Do I need to add any components to an N64 RGB lead?

I'm currently modding a PAL NUS-001 (FRA) console and have connected RGB with 75Ohm resistors inside the console, as well as cut the sync trace in pin 9 and soldered it directly to the RGB chip. I'm using some 3rd party RGB leads, which I believe have some resistors tying the RGB signals to ground, but do I need to add anything to the SCART lead?

Also, I know this may sound stupid, but how can I tell if the console is actually outputting RGB instead of composite? I don't have any Nintendo AV leads to compare the image and my TV accepts SCART RGB and composite over the same connection (I'll see if I can get any more picture info from it, but it just seems to show "AV1" regardless of what's plugged into it).

 

The RGB signals from the mod need to be amplified to get a picture from the N64. I forget what amp is commonly used for this mod...

 

It doesn't require amplification.

 

Huh. I did not know that as I deal with NTSC stuff only.

 

Technically it does require amplification for a proper video signal. N64 DACs should output a 0.7 V TV-ready signal that is not capable of driving a low impedance load such as a TV or monitor.

As you have it the 75 ohm resistor will divide the 0.7 V into 0.35 V at the display, assuming the DAC is even able to drive a 150 ohm load (the output 75 R and the 75 R load at the display input).

A video/operational amplifier will precisely amplify the signal 2x and the amplifier's feedback compensates for the amplifier's output resistance so that the 75 ohm output resistor is the only source of output impedance; this will precisely deliver the correct amplitude to the display as well as impedance match the line which prevents signal reflection on long cables.

 

The French N64 has an amp in it, the S-RGB chip. Which is what was used in the SNES IIRC.

The DAC is the VDC-NUS

 

If that's the case the resistors on the RGB signals in the cable should be removed. That topology is strictly for the PAL SNES's driver circuit.

amp -> 75 R -> cable -> display input

 

As you can see in this picture, it does have an S-RGB amp like ​Bad_Ad84 already pointed out: http://webs.ono.com/kwyjibo/front.jpg

But since you can improve the picture quality in a SNES by bypassing the S-RGB and instead using a THS7314DR video amp like shown here http://retrorgb.com/1chiprgb.html I think it would be best to grab the rgb directly from the DAC and feed it through a THS7314DR just like you do with NTSC consoles. After all what is better for the SNES should work better with the N64 too.

 

If the resistors tied to ground affect the signal, then I can change that later, but I've read conflicting information about whether I should add anything directly to the RGB lines in the cable.

Initially I would like to know if what I am actually getting is RGB - are there any connections I can physically severe to ensure that I'm definitely NOT getting composite? I tried just cutting pin 20 on the SCART output, but as far as I understand its required for sync over RGB.

 

Replace the composite video signal with composite sync instead. This will give you a black picture if the TV is using composite, but a fully working picture if the TV is using RGB.

 

Are there series capacitors in the cable? If not you might want to add 220 uF caps either in the console or cable if the picture seems dim, which could mean the signal has a significant DC offset and the amplifier cannot source the current. If possible leave the caps off because it will result in better picture quality.

If there are series 75 ohm resistors in the console, you must remove the 75 ohm resistors to GND in the cable, they will load the amplifier too much, attenuate the signal and screw up the impedance.

Again ideally the signal path is amp -> 75 R -> cable -> display input

The easiest way to check whether you're viewing composite or RGB is to short one of the RGB components to GND (after the series resistor) and note if the color is missing.

 

I just disconnected blue and the colour disappeared, so at least I know it was displaying RGB. I now have removed all the additional components from the RGB cable and the picture looks bright enough on the LCD on my living room.

I tried hooking it up to my monitor with a GBS 8220 CGA to VGA adapter, but the colour looks way off on it for some reason.

 

Does your RGB cable have 220 uF capacitors on the R G B lines?
Keropi mentioned he didn't notice any difference between different RGB mods for the FRA N64.

 

The image definitely looked bright enough on my TV without the need for the capacitors.

 

You said "I'm using some 3rd party RGB leads". You're talking about a (ready) Nintendo RGB cable, or you build your own one? Else I would expect the 220uF caps to be there, since only the French SNES RGB cables did not have them (unless you got a custom cable from Consolegoods or something).

 

I bought them a while ago from eBay, but can't remember what they were advertised as, probably as SNES leads though.

 

And did you open them to check if they had any caps or resistors?

 

Yes - no caps, just resistors tying the RGB signals to ground. I removed the resistors and directly wired the RGB ground connections to ground.