The backlight with this model is built into the LCD display assembly and draws power, through a separate backlight cable, directly from the device's power supply and depends on the logic board to provide backlight information.
Diagnosing the failure in this would require you to use a digital multimeter, set to DC, to check for 23.3-25.7 V between the chassis ground and the 24 V test point on the logic board. If no voltage is present then the power supply is likely bad. If proper voltage is present then you should inspect the LED backlight sync cable, which is attached to the back of the LCD panel, for damage and then reseat it. If the issue persists then the next step would be to replace the LCD panel or logic board, in either order since they are both costly components.
Hopefully that gives you something to go on.
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Here is a[guide|6525|teardown for the 27" Thunderbolt display], the backlight portions of the internals are very similar to your model.
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Here is a [guide|6525|teardown for the 27" Thunderbolt display], the backlight portions of the internals are very similar to your model.
The backlight with this model is built into the LCD display assembly and draws power, through a separate backlight cable, directly from the device's power supply and depends on the logic board to provide backlight information.
Diagnosing the failure in this would require you to use a digital multimeter, set to DC, to check for 23.3-25.7 V between the chassis ground and the 24 V test point on the logic board. If no voltage is present then the power supply is likely bad. If proper voltage is present then you should inspect the LED backlight sync cable, which is attached to the back of the LCD panel, for damage and then reseat it. If the issue persists then the next step would be to replace the LCD panel or logic board, in either order since they are both costly components.
Hopefully that gives you something to go on.
Here is a[guide|6525|teardown for the 27" Thunderbolt display], the backlight portions of the internals are very similar to your model.