Hi @northbeachdogwalker By any chance do you have Ambient Light Detection switched on in Eco settings. Just checking that you have the latest firmware, can be checked on the support site.https://www.samsung.com/uk/support/ Anynet+ HDMI control can also cause issues. If this is switched on toggle it off in Settings-External Device Manager.
Just check that cables/connections are secure, if using surge protector check that is ok. Holding Power Button in for around 5 seconds until Tv reboots or unplugging from mains for at least 30 seconds can sometimes fix issues ,though you obviously tried disconnecting Devices. A factory reset is another idea.
However if troubleshooting provides no solution possibly there is an issue with capacitor or main Board. Would need to get in touch with Samsung Support for options.
I do not work for Samsung or make Samsung Products but provide independent advice and valuable contributions.
My Device- S21 Ultra 5G ( SM-998B/DS ) CSC= BTU , 12GB/256GB
One Ui 5.0,Android 13 .
