I decided to fix this cheap unbranded scart switch. It has a function to share a signal from input B or C with the 1st input A which led to interference and
6 Port RGB Scart/RCA/Component Switch
RGB Pick Up Thread Of Swag! - Video Games - Retro Game Boards
Hama SCART Switch Box, 3-Way Black : Amazon.com.be: Electronics
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Scart-Umschaltpult »AV-100S«
Hama AV Selector 100S SCART RGB 3 in 1 out Switcher Retro Gaming for Sale in South Gate, CA - OfferUp