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Osgeld

Coleco Gemini Composite AV Mod basic idea

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I did a search on the topic and there's not much out there

Jess Ragan has a thread here which takes the complicated approach of starting to mess around with the R2R ladder dac with mixed results, another option is longhorn, which while a quality mod, but just for composite is a bit of overkill

so I got a set of geminieseseies off ebay needing love for a good price, thought to myself, heck it cant be that hard ... and its not. Since there is not even a rough guide out there outside of the post I mentioned above, I thought I would share my results

The gemini is just a 2600 boiled down to its bare bones, and a TIA chip with a pinout, its not really that complicated to figure out

!! NOTE !!

While I share this information I am not responsible for damage occurred while trying to perform said mod, its a community service to share my notes, not a tutorial, or sure fire method. My mod's were performed on 1 unit ... from Canada and it is a later rev unit, mileage may vary, feel free to ask questions, and good luck

 

in a nutshell here's the deal follow this image map, add a video amp, some RCA jacks and boom profit on a AV modded system

post-35237-0-26460600-1490757394_thumb.png

post-35237-0-56299700-1490757396_thumb.jpg

post-35237-0-49702400-1490757397_thumb.jpg

post-35237-0-72312900-1490757398_thumb.jpg

post-35237-0-88472300-1490757399_thumb.jpg

 

The video amp I use is a fairchild FMS6363 or a TI THS7314 video amp, do a search there's a bunch of people selling them on various game forums for cheap, or if you want you can use the standard transistor amp floating around on the fourms

 

as far as the FMS or the THS I used a 1k input termination resistor (ie from video out on the pic above to ground) to bring the signal in spec, if you use the transistor amp setup you may want to use a variable resistor on the input to dial it in

 

I know I am being not that descriptive but its an AV mod, there's what you need to remove, and there's where you need to hook stuff up

 

 

 

 

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That's a very good composite output.
Nice job, and thanks for sharing!


P.S.

The pinout for the NTSC version of the Coleco TIA (VIC E4002) can be found here:
https://console5.com/wiki/E4002

The source was the expansion module #1 service manual:
http://colecovisionzone.com/page/coleco%20industries/module1.html

(the doc is low res and someone thought it was a good idea to slap a huge watermark on each page to make it even harder to read, but it's still very useful...)

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Good grief, I was pretty far off the mark on some of those guesstimates. What's really galling is that this pinout existed a good six months before mine, which would have made my job a hell of a lot easier. If I had set the resistors in the right places, I suspect the picture quality would have drastically improved.

 

Your mod seems to be the way to go, although it's a lot more complicated than the one I attempted. Alas, my Gemmy (and soldering tools) are in Michigan, so I can't test it out for myself. That little IC above C43... did that come pre-built or did you have to put that together yourself? How much would one of those cost?

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Fantastic Osgeld; thank you for sharing. Was the one you sold me the same featured in this photo? :D

 

yes it was

 

Good grief, I was pretty far off the mark on some of those guesstimates. What's really galling is that this pinout existed a good six months before mine, which would have made my job a hell of a lot easier. If I had set the resistors in the right places, I suspect the picture quality would have drastically improved.

 

Your mod seems to be the way to go, although it's a lot more complicated than the one I attempted. Alas, my Gemmy (and soldering tools) are in Michigan, so I can't test it out for myself. That little IC above C43... did that come pre-built or did you have to put that together yourself? How much would one of those cost?

 

The little green board is one I made up as a generic amp for composite mods, though the chip is for component, doesnt matter its all pretty much the same I just use 1 channel instead of 3

 

I see some on ebay on a board setup for rgb applications around 20 bucks, which is quite a bit considering its about 2$ worth of parts, I havent considered selling the ones I made but I am sure I can let one go if you want to pay a few bucks for postage

 

the signal comming out of the r2r ladder dac on pretty much all atari's 5 volts at that point, you may be able to just use a resistor divider to drop it around 2v and be ok, I prefer the chip as they include buffering and filtering as well as being a 75ohm driver specificly made for video, and it tends to get a slightly better picture

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the one pictured above is a NTSC model, just a Canadian one

 

you should be able to trace the middle pin of q4 back to a bank of resistors near the tia clone chip, one of the two ends of that resistor will get you a non compatible composite signal, which you buffer down to ~1v (either with a video buffer driver like I use, or even one of those simple transistor amp mod's for 2600)

 

it may be 1v peak to peak at that point but it will be offset by 5 volts so you got to tone it down

Edited by Osgeld
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I have attached a picture of the motherboard. I am waiting for my scope to come back from repair. I build a simple resistor amp, but i am having hard time finding a 5v to feed the amp and the Video out.

I am from Canada as well, and did the mod on a Canadian model without any issues.

 

I

post-62089-0-53299000-1510753185_thumb.jpg

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I have attached a picture of the motherboard. I am waiting for my scope to come back from repair. I build a simple resistor amp, but i am having hard time finding a 5v to feed the amp and the Video out.

I am from Canada as well, and did the mod on a Canadian model without any issues.

 

I

I know this is an old thread, but I have questions. That board in your picture with the gold color....did yours have a metal RF shield? I've got the exact same board, but it didn't have the shield. The last Gemini I had had a tan/yellow board that DID have the shield. Just curious.

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Update:

 

retrofixerdude sent in his board so I had a quick chance to update this post for those with the bare copper top pcb older variants

 

I did not kill the coils in the RF section which does help (a very tiny tiny bit with noise if your eyeball is touching the screen) but it should give the reference points we are interested in for this board

 

please see below

 

post-35237-0-52065900-1538104138_thumb.jpg

post-35237-0-66225500-1538104163_thumb.jpg

post-35237-0-91487800-1538104155_thumb.jpg

 

Enjoy and happy gaming

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Buenos Días...

 

Estuve mirando la información que enviaron, realice los cambios que dijeron pero no tengo señal de audio ni de video, mi consulta es como puedo saber si por que no tengo señal, ya verifique que llegue los 5v al Ic U3 y U2, todos tiene los 5v.

 

Saludos desde Paraguay

 

 

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On 3/28/2017 at 10:26 PM, Osgeld said:

I did a search on the topic and there's not much out there

Jess Ragan has a thread here which takes the complicated approach of starting to mess around with the R2R ladder dac with mixed results, another option is longhorn, which while a quality mod, but just for composite is a bit of overkill

so I got a set of geminieseseies off ebay needing love for a good price, thought to myself, heck it cant be that hard ... and its not. Since there is not even a rough guide out there outside of the post I mentioned above, I thought I would share my results

The gemini is just a 2600 boiled down to its bare bones, and a TIA chip with a pinout, its not really that complicated to figure out

!! NOTE !!

While I share this information I am not responsible for damage occurred while trying to perform said mod, its a community service to share my notes, not a tutorial, or sure fire method. My mod's were performed on 1 unit ... from Canada and it is a later rev unit, mileage may vary, feel free to ask questions, and good luck

 

in a nutshell here's the deal follow this image map, add a video amp, some RCA jacks and boom profit on a AV modded system

post-35237-0-26460600-1490757394_thumb.png

 

The video amp I use is a fairchild FMS6363 or a TI THS7314 video amp, do a search there's a bunch of people selling them on various game forums for cheap, or if you want you can use the standard transistor amp floating around on the fourms

 

as far as the FMS or the THS I used a 1k input termination resistor (ie from video out on the pic above to ground) to bring the signal in spec, if you use the transistor amp setup you may want to use a variable resistor on the input to dial it in

 

I know I am being not that descriptive but its an AV mod, there's what you need to remove, and there's where you need to hook stuff up

 

 

 

 

Anything I should be aware of before doing this with a pre-assembled 2600 mod kit from online?

https://www.ebay.com/itm/272679598200 

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may require some fiddling as those single transistor amps can be of various quality 

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