I recently was repairing a 3 Coin Single Pay Line Model 809 Bally Slot Machine, After I went through the entire machine fixing cracked and broken wires adjusted the switch timing, replaced bulbs, lubricated the entire machine and installed a quarter coin mech I was able to slip a quarter in it and pull the handle first try! It worked!
After a few pulls I noticed the Coin Accepted and Insert Coin lights were not working, I checked them and they were both dead. Put in 2 more, pulled the handle and noticed that when I pulled the handle quickly the new bulbs would flash super bright almost like a small camera flash. I didn't realize they were getting this voltage surge till I had gone through 8 bulbs. It only did it when I pulled fast on the handle so I thought it was bad bulbs. Once I realized that there was a relation to the handle movement I started looking for a possible pinched wire or cracked jacket. Even though I had just looked over the entire machine I went ahead and looked, much to my amazement I did find 2 wires that where getting pinched or may have been pinched but they both were around the door hinge area.
They were not in the usual places where the wires are stretching but rather getting pinched by the reel assembly parts that stick out on top and the wire loom up against the door. Put a shrink wrap over the wire and moved the loom out of the way. The other was pinched by the hinge itself, I did the same thing to it.
OK now it's good to go, new bulbs and pulled the handle fast and Flash! There goes 2 more bulbs!
Recheck all wires and look at the hopper and find a few wires that were cut and no longer used from an obvious hopper upgrade, I cut the wires and pulled the pieces out of the loom and wire tied it all back up. Found a few more wires on reel assembly not being used and just cut off and tucked back into the loom, fixed them also. Then check the handle release wires and bell wires all switches and looks Like I got it now! But this time I used 12V lams instead of the 6.3v to handle the surge better just in case.
Pull the handle and super bright flash again, now I'm starting to think of all kinds of possible shorts but just none that made any sense. I then went through the entire machines switch contacts to check them again for timing figuring I must have bent something while I did the initial repair and setting. Of course I found a few that had improper aliognment of the contacts and looked like they were ner set up correctly since new judging by where the pit was on the blade and not on the point itself. Another switch set that rested on the left siode of the reels had the pushing arm with an insulator very close to the top of that plastic insulator and looked like it may be shorting out across the mushroomed head that holds the plastic cylinder on the pin post. Moved that switch assembly lower on the post. Once again confident I solved it, nut no such luck. Then I remembered reading about trying to look for arcing in the dark with the door open.
Finger on the door switch triggered the coin mech pulled the handle and there it was. Left side of reels saw the arc. Finally I got it. Looking at the area proved to show no signs of arcing on any parts? Did a slow pull of the handle with the reel assembly out and saw that the large spring that keeps tension on the linkage parts was very close to the Large switch called "B" but I didn't think it could ever touch it. A fast pull however moved the spring slightly a skew and the loop would move outward just a fraction and arc across that switch.
Now I'm out of bulbs but fixed the problem. This may apply for any machine that has that spring and switch on the left side they run parallel with each other and barely pass each other so it may not look like they could even be close enough to arc. Something so simple yet follows no sensible logic to troubleshoot. Maybe having a wire diagram may help but I'm very doubtful anyone would come to this conclusion by studying the schematic.