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THS7314 Atari 7800 S-Video Mod


the_crayon_king

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So apparently by separating the connection between C7 and C-8 and connecting Luma/Chroma with a THS7314 which has a AC sync tip clamp you can get S-Video out.

 

I will draw up what I have so far later.

 

Example of output picture:

aiy0yNZ.jpg

 

Ex circuit:

sXSw4p3.png

 

 

Location of connection:

lKd4uUb.jpg

Edited by the_crayon_king
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Where exactly are you tapping the luma and chroma as inputs into the circuit? Great work by the way :) Nice, simple way to get s-video!

 

 

It should be noted that you need a 75ohm resistor to ground on the luma line to dampen the brightness or else you will loose some color.

May be just as good to use a higher resistance value on the pull up resistor 3.6-5m ohm

 

I have circuit boards coming in to get improved pictures out of many consoles it will work for this.

I will experiment more on this and the coleco S-vid/Component mod once I get those boards in.

vlEzGos.jpg

Edited by the_crayon_king
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Very nice! Thanks for posting this. I saw your other post regarding the Intellivision s-video circuit. Was going to try that soon when I get some time :)

Yea I didn't get far; I think my intv is broke :( I think you may need a sync tip clamp circuit on the intv as well.

 

The circuit board I have on order is this:

https://easyeda.com/hotdog6394/THS7360_SD_Universal_Video_AMP-o2WQ8qHZT

 

It should work with almost every console I own.

So instead of designing a circuit board for each console I designed one that should be able to be configured for very many.

Everything I am working on should be transferable to the THS7314 and could be hand-built.

 

At any rate the circuitry I used is inline with the THS7314 and THS line of amplifier documents which means I can't license it.

I do protect the physical circuit board designs themselves under CC-BY-NC-SA 3.0

Basically with the specification that nobody sells them as a stand-alone kit but that any other use is fine refer to:

http://atariage.com/forums/topic/255584-mod-kits-for-various-use/

 

What I mean by this is anyone could print these boards and install them (even as a 'mod service')

I will be selling them at some point but Ill have to create guides for each variation of console.

Edited by the_crayon_king
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  • 1 month later...

I've been thinking about this circuit. Just a note for myself for later:

 

7knQhhI.png

 

 

Should be noted that if I pull up resistor isnt needed then the DC restore caps (on the right side) are probably not needed.

This is all the info I have atm as I am very busy on other stuff.

 

Rebuilding parts of the original circuitry (besides the linking of C7 to C8) may improve video quality.

Specifically 47pf >47pf to ground for chroma and 47pf to ground on luma.

Chroma should probably be connected to L1 not R2 (470ohm) as shown

Edited by the_crayon_king
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You're colors are off on those screen shots. At least as compared to the NTSC versions of those games. I also was able to get s-video from using the same simple 3904 transistor mod that is used on the Genesis and SMS. Basically did the same on tapping chroma and luma. However as I recall it was a very bright picture so very washed out and I believe it was a bit unstable as well.

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You're colors are off on those screen shots. At least as compared to the NTSC versions of those games. I also was able to get s-video from using the same simple 3904 transistor mod that is used on the Genesis and SMS. Basically did the same on tapping chroma and luma. However as I recall it was a very bright picture so very washed out and I believe it was a bit unstable as well.

 

I need like a color bar test of some sort. Also I messed with those two color pots and didn't mark the starting point (so that could be apart of the problem). I didn't grow up with an Atari so Ill have to put two side by side to figure out what its supposed to be.

 

With the THS7314 the picture is completely stable. The THS73XX line of circuits take alot of thought out of making things cause they seem to have a lot more wiggle room. Not to mention that I basically made a board that will work with everything that way I can use the same board for many different consoles. I don't really know if its better to say amplify something with a transistor then output v/s using a pullup resistor going into a video amp (or just a video amp).

 

The pull up resistor I used for the first time I did this was 3.6M ohm on the Luma line, I am pretty sure it needs to be somewhere around 5.1M ohm if needed at all (I am just assuming it is needed because I put it on there when I drew up the schematic initially but I don't remember) . That wrong value is what is making the picture look washed out. Putting those caps in listed in my last post should also change the colors/brightness.

 

Any info anyone has on the RFU may help as well as there is probably more circuitry in there that may need replicated

Edited by the_crayon_king
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I need like a color bar test of some sort. Also I messed with those two color pots and didn't mark the starting point (so that could be apart of the problem). I didn't grow up with an Atari so Ill have to put two side by side to figure out what its supposed to be.

 

Top color pot closest to the cartridge port adjust the 7800 palette only. Pot below it adjust 2600 palette only.

 

RevEng's 7800 Utility Cart has a color chart featuring matching Hues 1 and 15, per factory setting instructions.

 

Additionally, the 7800 Diagnosis Cart after displaying a blank screen for ~10 seconds while running its diagnostic, will also provide a way to match hues 1 and 15. A video of the Diagnostic Cart can be seen here.

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