How to check attenuation in CMVS setups without an oscilloscope.

We’ve recently had a lot of questions about making cables for MVS setups like CBOX or “insert setup here” scenarios where we just don’t know if the mod is reliably set up for proper compatibility with a TV/other TV tech like scalers, line doublers, switch boxes etc.

We don’t know what’s in your system and we can’t reliably provide cables that won’t result in a disasterous scenario if your setup is not attenuated. Nor can we provide cables that are guaranteed to work if you don’t know what’s in your actual console. We can be confident of certain MVS builds from some manufacturers but not others. Already we know that earlier CBOXs not attenuated, later ones are. So in these cases it’s really up to you to find out.

First off, checking csync *under load* ie either connected via a throughport Bnc from a PVM, or hooked to a plugged in SCART cable, or simply via a 75 ohm terminated hookup into your scope will give you about 1.75Vpp from an UNATTENUATED MVS. We’ve confirmed this.

When you don’t have a scope, things get a little more tricky but you can still find out! I’d use a cheap multimeter to check things for certain in this case. To that end here is a guide to finding out what’s in your CMVS.

This requires taking your system apart and checking it for resistors. We’re not responsible for any damage you do in this scenario so please be careful and follow correct practices! If you don’t know what you’re doing here it doesn’t come recommended but the alternative might be breaking your OSSC or G-Scart with the wrong cable and we really don’t want that scenario either.

There actually is an attenuating resistor on the MVS board, but it’s not high value  enough.

R51 is the series resistor on csync in an unmodded MVS board. It is very difficult to see as it’s a super tiny SMD but it should have “101” written on it. This is a 100 ohm resistor. Citrus3000 wrote a post on Twitter about changing this to a 620... now I don’t know if people who make CMVS setups actually do that or whether they attenuate their own additional boards that generate the video output.

So the resistor you are looking for is circled in red. If this says 101 on it (use a magnifying lens or camera to read it unless you have superhero style vision) then this is the stock resistor that these boards had from manufacture. It doesn’t mean your setup is unattenuated for 75 ohm sync, because there may be extra resistors elsewhere.

You can check resistance from the right side of this resistor into where csync connects to your AV port. If you get a straight 100 ohm (or very near it) then this is an unattenuated setup and you need attenuation in the cable.

If you get no results then.... your csync signal may well be going into some video processing somewhere and we just don’t know. Look for things like sync strippers, common buffer ICs, inputs into video amp ICs and check for continuity from R51. You can probably work something out from that if you know what these components commonly output - or send us a photo with descriptions and we might be able to tell.

Here’s another picture from an *unattenuated* CBOX. R51 IS connected directly to the csync in the DIN on this particular system, but you might also see this little red board in these units.

This is the pcb the CBOX uses to generate svideo and composite video. I’m pretty sure this is a generic board normally used for something else because of the way it’s spliced over the pcb underneath it and the unpopulated bit for an IC that looks like a sync stripper would normally go there. Now the bottom solder pad is connected directly to the csync pin in the port. Actually it’s connected to BOTH the csync pin and where composite video would normally be in a Neo Geo. So you can check resistance between this board and the accessible pins on the opposite side of the din to what you see here, but I’d also check from R51.

You can see in the picture that the creator of the CBOX has made some later modification to connect stereo audio to the din.

Perhaps in later units there is a resistor between that bottom pad and csync in the port. You might even be able to physically see a modification. Point is: we don’t know. This is an early CBOX that was loaned to us and there was nothing in this particular unit, no attenuation at all on the output video board.

At any rate I hope this helped! 

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