My first reaction is that it is the lock relay, but looking at the schematic it appears that Bally changed gears with Old Chicago and didn’t use the “hit the left flipper to turn on the GI” idea. As far as I can tell the lock relay is energized automatically as soon as power is applied and stays energized all the time. Also there are no lock relay switches in line with the GI like many other Ballys of similar vintage. So really there doesn’t seem to be anything there other than the fuse (I think it should be the one with a blue wire). Freds ideas above should be the best path.

Those are 2 separate circuits, top and bottom, its a bit confusing the way it is drawn but they are not related to each other, other than using one common leg of the power supply. It would have been better to stagger those a bit more away from each other to prevent that confusion.

The 455 bulb is a somewhat clever way of making the delay circuit work. When a slam tilt occurs, this will energize the delay relay, which then locks itself on via one of its switches (its the one right above the 455 bulb in the schematic). This switch is connected through that blinking 455 bulb, so for some period of time (not long, just a few seconds) the machine will go dead, but then when the light blinks it will cut off the lock-in and it will come back to life.

As always use caution when poking around in there with the power on! Be careful.

You are looking for 6 volts AC, be sure the meter is set for AC. You can put one lead on the yellow common like zacaj said. either at the transformer or anywhere else you can safely grab it, then the other lead right onto that wire that ties the two 6V fuses together on the left side. You gotta have 6 VAC there!

Then move that lead over to the other side of each 6V fuse, you should also see the 6VAC there as well. This will tell you its getting through the fuses OK.

Yech, I think you are right. This will need to be checked too. I am pretty sure this is the same connector I put next to the photo from the ipdb. That's pretty ugly looking, would be good to get more pics of that front part of the relay board.

Old Chicago hack (resized).jpg