If you can't find a box with an RF option, there's an easy solution. I've got a Sony B+W Portable with 4" screen in Rammy. It sits on an old VCR that doesn't work mechanically. I feed a DVD player in through the VCR Scart to get an RF coax output for the TV. Any old Free View box, with Scart Out, would work in just the same way. I find that WW2 films looks particularly effective in BW "Tiny" Screen for some reason
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Dave W