Imac replacement screen not working

mikesmithccs
mikesmithccs used Ask the Experts™
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Guys,

I have an Imac that appeared to have a bad screen. I changed out the screen and still no video. I can plug an external monitor up to the thunderbolt port and get video on an external monitor fine. Any suggestions?
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Business Consultant (Owner)
Most Valuable Expert 2012
Expert of the Year 2018
Commented:
Is it LCD or LED?  Assuming LCD (I am not sure why):

Screen (you just replaced)
Inverter (to run the screen)
Cold cathode tube (to light the screen)
Interconnections to all these.

Not likely the GPU as the external monitor would not light in that case.
mikesmithccsOwner/Operator

Author

Commented:
It is a late model IMac with 27 inch 4k retina display. Not sure about whether LCD or LED, I would also assume LCD. Everything is connected and appears to be seated well. The screen never lights up or shows any backlight. However, an external monitor works just fine. Im wondering if one of the connections on the board has failed.
JohnBusiness Consultant (Owner)
Most Valuable Expert 2012
Expert of the Year 2018
Commented:
I would suspect the Inverter, Cold Cathode tube if LCD.

If LED, I agree it could be a connection on the board (say where the video cable sits)
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JohnBusiness Consultant (Owner)
Most Valuable Expert 2012
Expert of the Year 2018

Commented:
Also, I am assuming "screen" meant the glass piece. If you replaced the entire unit, that would include the parts I mentioned. Sorry for my own confusion

So yes, look at the connection points on the mainboard.
if it is Inverter anyway there will be video on screen just very very dim.
mikesmithccsOwner/Operator

Author

Commented:
At this time I have sent the "replacement" screen back, as I believe it may also be bad. Going to try another and see how it goes.

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