Just two things then call it in.
1. Be sure the firmware is current. If you don't know how, call in and have Samsung walk through the steps.
2. Be sure the antenna is attached. Maybe try anther antenna. Rotate the antenna direction and try again.
Samsung UN55MU6290F Mfr Jan 2018
I’ve had OTA on it before. Now it won’t work. It won’t let me auto program. When I go to SETTINGS—-BROADCASTING the three options are Auto Program, Program Rating Lock Settings, and Audio Options. Auto Program is grayed out. If I select it, I get a message that “This function isn’t available.” I’ve unplugged it for 2 hours ... nada.
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