I'm a newbie here and need your expert help. I have a 1976 Bally Flip Flop that has been in basement storage for 25 years and I'm trying to get working.
At first all the lights would come on but the credit/start button did nothing. I found that the common electrical tab had broken off one of the coin switches. After replacing the switch, the credit button worked and the machine reset, but the left kicker began firing and would not stop. I shut the machine off and checked the kicker contact switches to make sure they were not touching continuously. Turning the machine back on, only the coin door lights would light; nothing else works and the lock solenoid does not engage. Manually activating the lock solenoid allows all the playfield and backbox lights to come on. I think the problem is with the Game Over Interlock Relay, but I'm not sure. I included a pic of the Game Over Relay.
I have the schematic (I just so-so being at able to follow it). Any suggestions on what to look for or test would be greatly appreciated. I can also take whatever pictures anyone needs to help troubleshoot. I'm really looking forward to being able to play this classic EM pinball machine again. Thanks in advance.

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With the machine off, unplugged and the coil still wired, I'm getting a reading of 47.7 ohms across the 2 terminals on the lock coil (and about the same on the neighboring credit & reset coils). Also - the 4 slam switches (coin door, bottom of playfield, 2 in cabinet) are all open.

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I'll check the delay relay and report back as soon as I get home later this afternoon . . . The machine is acting exactly like it does when a slam switch is closed except that the timer bulb is not lit.

I manually moved the delay armature, cleaned the contacts and the machine fired right up! thanks a million. Most game elements are working after a 25 year hiatus, but one thing is not. The "Game Over" light on the back box does not light when the game is over (it had been lit previous to this). The ball count works fine and the actual game over operation is fine (flippers and playfield switches are off), but the "Game Over" light flashes very briefly at the end of the game and then stays out. Any suggestions on what I should check?

I got the Game Over light working. It was just the leaf switch on the game over relay that needed adjusting as you suggested (how do these things move after working fine?). Since then the outhole kicker stopped working, but I got that restarted. And now the ball count lights stopped working, but the machine does continue to "know" the ball count because it provides the required bonus on balls 3 & 5 and ends the game after ball 5.

That worked. Thanks again. Only 1 remaining issue. Occasionally, when I press the credit/start button, the machine resets (scores) and the credit wheel reduces by 1, but the machine stays in Game Over mode. Pressing the credit button again starts the game as intended.

Another fix and another problem. I cleaned the leaf switches and the credit reset problem has gone away, but now one of the score reels (thousands on player 2) doesn't always reset to 0. The machine thinks everything os okay and will start a new game even though player 2 is starting with thousands of points.