relay with a finger push it seems to always award credits. Sadly, my arms aren't long enough to hold the cam in a position where the 25c relay is energized but not awarding credits and pressing the relay plate.

Adding that if I block the NC coin switch it behaves the same way as when zero credits are awarded - the 25c relay engages but the score motor does not run. It makes sense that the score motor would not run if either of the two contacts circled are not made. But no amount of adjusting these seems to work.

Williams GP Coin Switch (resized).png

HowardR - Sorry if I'm using the wrong term. I've attached a photo of what I'm calling the "cam" that I'm rotating to cause the coin switch to engage. - The same as pulling the coin trip wire down.

20240602_123232 (resized).jpg

I did re-check the 5B contacts. Attaching a photo with them in both positions.

WilliamsGP-5B -OpenClose (resized).png

I did try lowering that upper leaf contact to ensure a M/B but still get the same behavior. I'm also attaching a gif animation of the coin dropping through when the 25c relay clicks but no credits. (Hoping the animation works).

20240602_125144_1_1_1.gif

pinballdaveh - Good question. The issue is that the 25c relay is not holding on the initial coin drop for most of the time. So as the coin hits the switch it quickly pulls the 25c relay then releases. So no hold is activated. If I carefully pull down the wire in the coin slot it always triggers the score motor.

I'll point out that this unit was set for free play when I got it. It was well maintained but I don't think they ever bothered to check the coin action since there were no coin acceptors in the game when I got it.

Confirmed on both tests - with the score motor unplugged both relays hold and release when cam 5 drops. Adding that if I manually push the 25c relay it stays locked until cam 2 drops.

I think I'm getting closer to resolving the problem. While looking at your suggestion I've tried bypassing/jumpering a number of things that might impact. I also noticed empirically that the position of the score motor cams (either 0 degrees or 180 degrees) definitely determines if credits are missed. For instance if the motor is positioned at 0 degrees the 25c relay does not hold. But if the score motor is at 180 degrees when a coin is dropped it always awards the correct credits.

So it is clear there is a switch that is not making good contact at the 0 degree position.

Narrowed it down further to the switch stack on cam 4. Just by applying a little downward pressure on the stack I was able to consistently get credits awarded in both the 0 and 180 degree rotation position. While I cleaned / adjusted 4E multiple times it became clear this set of contacts needed a much more agressive cleanup of the contacts.

Bottom line is that 4E was only making contact (on the make side of the make/brake switch contact) when the cam was sitting in the 180 degree position. My guess is that in the zero degree position is was just slightly not enough pressure to get through what ever layer of oxidation built up over the years.

Thank you for the help.