Design for a Neo Geo MVS RGB Mod

[Jumpingmanjim]

I have tried to design an RGB circuit in Kicad using the THS7316 for the Neo Geo MVS.




It uses voltage dividers to lower the input from 4.1V (I found that voltage here) to 0.7V. The THS7316 then amplifies it 2X to 1.4V, and then the 75 ohm resistors form a voltage divider with the 75 ohm termination in the TV/scaler to divide that back down to 0.7v. The capacitor on 5V and GND is there because the data sheet recommends it to prevent ringing.

Sync is based on the assumption that the MVS produces 5v TTL sync, and then I use a voltage divider to take it down to 0.7v

There is a lot of confusion about what a proper circuit is for the MVS. I have seen people just use resistors and leave it at that. At least with this design you will be blowing up the chip instead of your framemeister

If anyone could offer some improvements that would be greatly appreciated.

Presently i'm unsure about:
- Capacitors on the THS7316 output (I think the MVS is DC Coupled???)
- Correct MVS output voltage

[Konsolkongen]

I’m actually looking for a correct way to do this as well. I got basically the same circuit in my cMVS, but with slightly different resistor values on the voltage devider. Mine does seem a little off, so yours may be the correct values. I’m using a 1C board if that makes any difference.

To measure the correct voltage on the RGB line, I would need the MVS to display an all white screen, right? Not sure what the best option is for that :/

[Kez]

There is a 240p test suite equivalent for the Neo Geo. If you happen to have an expensive flash cart that would be the easiest way. Looks like you can also write it onto a donor cart if you have the right equipment.

I consolized my MVS recently and couldn't find a lot of information regarding proper RGB circuits, and wasn't happy just using series resistors. In the end I used an AV driver I had going spare and used the supergun circuit Tim posted on his site, so I can adjust the brightness manually on the back. I have found a setting I am pretty happy with, so I guess I could just measure the pot and replace it with a voltage divider but I already have a hole in the case so not much point now.

[Konsolkongen]

Unfortunately I don't own a flash cart for my Neo Geo, and I don't have a game I'm willing to sacrifice either

[Jumpingmanjim]

I’m actually looking for a correct way to do this as well. I got basically the same circuit in my cMVS, but with slightly different resistor values on the voltage devider. Mine does seem a little off, so yours may be the correct values. I’m using a 1C board if that makes any difference.

To measure the correct voltage on the RGB line, I would need the MVS to display an all white screen, right? Not sure what the best option is for that :/

Have you got the unibios? You could use the test function to show the colour bars at least.

[Konsolkongen]

I do, but I’m pretty sure that the whole screen needs to be a solid white colour or it wont work.

[nam9]

I've done this for my own CMVS builds.

Same idea as yours with respect to resistor values, but I just used the the standard PCE RGB Amp from mickcris with a few modifications:



(6.8k ohm (R1) and 1.4k ohm (R2) which gives 0.7v output).

That takes care of the RGB lines nicely. My main issue though is that CSync is not treated properly in this circuit.

Given that this is an MVS and you need to buffer/amp RGB, Sync and Audio, personally I would take a look at Tim Worthington's A/V Driver.
That should be able to do everything you need in a cost effective manner compared to a bunch of individual circuits...

(One caveat being you would still need to pre-mix the L/R/SFX audio into 2 channels before the A/V Driver...).