PAL MD v1/HDG - Jailbars

Discussion in 'Repair, Restoration, Conservation and Preservation' started by AN1, Sep 13, 2017.

  1. AN1

    AN1 Member

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    Hi all,

    I just got my Mega Drive! From what I can tell it functions beautifully, controllers all work, cartridges all boot up instantly and play fine, sound is great.

    I was planning on installing a region switch and then go EverDrive + NTSC ROMs, but seems like I should look into another issue first. I got some pretty severe jailbars. Federal prison strength bars even, I'd say.

    What's the consensus / state-of-the-art opinion on this subject? I remember reading about various fixes. I'd love to get rid of this artifact.

    Thanks! ;-)
     
  2. andynumbers

    andynumbers Active Member

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    If it's jailbars in the blue only over an RGB connection, it's a well-documented case of the Sony encoder interfering with the blue channel. The quick fix is to lift pin 6 of the encoder and tie it to ground, or lift pin 50 (I think?) of the VDP. Of course, you lose your composite video if you do this.
     
  3. AN1

    AN1 Member

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    I'm using a quality SCART RGB cable, but the jailabars are in most colors, perhaps strongest in blue/purple hues. I tried to take some pictures. As always, it's incredibly obvious in person but sometimes downright invisible trough a smartphone camera. But here:

    https://imgur.com/a/4v8Tq

    Most obvious in Steel Empire, those hues/large colored blocks really make the visible.

    Not sure if it's the RGB interference thing. It's not just in blue and the pattern is also different than what I've seen in other pictures it's two bars and then a space.

    I'd be grateful for any suggestions or anything else I can do to diagnose.
     
  4. rama

    rama Peppy Member

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    The problem is that several fast digital signals radiate into the analog RGB.
    Earlier MD models suffer the most from it, while later models (MD2) had improvements but are still affected.

    There are 2 ways you can go about it:
    - RGB bypass mod
    - shielding the RGB lines + disconnecting the color carrier clock (pin 6 mod) + better power supply bypassing

    I recommend the bypass mod. It's far easier :)
     
  5. AN1

    AN1 Member

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    I would've gotten a model 2 but the ones with good sound were too difficult to find :/

    You mean this bypass mod:

    http://retrorgb.com/genesis1rgbbypass.html

    ?

    That doesn't seem easy at all! ;-) Certainly beyond my skills.

    So just doing the pin 6 mod is not enough? What's the power supply bypassing thing?
     
  6. rama

    rama Peppy Member

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    There's no easy way to explain it all. Look up some guides for the color carrier disconnect mod and try that.
    Hopefully it's enough for your setup. You will loose composite video though!
     
  7. AN1

    AN1 Member

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    I haven't found any detailed guides. Basically, disconnect pin 6 on the video encoder or the connected pin on the graphics chip. Somewhere I've also read to connect pin 6 to ground. Never heard about the PSU bypass :/

    Any detailed instructions/references would be very helpful!

    I'd consider the RGB bypass with a separate amp, but that mod is labeled 'experts only' and as usual the board is sold out everywhere.

    I won't miss composite / S-video / RF ;-)
     
  8. AndehX

    AndehX You got boost power!

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    if you're gonna disconnect any pins, go with pin 50 of the VDP chip. I tried disconnecting pin 6 of the sony encoder chip and it made no difference at all. Apparently the interference is happening in the traces on the board and not just in the encoder chip, so stopping that clock signal before it's even left the VDP chip is apparently much better, although I've yet to try it on mine.
     
  9. AN1

    AN1 Member

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    So, yesterday I gave it a shot and disconnected the trace of VDP pin 50 on the top side right next to the leg. Unfortunately, while it did have some effect, but not much. Here's a before & after:

    https://i.imgur.com/547TkKB.jpg
    https://i.imgur.com/YnbvYHl.jpg

    It looks a lot worse in person and is still quite bad for certain color / picture combinations like Steel Empire here. It's certainly different, those doubled up lines changed into a more regular pattern.

    This is not from my MD, but it now looks exactly like that:

    https://imgur.com/a/QENR6

    Notice that the bars seem to fade in and out with a sine wave like intensity along the horizontal axis, very apparent in the blue strip at the top.

    It seems this fix is confirmed to work on a VA2, but I have a VA4 board revision that looks quite different. Any other suggestions for what to do about those jail bars much appreciated ;-)
     

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