Hi, welcome to Just answer
How old is it ?
The problem is most likely a bad board (Probably bad SSB board) that is causing this issue and will have to be checked
I mean you have already checked it with set top box and non HDMI cables
how good are your DIY skills ? are you willing to replace/repair the part yourself ?
If you get it replaced professionally then it will cost around £140-£150 depending on the part that is replaced including everything
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