Couple of questions about building a Supergun for MVS boards

[Marmotta]

I was planning on making a Supergun to test MVS boards, using the upcoming Retro Gaming Cables Mini DIN passthrough cable, but had a few questions about what components to use on the board, as the cables themselves will supposedly be completely passive...

For CRT use, does 75 ohm on the RGB lines, 100 ohm on the sync line and 180 ohm on the +5V line seem right? Would any or all of these lines also be grounded, as I usually just see resistors added in-line on SCART cable mods? I saw Citrus300psi posted a while ago about the unregulated voltage on the MVS composite sync output was far too high, would the aforementioned 100 ohm resistor be enough to bring this down to safe levels?

Thanks for any replies, I just want to try and get this right first time round.

[Frank_fjs]

Grab a Multimeter and start measuring voltages. Scope would be better if you have one.

Then you can calculate the resistors required.

http://www.ohmslawcalculator.com/voltag ... calculator

From memory, my MVS board outputs around 3VPP, your monitor will be expecting 0.7VPP.

So, using 75R on the RGB lines will give you an output of 1.5VPP. Double what you want.

Meaning you'll need to use a higher value resistor which will work in terms of voltage and brightness, however you will create an impedance mismatch. Your monitor is 75 Ohm terminated and is expecting a 75R in series along the RGB lines.

Put simply, you can't just use resistors if you want to do it properly. You will need to use a video buffer.

[Mobiusstriptech]

Yeah. What he said. If you want to do it right the first time. It's time to bust out the scope and design a circuit that outputs .7vpp 75ohm impedance signals. If you only plan to e er use the supergun for MVS boards, you are in luck. You can probably just figure out the necessary components and use the same device for all of them. Just remember you need to account for the boards that support stereo sound aka MVS pinout vs standard JAMMA which only has mono.

[Frank_fjs]

Someone has already done the hard work for you:

https://oshpark.com/shared_projects/R8NJ9UQ4

Order the PCBs and components and you have video taken care of.

[Mobiusstriptech]

That board needs a remix something fierce. Looks like borti made the schematic, so with any luck it is correct. That's a pretty reputable source for the circuit design.

[Marmotta]

Thanks for the replies guys, I was thinking there should be more to it, but have just seen so many CMVS guides out there that suggest just adding a few resistors to the relevant lines.

I don't have an oscillator (or access to one), but was hoping the MVS boards were ubiquitous enough that someone had already made a suitable circuit, which seems to be the case. As I'm going to be using exclusively with MVS boards, I've no need to account for much variance between signal sources.

I'd rather incorporate the design into my board, but couldn't find a schematic on Borti's github, so I've drawn a quick sketch of what I think it should look like (obviously RGB lines will be separated):



As for audio, I intend on taking stereo audio from the 9480F sound chip on the MVS board, since JAMMA doesn't support stereo input, as far as I'm aware. Are the audio signals "safe", as all the stereo mod guides for CMVS builds don't mention adding additional components.

[Mobiusstriptech]

It depends on the board. Not all of the boards support direct stereo at line level without a circuit. The mv-1fz is an example of this. For cases where line level stereo isn't available directly, there are usually options available already. I have my board up on oshpark for the mv-1fX if you want to use that.

[Frank_fjs]

There's so many variations of MVS boards so this won't apply to them all...

Some single slots have stereo audio output from the jamma edge. They have a switch which let's you select either standard mono output or stereo output, which outputs one channel over pin 10 and the other over pin L with a common ground. It's amplified speaker level output so a line level conversion is needed.

[Marmotta]

I'd most likely be using MV-1C boards and getting stereo sound using this guide from Jamma Nation:

http://www.jamma-nation-x.com/jammax/cmvs.html#stereo

Is there any advantage to keeping video ground separate from common ground in my design? There seems to be continuity between the pins on the MVS anyway.