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Booting a Pac-Man PCB from the Dead

Written 3/14/2009

As part of an electronics parts-n-stuff trade, I got an original 1979 Pac-Man Arcade PCB by Midway. Other than a hokey speed-up hack (which I promptly removed), all the parts were the original ones. Unfortunately, I don't own a Pac-Man cabinet, let alone an RGB monitor, power supply, or wiring harness. Eager to try the board out, I set out to hook it up in the quickest way possible despite these issues.

Looking up the pinout of the PCB, I found that it would not be too difficult to hack up an old external hard drive PSU I had lying around for the purposes of playing Pac-Man. The original board expects 7.5VAC and 12VAC, but that's ridiculous and I won't have that in my house. It turns out you can just throw 5VDC and 12VDC at the board and it'll run just fine. Looking at the edge connector on the Pac-Man PCB, there are small test points to solder to. Lovely; I won't need to deface the connector.

I hit a snag, though. For testing's sake, I only soldered on a little speaker to hear if there was any output to see if it was running, as I hadn't figured out a monitor nor controls ( I could coin up and start the game using a jumper wire between the coin input and ground, so that was a non-issue ). All I heard when I turned it on was a small pop (so the sound amp did come on) but no game running. There was a smaller pop that occured 4 times a second.

"Hmm", I thought. "What would make a small click four times a second?" I then remembered some helpful technical documentation I had read a long time ago about the watchdog installed on the Pac-Man PCB. I suppose that must have been getting power, because it indeed polls 4 times a second and reportedly makes a small click when it does so - many repair logs cite these same symptoms when the Z80 CPU isn't running, and hence the watchdog is trying to reset the board.

I checked with a multimeter on my 5V line, and sure enough, there was nothing. Looking at the beefy power circuitry that was designed to drop 7.5VAC down to a recified 5VDC, I saw that one huge white resistor (the big blocky kind, not the nice little ceramic kind) had a lot of burn marks near the PCB; I'm sure it got quite hot in the past. Testing it showed that it had gone totally open. Since this circuitry isn't being used any more to drop 7.5VAC down, I simply bridged the resistor.

Presto, the board booted right up. Without a screen, nor proper input, I cautiously jumpered 0V (ground) and the pin on the edge connector that corrosponds to the coin 1 input. Sure enough, blasting out of the speaker came the coin-up sound. Jumpering the start input had the game start up, intro music playing and pac-man chomp sounds all intact. Awesome!

At this point I really wanted to actually play it. Without a proper RGB monitor in the house, I was kinda stuck at this point, until I remembered that the Sega Nomad uses analogue RGB signals internally for its LCD screen. As crappy as an LCD of it is it'll do just fine for a non-scrolling game like Pac-Man. Grabbing some wire and an iron I took off the MD logic from the Nomad and wired up the RGB+S lines to the Pac-Man board.

As for controls, I had a bunch of old Sega Genesis/Megadrive pads lying around. On a day where I was more patient, I might have constructed proper circuitry to interpret the pad's signals, but on this lazy day I just ripped it apart, cut out the original pad lines, and soldered straight to the controller board and the test points on the Pac-Man PCB's pins. I only needed directions, coin, and start anyway. Eh, it works.

Lastly, I grabbed a real speaker and hooked it up (the sound amp on the Pac-Man board is impressive for what it is!), and lo and behold everything worked properly on the first try. Some day I hope to get a nice Cocktail Midway cabinet and make it into a Pac-Man machine using this board, especially now that I've given it a much more efficient power source than that awful stock one these came with.

Now it's picture time!

Here's the setup in its utilitarian glory, complete with one half of a Sega Nomad and octopus of wires going to the control pad. Note how the LCD drops some horizontal lines as the resolution and sync information isn't perfect for this display.

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