I am not an electrical engineer, but I would say it’s the cables. Very cheap AV cables often have no shielding whatsoever, and RGB SCART cables, in particular, are susceptible to noise from the composite video line, which can manifest as visual noise (such as what you’re seeing) and audio hum, notably on all-white screens.
I recommend quality, fully-shielded cables from Retro Gaming Cables in the UK or Retro-Access in the US. To make sure that it is indeed the cables, I would pick one console and order a quality cable for it; if the better cable solves the noise problem, go ahead and order cables for the rest of your consoles.
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