There are no production games called "Single Coin" - they are modified (usually) Bally six cards.

I'm guessing this is a 1970s model since there is a c button on the lock bar to press.

The typical operation is such that the user presses the C button, the search discs turn, and when three or more relays are engaged at once, the search index relay pulls in, stopping the search disc at the point where the user has a winner. The backglass will flash double or nothing, and the user needs to press the D button for a chance at double or the R button to take the regular amount of replays.

These single coin machines were changed in many different ways from the standard method of operation. Looking at the photo, I know a couple of machine that it is not, but I don't recognize what machine it is right off the bat. Are there 28 holes on the playfield? This will help me to pull the right paperwork for review.

Shoot-A-Line was the only 28 hole machine, and the scoring works a little bit differently than in other games. There is also no D or R button side by side on the left, only an R button on the right.

Disabling the timer stepup in that machine can cause other problems, potentially. Knowing the game I can advise for sure.

On Shoot a line, the timer is less useful than just about any other game. If the search relays are chattering, it should be ok. If it just won't score, make sure the timer is on the 4th position, just to rule that out.

I wouldn't run it that way, but shoot a line has a problem where it will time out and cannot be revived even if you have a winner. Not a great design. It's the only game like that. Others can restart the search even if they time out. So I can see why the customer would want it disabled.

Shoot a line scores by latching various parts of the control unit in a similar way to the other games. It has two coils that are in a hard to access area with switch stacks whose adjustment is critical---! to making it score.

These are located at the top of the back door behind the control unit. There are coils for 2x and 4x. Both need to be able to pull. IIRC, 4x only pulls on yellow line wins, and 2x pulls for any other win.

Test those coils. Make sure if they pull, they change the state of the switch stack appropriately. Again, if I recall, the coils pulse, the unit rotates, and the switches remain in the active position until the cam finishes rotating. The coils may not be made to be active for very long.

The manuals and schematic for shoot a line are on bingo.cdyn.com. Good luck and let us know if we can help further.