Just speculation, but I had a DK machine with jailbars that ended up being due solely to the brightness being out of whack (recapped and then had to adjust the coarse brightness control on the main board).
(the problem manifested as a white-ish tint to the whole screen and distinct pink jailbars)

It's possible that a raw RGB mod commands too much total brightness for all 3 channels and needs to be dialed down a notch, overall.

This could be why the Japanese booster is the best one to use, all the homebrew self made ones do not compensate to a fine enough brightness.
With that being said, the jailbars are not a product of wiring or interference but actually a distortion created by brightness levels?

I'm 99.99% sure that video jailbars are caused by noise distortion on the pcb itself. You'd need to rewire the entire pcb to fix this. The reason I'm sure is:

I've done 3 "clean composite video" mods to nes 2 systems. These systems are known to have jailbars over their default rf circuit. When I did the composite mod I had everything wired up and noticed no jailbars. Then I hot glued the video out wire from the ppu against the pcb and suddenly there were jailbars. Then I lifted the wire off of the pcb and the jailbars were gone. Then I had the system running, with it running I slowly brought the video out wire closer to the pcb and then further away. I literally would real time watch on my tv the jailbars get worse and better as I moved the video out wire closer to the pcb and further away from it.  It's a noisy pcb and that's all there is to it.  I've built 5-6 different types of rgb amps and they all had the same problem.

On the nes2 its definitly interference...dunno about the rgb though.

Can't you just use shielded wires?

First thought I had, too.

the wires don't need shielding it only picks up interference when it's super close to the pcb. Trust me I can have the s-video and rgb wires go completely away from the pcb and the interference is still there. The interference is all over the system you just don't normally see it because composite video makes it too blurry to notice.

The NES is digital, the only interference that would hurt the PPU's video is interference on the video output pins... -facepalm- So...you're using logic wire to hook up video? heh.

Not entirely true...shit like interference on the data lines and adress lines (and most importantly power lines) can all contribute to the video being fuzzy/liney etc.

Is there any sort of shielding mat you can buy to cover the PCB.

Maybe this is why Nintendo had the extensive shielding within the toaster, so all the rest of the electronics around the NES wouldn't pick up the excessive noise it output.

glad to know someone here has half a brain

you can surround the nes or famicom pcb in as much shielding as you want.  That won't change anything.  The interference comes from the pcb layout itself.  To fix it you'd probably have to rewire the entire pcb...it's not worth the effort.

So when you raise the PPU off of the board and then run the wire of the PPU that outputs part of the signal away, it's interference from the board? Apparently the board you're using to mod wasn't designed good....but yet shielding wire would make all that irrelevant no matter what. How about you try it?

we have tried shielding.  The kits we use already have the ppu raised off of the pcb.  This has nothing to do with the kit pcb I had the same amount of jailbars before I started using these kits

But you said that if you kept that one wire off the PCB then there was no issue, correct?

If that one wire is properly shielded, it's distance from the board should be irrelevant.

That was for a clean composite video mod.  And the interference only affected the wire when it was pretty much right against the pcb.  Also composite is very blurry had it been rgb I'm sure you would have seen interference no matter where the wire was.  I wired up a rgb setup with the wires not coming anywhere near the pcb and still the interference was there.  The interference is in the entire system not just the video out wires, like I said.

So isn't the "easy answer" to consolize an actual PC10 rather than waste all this effort swapping PPU's?