Recently my external 49” 4K Samsung TV is not being recognized by my iMac. When I remove or plug the cable on the back of the iMac it seems to know that an external monitor is present because the iMac screen responds / flashes.
Usually after several restarts of the iMac the proper connection will restore. It seems as time has gone one it is requiring more and more restarts to get it working.
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Try a new HDMI or VGA adapter.
Sorry to say its starting to sound like your logic board needs replacing as the GPU and the Thunderbolt port logic is soldered to it.
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Using a Thunderbolt to HDMI cable from Monoprice. Tried a new cable from Best Buy also rated for 4k and had the same issue.
Did you try the other Thunderbolt port?
I have tried another thunderbolt port. Same issue. I should also mention that when starting the machine in safe mode or recovery mode both the built in and external monitor to not display the graphics properly. The if I unplug the monitor it displays properly. What’s odd is that over time the issue seems to be getting worse. More frequent and requiring more restarts to make it work properly. Over the past year I would say.
Past 24 Hours: 0
Past 7 Days: 0
Past 30 Days: 0
All Time: 73
