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Thread: Sega Genesis Model 1 S-Video Mod Problem

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    Default Sega Genesis Model 1 S-Video Mod Problem

    Hey guys, today I modded my Sega Genesis Model 1 (HDG) that has a Sony CXA1145 encoder with S-video and it looks great (with the exception of the rainbow banding). The only problem I have is the intro to some games (Sonic games are most notable) have really bad interference and/or the picture cuts out for a second and then comes back. I tried this on a second model 1 and it did the same thing. I used two 75ohm resistors and two 220uF 16v capacitors. Please help and thanks.

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    The medium-sized mang. Raging in the Streets Lastcallhall's Avatar
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    It's because you're using a cap/resistor combination on your luma line. Try using a 2N3904 (or compatible) and a 33 ohm resistor (or so) for the line instead. input into the base, +5v into the collector, and the resistor --> output on the emitter.

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    I actually ordered a crap ton of 2N3904 transistors and 27ohm resistors a few days ago, they should be here tomorrow. I'm messing around with different S-video circuits to see which one is easier, cheaper, looks better, etc. Thanks for the info.

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    Master of Shinobi MaxWar's Avatar
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    My favourite version of the mod is this one ( minus the cap that is in reverse )
    This pretty much amounts to the adjustment LastCallHall suggested


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    The medium-sized mang. Raging in the Streets Lastcallhall's Avatar
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    About the cap... I asked my EE instructor who holds a masters in EE and ha 30+ years experience about that, and he said essentially that there's enough negative biasing to make it not really matter which way the cap is pointing. Still, I tend to follow the CXA1645 plan and go from chip --> positive --> negative --> out

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    Master of Shinobi MaxWar's Avatar
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    I had tried both orientations and could no observe any negative side effect in the signal. I have been told that It was just going to make the cap age faster.
    Honestly I lack the theoretical knowledge to explain why that would be so but I tend to always respect the orientation.

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    Default Sega Genesis S-Video Diagram

    Alpha, try the following;
    Genesis Svideo_1.JPG
    Last edited by fsa; 10-31-2013 at 11:36 PM.

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    Quote Originally Posted by fsa View Post
    Alpha, try the following;
    Genesis Svideo_1.JPG
    Otherwise known as 3 posts above.

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    Master of Shinobi MaxWar's Avatar
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    I kinda like that second diagram better though
    Less junk on it.

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    I did notice that the first diagram calls the voltage regulator a "transistor," but otherwise I prefer that diagram instead. The more information the better, IMO.

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    Master of Shinobi MaxWar's Avatar
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    Also, I like to use a trimpot on chroma instead of a resistor. So I can finetune the value to minimize jailbars.

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    Quote Originally Posted by MaxWar View Post
    Also, I like to use a trimpot on chroma instead of a resistor. So I can finetune the value to minimize jailbars.
    That's such a simple idea that I never really did for the s-video circuit. I just accepted the jailbars as part of the experience. I'll have to start doing this from now on.

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    Master of Shinobi MaxWar's Avatar
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    Quote Originally Posted by Lastcallhall View Post
    That's such a simple idea that I never really did for the s-video circuit. I just accepted the jailbars as part of the experience. I'll have to start doing this from now on.
    You can make it pretty good but never perfect though.
    If the chroma is too low, you get noise in the picture. If it is too high, you get jailbars.
    The problem is the noise start appearing before the jailbars are completely gone.
    But if you lower chroma level to just before the noise becomes visible, you get minimal jailbars.

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    You could try a stronger resistor on chroma as well, I've found that 330 ohms is a nice sweet spot with minimal jailbars and little to no noise.

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    Just used the circuit Lastcallhall and MaxWar suggested, and the interference is gone! :-) Thanks for the suggestions and info guys.

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