Q*bert knocker repair

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May 162020

For those that don’t know what I’m talking about, a knocker is basically a bit of a gimmick that was installed into a Q*bert cabinet. The idea is that when Q*bert falls to his death off the pyramid, the game gives out a thump as he hits the bottom of the arcade cabinet using a coil to fire the knocker. Clever huh?

Since I’ve owned this cabinet I have never been able to get the knocker working, so this weekend I decided that this was going to change.

I had previously purchased a replacement known working knocker, as the plunger that makes the knocking noise, on my original knocker, had totally seized up. When I got the replacement I hoped that it would be a case of just swapping the part and enabling dip switch 6 on the main PCB and that was it. Sadly I was very wrong!

So the first thing I did was grab a copy of the manual and have a look a the wiring diagrams for the knocker area. I also confirmed that the plunger was freely moving, which it was, and that the 1 amp fuse at F6 had not blown, which it hadn’t

Next I started to look at what voltage was required to power the knocker, and started to trace it all the way to the PCB from the Power supply. I attached my black multimeter wire to the ground strap and then I could see +30v DC at the soundboard (A6, J1-P1), and the sound worked ok, and from there went backwards through all these locations:

  • Video Power supply pin 6
  • Filter board (A8, J3-P3) pin 10
  • Filter board (A8, J10-P10) pin 10
  • Knocker connector (A7, J1-P1) pin 2
  • Knocker connector (A7, J1-P1) pin 1
  • Filter board (A8, J10-P10) pin 9
  • Filter board (A8, J3-P13) pin 9
  • Main PCB (A1, J1-P1) pin 6

At every location I could see +30v DC so was a bit stumped at this point as to why things weren’t working. What I decided to do next was to verify that everything worked beyond the PCB, and thus proving that the PCB had a fault, so what I did was ground the tab of the 2N6044 transistor @ Q2 on the main PCB, and nearly got the shock of my life when then coil activated and the knocker fired. This confirmed that the fault was on my main PCB.

I didn’t know if the fault was now a logic one, or an issue with the transistor at Q2. So I hooked up my logic probe to the main PCB and attached it to the 74LS377 @ A8, pin 15. This is the chip which sends a signal to the transistor in order to activate the knocker. I then played the game and watched my logic probe as Q*bert fell of the pyramid. I could see that the logic changed from a low state, to a high state confirming that it was trying to activate the coil and knocker.

This then led me to believe that the transistor @ Q2 had failed and this was where the problem was. Unfortunately I did not have any spare 2N6044 transistors but noticed that the same transistor existed on the PCB @ Q1 and Q4 and these were optional transistors for coin 1 and coin lockout, so I used one of these to replace Q2. I also confirmed with a tester that Q2 had failed, and it had:

Once the new transistor was in place, I put the main PCB in the cabinet, powered up and the game, and was immediately greeted with the sound of the knocker. I then played a game, and when Q*bert fell off the pyramid was delighted to hear the sound of him hitting the bottom of the cabinet. Knocker fixed!