I have had good experience with this AVR for years. Now I have a new TV and 4K does not work despite activated passthrough. New firmware is also no longer downloadable from the master. TV & Apple TV 4K run directly connected to each other in 4K without any problems. If I go through this AVR, I can reach a maximum of 1080 P. I bought expensive Apple Belkin HDMI cables. No change My TV is a Phillips Ambilight 50PUS7304/12. Does anyone have any tips on what else I can try or do I need a new AVR?

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first of all, have you chosen the right connection on the back of the AVR? not all of them support 4k. 
possibly test with another source. possibly a setting on the tv? some tv devices allow settings per connection.
or at denon - Video | Output Settings - Video Conversion - "Resolution (HDMI)" -> Auto.

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