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Thread: Dim RGB picture with Genesis on NAC cab...

  1. #1
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    Dim RGB picture with Genesis on NAC cab...

    SO...here's the info...

    I'm using a Jammacon adapter board (sold on arcadeotaku) with a SCART cable (sold by retro_console_accessories on ebay) on a model 1 hi-def logo Sega Genesis...

    It's being hooked up to a New Astro MS9 monitor. The picture is super sharp looking...but rather dim. It's also a tad dim using an RBG modded NES top loader. With the Genesis I have to have my contrast maxed out with my brightness cranked up a fair amount as well...which washes the color out, so then I need to bump up the RGB pots as well...

    I looked inside the SCART cable for the Genesis...she put 220uf 6.3v caps on each of the RBG lines. No resistors. Same as the NES cable she made as well.

    Anybody have any thoughts on why the image is dark on my monitor?

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  2. #2
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    Those cables are supposed to have the caps in them, but you're not using a standard RGB application so you may try removing them and see if it helps. Without the caps a Genesis will be way too bright on a regular RGB monitor.

  3. #3
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    Quote Originally Posted by GohanX View Post
    Those cables are supposed to have the caps in them, but you're not using a standard RGB application so you may try removing them and see if it helps. Without the caps a Genesis will be way too bright on a regular RGB monitor.
    http://members.optusnet.com.au/evilt...scart.htm#snes

    I found a link with a write up from eviltim with various schematics for RGB SCART cables...looks like he recommends that the Model 1 Genesis should have both 220uf caps and 75ohm resistors...not sure it that applies to arcade monitors specifically though, looks like he's linking to another page with some kind of arcade amp as well...

    So anywhos, you're saying that since I'm using an arcade monitor...that the 220uf caps in my cable are what's causing the picture to be dim, correct? What specifically do the caps/resistor serve in the SCART cable anyways?
    Last edited by ahcmetal; 06-01-2016 at 04:53 PM.

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  4. #4
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    The resistors/caps in the cable bring the signal down to a proper level for SCART displays (or PVMs).

    Without them you will have a more amplified signal... but probably not amplified enough for an arcade monitor. If it's already too dim without the resistors, removing the caps probably won't help you. There's a big difference between the .7vpp of "normal" video and the 3-5vpp that JAMMA uses.

    Talk with someone who's made a JAMMA genesis, you're probably going to need some sort of RGB amp to make this look right.

  5. #5
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    Quote Originally Posted by ahcmetal View Post
    http://members.optusnet.com.au/evilt...scart.htm#snes

    I found a link with a write up from eviltim with various schematics for RGB SCART cables...looks like he recommends that the Model 1 Genesis should have both 220uf caps and 75ohm resistors...not sure it that applies to arcade monitors specifically though, looks like he's linking to another page with some kind of arcade amp as well...

    So anywhos, you're saying that since I'm using an arcade monitor...that the 220uf caps in my cable are what's causing the picture to be dim, correct? What specifically do the caps/resistor serve in the SCART cable anyways?
    The 220uf cap and 75 ohm resistor combo is what is commonly used in the aftermarket scart cables based on the video encoder specs (also however seen schematics which show 470uf / 75ohm). However, the official Sega scart cable (adapteur r.v.b.) uses a different design:

    md scart cable.jpg
    segargb1.png

    That said I do not think the official cable would help you out in this situation - for your particular problem an RGB amp may be the only feasible solution. Removing resistance from the video lines would brighten up the image, but in your case the cable doesn't have the resistors. So resistance wise you could look at your wiring (cable quality, length, verification that there are no other resistors on the path to the monitor). If this was a PVM style monitor with video out terminals you could remove the terminator plugs on those and that would noticeably brighten up the image but its unlikely the case with your monitor.
    Regards, MtothaJ

  6. #6
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    When I build a Jamma Genesis I tap RGB from in between the 5.6K and 4.7K resistors. Then pass through 100ohm resistors and 220 caps.
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  7. #7
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    Quote Originally Posted by bustedstr8 View Post
    When I build a Jamma Genesis I tap RGB from in between the 5.6K and 4.7K resistors. Then pass through 100ohm resistors and 220 caps.
    ^the jamma genesis thread actually covers this IIRC.

    http://www.neo-geo.com/forums/showth...-Jamma-Genesis
    Last edited by Lemony Vengeance; 06-05-2016 at 05:17 PM.
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  8. #8
    Choi's Barber
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    How does the Saturn look through the jammacon?

  9. #9
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    Quote Originally Posted by kuze View Post
    How does the Saturn look through the jammacon?
    I think it looks great!

    The jammacon board actually had 2 pots on it, one for birghtness and another for contrast adjustments which I hadn't noticed. Once I started using those I barely had to touch my adjustment pots in the cab at all. The RGB modded NES looks great, SNES and Saturn both look super sharp with good colors and brightness. The genesis stuff looks great as well with a few tweaks of the pots...as far as simplicity goes though, plug and play and everything looks damn good, this adpater board is pretty slick.

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  10. #10
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    Nice, thanks for the rundown. I'd been considering picking one up mainly for Saturn shooties, but the multi console support makes the jammacon even more attractive.

  11. #11
    Another Striker
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    Here is a pic of a SEGA Saturn hooked up to my New Astro City via JammaCon device. So far, very happy with the result! I also bought the Pad Hack daughter-board, and I will be using this with my CMVS. I just had to tweak the brightness and contrast pots, and fiddle with the screen geometry on the cab. All good!

    jamma2console.jpg

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