Looks like the board also has a brand.
It seems Italian.
The tube is 28". Why does the board says 17"?
It seems Italian.
The tube is 28". Why does the board says 17"?

Try checking voltages in different places (like PC case, TV video ground, grounds on transcoders, GPU video outputs, cable "shields") to see what is buzzing. Check AC voltage too. To pin it down, then unplug connectors to see if the voltages are still present.
A new PSU may improve things. PCs can leak significant current to earth ground - it depends largely on the power supply. This might not be an issue if all is grounded properly. Is it grounded properly to and through the mains? Some electricians cut corners and don't bother. I've measured 220v at a PC case before (in Thailand, usually no ground pin available), but measuring ~10-12v and zaps through forearms when using laptops is not unusual.
How are the transcoders powered? From a separate power brick? These can give very variable performance. It may be better to power directly from PC via USB or molex.
I wonder if your "Bandridge switcher" was providing enough AC coupling and ground isolation or whatever to protect your transcoders.It's an auto-scart switch, maybe. One of these: https://videogameperfection.com/2016/10/20/bandridge-selector-review/