Coleco Gemini A/V mod

[Jess Ragan]

Well, it took a lot of online research and a little trial and error, but I did get a simple A/V mod to work with the Coleco Gemini. I've published my findings here:

 

http://kiblitzing.blogspot.com/2013/08/coleco-gemini-second-encounter.html

 

My only beef is that the contrast is poor, making games set against a black screen tough to play. I think using different resistors might help sharpen things up a bit.

[CatPix]

The signal need to be amplifier via transistors and filtered after that to deliver a stronger signal (funny fact : on B&W early sets, the contrast knob was just filtering more or less the composite signal coming from the RF. Nothing else.)

[Jess Ragan]

That's entirely possible. I know there are other mods out there that are more complicated than this one, and likely achieve superior results. However, I did the same mod on a Light Sixer, and the picture is WORLDS better. There's just no comparison. Blacks are blacker, the artwork is sharper, the whole nine. I suspect that the mod on the Gemini wasn't done properly... I was basically spitballing for video pins and happened to get a picture because wires were connected in the right places. I'm sure I've got CSYNC, COLOR, and the LUM pins connected, but I don't think I have the resistors in the right place. The only way I can find them is to use a prototyping board to swap out resistors on the fly. The process of unplugging the system and monitor, plugging in the soldering iron, swapping resistors, and plugging back in the system and monitor is hugely monotonous and takes entirely too much time.

[CatPix]

And rise up the risk to damage something.

No problems your mod is the prime basis for any other modder to build out something else from it. I suppose that any Atari A/V mod may be adapted on the Gemini, with maybe some tweaking to ajust on different output power from the chips.

[jhd]

Finding space inside of the case may be an issue, but what about using potentiometer(s) in place of the fixed resistors? This should make it easier to fine-tune the colour.

[Jess Ragan]

Finding space inside of the case may be an issue, but what about using potentiometer(s) in place of the fixed resistors? This should make it easier to fine-tune the colour.

 

That's the way the original mod was designed, yeah. I was actually thinking that I could connect each of the wires to a potentiometer, dial them to the proper resistance, and measure each one with a multimeter so I'd know which resistors to replace them with when I was done. I think I've got some pots around somewhere that I could use for that purpose, but they're extremely tiny.

[Ghrayfahx]

How did this turn out? I just found a Gemini, 10 controllers and 65 games for $40 at a yard sale! The RF output leaves a LOT to be desired. Plus, my main TV is a multi system one with a PAL style RF connector. (I live in the US and don't have any more PAL adapters) I definitely would like to look at modding the system. It's not a real 2600, so I feel a lot better doing it on this than the real thing.

[Hyperboy]

I just found a Gemini, 10 controllers and 65 games for $40

Nice freakin' score!!

[Ghrayfahx]

Thanks. Its pretty awesome. I don't have anything right now to convert 300ohm to 75, so it looks like CRAP right now. Still, it's great to have some classic gaming in my house again.

[Jess Ragan]

Clearly I need to pay more attention to old posts.

 

It seems Osgeld found the secret sauce to make this mod work properly. His post is here:

 

http://atariage.com/forums/topic/263822-coleco-gemini-composite-av-mod-basic-idea/