when using imac as second external display, the screen renders but brightness is set to zero. I see no way to adjust this. Does anyone know how to correct?

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jkborys Author

Level 1

8 points

when using imac as second external display, the screen renders but brightness is set to zero. I see no way to adjust this. Does anyone know how to correct?

I am using two iMacs:

Primary iMac is iMac (Retina 5K, 27-inch, 2017)

Secondary iMac is iMac (27-inch, Late 2013)

both are running Mojave 10.14.6

When I plug my wireless keyboard directly into the Secondary iMac the display works properly. SOMETIMES. Sometimes it just stays black.

I know the display works, because in bright light from the window, i can actually see the outline of the background image that I use on that secondary display. But I see no way to turn up that brightness. brightness in preferences is already set to 100%.

I have searched forums and seen some work arounds, none of which have worked.

Help?

iMac Line (2012 and Later)

Posted on Sep 23, 2019 9:00 AM

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Answer 1

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ProbsRus

Level 4

1,126 points

Sep 23, 2019 10:05 AM in response to jkborys

Does the second iMac display backlight operate without any issues when booted as a standalone iMac? Is it possible that the second iMac has a hardware issue with display that is contributing to the problem? The fact that you can see that the image is present on the second iMac even when the backlight isn't operating indicates to me there might be other issues with the hardware.

It wouldn't hurt to try resetting the NVRAM on the second iMac to see if that makes a difference, although the intermittent nature of the issue is a bit confusing to me and again indicates a possible hardware issue.

Reset NVRAM or PRAM on your Mac - Apple Support

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Answer 2

jkborys Author

Level 1

8 points

Sep 23, 2019 10:17 AM in response to ProbsRus

There are no issues with the iMac itself. It runs beautifully as a stand alone iMac. Right after I posted this, I attempted to use the command+F2 command again, without the keyboard attached, and the screen displayed perfectly. The only thing I can think of is the cable and or adapters to convert from thunderbolt to usb-c. But I am baffled by the intermittent nature of the problem.

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Answer 3

ProbsRus

Level 4

1,126 points

Sep 23, 2019 10:27 AM in response to jkborys

In my opinion, the fact that you can see the faint image showing the normal outline that you expect to see on the second iMac tells me that the adapters/cables are probably OK and that the two iMacs are communicating properly. The mystery for me is why the backlight circuitry doesn't reliably turn on when the second iMac is used in Target Display Mode, but works fine in the normal iMac mode. I will continue to ponder this...

If you haven't already done so, these options may help rule out connection issues:

Try moving the Thunderbolt cable on the second iMac to the second Thunderbolt port in case

Try using a different USB-C/Thunderbolt 3 port on the primary iMac

And of course, the previously suggested NVRAM on both systems, just in case...

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when using imac as second external display, the screen renders but brightness is set to zero. I see no way to adjust this. Does anyone know how to correct?

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