I have some data to provide that might or might not be helpful since it's more general and not specific to the FU-RGB board (I don't have one yet). Short version is that
even with all the SSDS3's video components on top removed (are there any on bottom?), the mini-DIN isolated from video ground, ALL video related pins on the SSDS3 female expansion port disconnected, and using Voultar's internal RGB amp in my Core Grafx....the SSDS3 _still_ leaks noise back into the internal RGB that isn't there when the SSDS3 isn't connected (or is connected but a real hucard is being used, thus basically turning off the SSDS3 from my understanding).
NOTE: My Core Grafx has been recapped and had the voltage regulator replaced with a L78S05
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Longer version previous posts from the main SSDS3 thread:
1st:So I tried to replicate FBX's work on the video circuit to clean up the video signal on my PC-Engine when using the SSDS3. Like him I've got a Voultar PCE RGB amp in my PC-Engine, but mine is a Core Grafx rather than a Super Grafx like FBX has (seems like people with a Super Grafx are having better luck with video quality in general). I've also been turning off the video low pass filter (LPF) on the OSSC to see just how much noise is being introduced.
I got in my hot air station today and removed the video components from the SSDS3 and there was little or no visual difference, as expected basically. Next I was going to clip the video pins, but I can't get to all of them while the expansion connector is still attached to the SSDS3. I was able to clip RGB and csync however, so those pins never make it to the SSDS3. However, I can still see interference added by the SSDS3 even when loading hucard games from the SSDS3. This interference is far reduced (and not even the same pattern) when loading a real hucard.
All that's left for me to cut/disconnect is composite video A22 and analog video ground A21. To do this I'll have to pretty much desolder the expansion connector. Before I do this, I have some questions about analog video ground that I'm hoping FBX or someone can answer.
(Note, I am getting RGB from the PCE DIN port, and audio from the SSDS3 mini-DIN port)
So when I was checking things with a multimeter on the SSDS3, it appears only analog video ground (A21) is connected to the SSDS3's mini-DIN. Analog audio ground (A2) was not connected to the mini-DIN. If I disconnect A21, will that not cause some audio issues or at least make the audio cable shielding worse since the mini-DIN's ground would effectively be floating? I know eventually that the mini-DIN would need to be removed and remounted in a way that it's not connected to the SSDS3 in the future if I were to ever put a RGB amp in the SSDS3, but I was hoping to avoid that for now if possible while I'm just getting the video perfect and waiting for a thorough audio solution.
2nd:Well, I got impatient

. I desoldered the mini-DIN and expansion connector on the SSDS3, clipped composite video and analog video ground, glued the mini-DIN back in place on it's side with a piece of electrical tape between it and the PCB so ground would not reconnect, reconnected audio, and connected analog audio ground to the mini-DIN........ and it didn't really seem to do anything :\ (I also tested before putting the mini-DIN back in and it was the same).
I'm wondering if the Super Grafx just has more filtering or something inside the console. I recorded a video, but the recording plus youtube transcoding softened the noise effect some, but it's still visible.
https://www.youtube.com/watch?v=Kmyds4gSsLc . When playing a game on the SSDS3 with the OSSC LPF disabled, there's 'digital looking snow', kinda like the 'snow' of a old bad antenna signal on an old tv. There's also some diagonal lines going across the screen at different speeds.
First game loaded is direct from hucard. The other ones are from the SSDS3. The LPF on the OSSC is off most of the time, but I turn it on for a couple of seconds twice or so (I was going to add annotations to the youtube video specifying where, but didn't realize they've discontinued the ability to do that).