Deciphering schematics like solving a puzzle

April 16, 2013 Dan KB6NU

My brother, Mike, goes to estate sales. (He also blogs about estate sales, following a fine family tradition.) He finds all kinds of cool stuff at these sales, but for the past year or so, he’s been most keen on acquiring pinball machines. He now has a shuffle bowling machine and a Merry Go Round pinball machine.

Mike is an accountant, and because he had very little technical training, he asked if I would help him troubleshooting the electrical parts of the machine. Being a former pinball player myself, I said sure. It’s been a lot of fun. Deciphering the schematics—and, fortunately, the schematics for most of these machines are readily available—is like solving a puzzle.

The Merry Go Round machine is a good example. Below, is the schematic of the primary circuit of the machine. (Click on the image to get a larger version of it.)

Figuring out how this circuit is supposed to work was a lot of fun.

Figuring out how this circuit is supposed to work was a lot of fun. (Click on the image to get a larger version.)

Shortly after he bought this machine and got it down in his basement, he called me and said, “It doesn’t work. I plug it in and nothing. No lights. Nothing.” I told him to take a picture of the schematic with his camera and send it to me. I got an email with the image above minutes later. Now that we were literally working off the same page, we could figure out what was wrong.

One of the first things I noticed is that the component symbols are drawn a little differently than the symbols we use in electronics. This can be seen on the bottom line of the schematic. The switches almost look like capacitors.

Another difference is that both mechanical switches and relay contacts are drawn the same way. The switches are given a name, such as “ANTI-CHEAT SW,” while the relay contacts are labelled with a single letter. The relay coils are also labelled with a letter making it easy to determine what relay coil closes which contacts.

Once I figured that out, I started decoding the circuit logic. Unlike some machines, this one does not have an on-off switch. After staring at the schematic for several minutes, I figured out that the key was the COIN CHUTE switch. This switch is supposed to be activated when someone puts in a dime. Closing that switch allows current to flow through the K relay, which closes the K contacts. One of the K contacts applies power to the transformer primary.

I had my brother plug in the machine, then very carefully close the COIN CHUTE switch with a screwdriver. This was necessary because the coin mechanism wasn’t working right. Unfortunately, nothing happened when he did this.

Next, I had him check the switches on the very bottom of the schematic: ANTI-CHEAT SW, SHUTOFF SW, and the BOUNCE SW. As you can see, all of these have to be closed for power to get to the transformer primary. I told him to unplug the machine and then use his multimeter to measure the continuity between the switch terminals. After a discussion of what range to set his multimeter to, how the meter measures resistance, and why unplugging the machine was necessary, he did so.

At first, he reported that all three switches were closed. We were doing this over the phone, so I had to take his word for it, but that just didn’t seem right. If they were all closed, something should have happened.

I thought it might be possible that the K relay coil was open or maybe shorted, so I had him check its resistance. It measured about 400 ohms. I then had him measure the resistance of the E coil. That also measured about 400 ohms. Since the both measured about the same, I guessed that they were probably OK.

So, I had him check the switches again. This time, he found that the BOUNCE SW mechanism seemed stuck open. He fiddled around with it, and when he thought he’d gotten it closed, he plugged in the machine again and tried the COIN CHUTE switch again. Success! Lights came on and the motor started turning.

He’s got a lot more work to do before we can start playing the game. Bumpers need to be replaced and the scoring wheels need to be cleaned and lubricated, but we learned a lot about the machine with this exercise. It won’t be long before we’re playing pinball.

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