What cap and resistor ratings do I use to mod my JP21/scart cable to get 75 ohm?

[davemarcotte]

Hi guys,
I’m new here and to a lot of this stuff.

I just received my OSSC 1.7 this week along with a JP21 to SCART adapter cable from Matt.

I have a ‘The Special Version of Combo AV’ supergun here I’m trying to get to work with the OSSC.

Matt mentioned I need to be careful and be sure it’s outputting 75 ohm. I need help getting there.

I watched the RetroRGB video (https://youtu.be/Z3sE3uk5kPU) on the topic and basically understand I can add a resistor and cap in line on the sync line. But his page only lists the appropriate cap and resistor values for particular consoles.

I’m ready to solder in a cap and resistor right now today but those pages don’t list the exact sized resistors or caps I’ll need for this supergun to make sure my new OSSC is receiving an exact 75ohm signal. Do you guys know what resistor and cap ratings I should use?

I don’t have an oscilloscope. Only a multimeter.

When I read the ohms on the bare cable it’s 0.5 ohm and goes up to 2.5 ohm when adding in Matt’s JP21 to scart adapter cable I just bought from him.

I’m thinking I’ll add the cap and resistor at the output end of the jp21 to scart cable. I just need to know the proper cap and resistor ratings to use.

Can anyone help me determine?

Thanks,

PS. I also think this supergun might be TTL. I don’t know how to test to be sure. Nor what voltage TTL puts out.
I am getting no picture on the OSSC from the supergun when connecting up the JP21 to scart adapter from the supergun.
I get the gray start up gradient screen, then hit button 1 and cycle through all the different options but nothing outputs.

[BuckoA51]

Even if it was TTL voltage level sync you should still get something from the OSSC, just don’t leave it on too long until you’re sure.

TTL sync voltage is between 0.8 and 5 volts.

[davemarcotte]

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.

[davemarcotte]

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.

[davemarcotte]

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.

[BuckoA51]

Do you know for certain the Supergun is outputting TTL level sync?

[davemarcotte]

I don’t, no. How do I test for that?

[BuckoA51]

You can only know by using an oscilloscope unfortunately.

I would suggest adding the resistor, if it doesn’t work it probably was properly attenuated to begin with and you can remove it, if it does, then it wasn’t.

[davemarcotte]

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.

[BuckoA51]

check that you have the RGB lines connected correctly. Try a different JAMMA board.

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