Wednesday, August 7, 2013

Coleco Gemini: The Second Encounter

You remember this guy, right? Short, black, and ill-tempered, like Gary Coleman after Diff'rent Strokes was cancelled?


Well, after some encouragement from the fine folks at AtariAge,I decided to dig it out from the shed and take it apart, along with the mysterious project box attached to it. It turns out that the box, which I hoped would let me connect the Gemini to a television, offered nothing but an alternate power source, along with plenty of regret for those foolish enough to plug it into their big screen LCD television sets. Iesposta from the AtariAge forums offered this warning:
"Don't use that project box. If that 110V power soldered to the yellow transformer lead touches the Coax Connector and that is connected to a TV, well, it would be very bad, unless by 50/50 chance it was plugged into the house ground. 
If 110V did get into the TV or the RF of the Gemini, it would probably be powdered toast, man."
When I get a word of caution with a Ren and Stimpy reference in it, you'd better believe I listen! So I chucked the awful thing in the shed and turned my attention to the Gemini itself. Thankfully, the wires inside the machine leading to the project box weren't permanently attached... they slipped out of the system almost immediately after I opened it. That was a welcome surprise!

That was the easy part. However, hacking a system with little available documentation is considerably trickier. Sure, A/V mods for the standard Atari 2600 are all over the internet, but there's something you've got to understand about the Coleco Gemini. For all Coleco's efforts, the machine is not the same as a genuine Atari 2600, and if anyone tells you otherwise...


I quickly learned this lesson when I connected Ben Heckendorn's classic 2600 composite mod to the system in the usual way. Instead of a picture, I got a rolling green screen full of gibberish... a clear indication that the Gemini was not the twin of the Atari 2600 that Coleco had made it out to be.

That meant that I had to do a little detective work to discover the functions of the pins on Coleco's TIA knock-off, shown here:


Before I proceed, here's the 411 on the TIA. The Television Interface Adapter is the muscle of the Atari 2600, handling its video, audio, and input/output functions. Anything you see on the screen or hear from the speakers of your television set can be traced back to this little beauty. A real Atari 2600 uses a 10444 for its TIA chip; the Gemini has an off-the-shelf counterpart called the 73192. And that's where the headaches start...

The 73192 is wired differently than the 10444, with several key pins swapped. I was able to find half of them by tracing them back to the 6507, a budget-priced version of the 8-bit processor that all the cool game systems were wearing back in the 1980s.*

And some robots a thousand years later, apparently.
(courtesy DigitalRetroPark.de)
* The guys with Z-80s were all in the chess club, raising a pair of taped-up glasses with one hand while pulling down their wedgied underwear with the other.

The other pins had to be sniffed out with sneakier methods. For instance, the paddle inputs were tracked down by connecting a multimeter to the pins and looking for changes in voltage as I cranked on the dial of the Coleco Gemini controller. 

That wasn't the brass ring, though... I was after the video pins. I only managed to find those through a process of trial and error. Solder a wire here, get a dim monochrome image, solder another wire here with a resistor on the end, get a few more details, etc. Here's how it looked when I finally found the sweet spots on the underside of the board. You'll pardon the annotations!


After wiring up the prototype, I replaced it with something tidier and more permanent, as shown here:


The mod works as shown, but there's still room for improvement. Colorful titles like Pitfall! look great, but games set against a black background, like childhood favorite Crystal Castles and my own Solar Plexus, aren't as sharp as they ought to be. Crystal Castles in particular becomes an annoying challenge of hunting for barely visible gems, an issue I didn't have with the Light Sixer mod I'd done the week before.

You'll pardon the scanline. I chased
the beam, but it got away from me...
There are gems here somewhere! I, uh, think.
So hey, mission accomplished! Sort of. By the way, here's the pinout for the Coleco Gemini's TIA, since it seems to be such a closely guarded secret. The pins listed in red are guesstimates, but since I've got my system running composite video, I'd say they were pretty close to the mark.


All right, all right. I've been hitting the mod talk pretty hard lately, so I'll give it a rest and review some of the games I've been playing on my Atari systems in the next installment. Until then, folks!

1 comment:

  1. did you use a video amplifier chip (??forgive my ignorance) or are these wires going strait into rca outputs? ive recently wanted to fire up my gemini but dont have a crt of course.

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