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Thread: MD1 PAL sync issue after 60hz mod

  1. #1
    Master of Shinobi
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    Default MD1 PAL sync issue after 60hz mod

    I have my primary Megadrive, a PAL VA6 unit, outfitted with a standard fare region mod: on/off/on switch wired to the appropriate three locations to create JP, USA, and EU modes (USA mode is if you leave the jumpers floating, JP and EU if you apply the appropriate voltage or ground to the appropriate jumper). I'm using a RGB Scart cable from Retro Computer Shack, and I have the ceramic cap C28 removed to prevent one of the clock signals entering the CXA video encoder, thereby almost complete eliminating jailbars (but also removing composite or RF video, forgot which, it doesn't matter to me).

    The problem is that when I have the machine running in 60hz mode, it occasionally starts giving sync issues. Most often it will just pad the entire screen to the left by a significant margin, like 30 pixels or so. But sometimes, in extreme cases, it will also go completely crazy and drop sync entirely, having the screen roll around in semi-random patterns. The problem takes a while to happen, sometimes it doesn't happen for an hour, sometimes it just decides to do this very often. I found that it most often happens either if:
    - the machine has just switched resolutions from 256 to 320px mode or vica versa
    - if I switch from 50hz to 60hz once the machine is already running - it takes much longer for the image to center in this case.
    But it can also happen completely randomly out of the blue which is extremely annoying.

    It doesn't seem to have any other problems if left running in 50hz mode.

    I've just recapped the entire machine, and it made no difference.

    Also, the colour contrast is a bit dull, compared to my VA0 JP model, but this is a minor problem.

    What could I do to get the image stabilized in 60hz mode?

    edit: I should mention, that the problem happens less often over time, once the machine has been turned on for long enough.
    Last edited by zyrobs; 10-01-2014 at 05:27 PM.

  2. #2
    Master of Shinobi
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    I've tried wiring c-sync (from cxa1145 pin 11 with a 220uf cap) into the scart output instead of composite, and now the screen is stable at all times, but always pushed to the left.

    Any ideas?

    Edit: since the machine is stable and centered with composite video as sync, but only after it "warmed up", I'm thinking some of the ICs may be dying - my first guess is the voltage regulator 7805.
    Last edited by zyrobs; 10-02-2014 at 10:40 PM.

  3. #3
    For great justice! Outrunner Tor Landeel's Avatar
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    Scart screen is always decentered to the left to the point that many tvs crop a portion of visible area.
    Other connections do not suffer this problem.
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  4. #4
    Mega Driven Raging in the Streets cleeg's Avatar
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    I've had big problems with this on my Panasonic LCD and SCART. Only with the MD at 50Hz I must say though. If you can afford to shell out on an XRGB framemeister mini HDMI upscaler I'd recommend one. It solves all these sorts of problems. Let me know if you need any help, or go to this thread...

    http://www.sega-16.com/forum/showthr...893#post685893

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