davemarcotte

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  • July 18, 2022 at 10:11 AM in reply to: What cap and resistor ratings do I use to mod my JP21/scart cable to get 75 ohm? #54162
    davemarcotte
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      Okay. Thanks so much. I’m going to research what the best value o-scope is to buy right now these days and learn how to use it. I’ve always wanted to anyway and this gives me a reason.

      No change after. Still noisy/blurry picture in the reddish tint zone. No idea why. More to learn and test I guess. Thanks for taking the time for me.

      July 16, 2022 at 2:36 PM in reply to: What cap and resistor ratings do I use to mod my JP21/scart cable to get 75 ohm? #54140
      davemarcotte
      Participant

        I don’t, no. How do I test for that?

        July 16, 2022 at 12:14 PM in reply to: What cap and resistor ratings do I use to mod my JP21/scart cable to get 75 ohm? #54136
        davemarcotte
        Participant

          Well, perhaps I’m having trouble because I’m testing with a Pandora Box DX Special with a JAMMA connector.
          When I set it to 15hz, I get nothing from the OSSC. When I set it to 720p 16:9 I get video and audio out of the OSSC. It’s just reddish and blurry.

          Setting the DX to 640×480 4:3 I get nothing out again although I was earlier.

          Not sure what’s up. Done for today though. More trying tomorrow.

          July 16, 2022 at 11:59 AM in reply to: What cap and resistor ratings do I use to mod my JP21/scart cable to get 75 ohm? #54133
          davemarcotte
          Participant

            So why does the OSSC wiki say this, “A 470 ohm series resistor on the console side of the cable is generally a good solution when using cables which are wired for the TTL-level sync output of a console.” ? https://junkerhq.net/xrgb/index.php/OSSC

            If we’re trying to get 75ohm output? Is it because even though the cable reads 470 ohm between the source pin and the input side of the capacitor, if read with an oscilloscope it’s actually at 75 ohm because of the variable nature of video signals?

            I am getting the same red, noisy/blurry image as before still with them in line. Not sure why.

            July 16, 2022 at 10:51 AM in reply to: What cap and resistor ratings do I use to mod my JP21/scart cable to get 75 ohm? #54132
            davemarcotte
            Participant

              Thanks Bucko. I tried again today and it is giving me video and audio output after all. So that’s good news.

              It does look overdriven. Noisy and slightly blurry and in the reddish tint.

              I think I’ll solder in a 470ohm resistor and a 220 10v cap and see what it looks like.

              Does it take an oscilloscope to see what the supergun is outputting through those two (cap and resistor) to tell if it’s optimal?
              If so, I guess I can begin budgeting for an o-scope.

              But what is an optimal voltage output? I know I’m shooting for 75 ohm output resistance, right? Anything else I need to track or consider here?

              Thanks so much.

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