Black screen on Imac with new LCD

1tomcat1

macrumors newbie
Original poster
Mar 3, 2017
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I just replaced a severely broken 21.5 inch 2012 A1418 13,1 iMac screen with a new LCD. The original broken LCD didn't display anything....just a black screen! I don't really have any history on the machine since I bought it at auction several weeks ago with the damaged screen.

The machine boots/runs fine, hooked to a alternate monitor. The newly installed LCD screen is black with no visible image even when I shine a light on it. Since it works flawlessly with the attached monitor, I think I can safely assume the problem isn't on the logic board or the GPU. Also Since the invertor is new, it shouldn't be bad. I now wonder if the lvds video display cable is damaged.

Does anyone have any other ideas as to what the problem may be before I order a replacement cable?
 
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nambuccaheadsau

macrumors 68000
Oct 19, 2007
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Nambucca Heads Australia
Pity it is a 2012 model in this instance as each time you take it apart it needs the new sticky surround. I would think there is something not connected properly. Have a look at the ifixit tear down steps when I edit this and attach

https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+EMC+2544+Hard+Drive+Replacement/16729

Steps 19 and 20 show in particular disconnecting power cable from the LCD. Suggest that is where the problem lay.
 
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1tomcat1

macrumors newbie
Original poster
Mar 3, 2017
24
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Oh sorry... I should have said, I haven't permanently secured the new LCD screen to the base yet with the sticky foam tape. I tested the screen in place secured to the base with temporary scotch tape and determined that when it booted the LCD remained black. I have checked and re-seated both cables to the logic board but still no go...

I did the PRAM and SMC reset with no luck!
 

nambuccaheadsau

macrumors 68000
Oct 19, 2007
1,835
433
Nambucca Heads Australia
Okay.

Double sure you locked the retaining tab as shown in pic # 20?

Tomcat you are braver than me undertake this job. This and later model iMacs are considered so touchy Resellers Down Under often send the units back to Apple in Sydney, Australia, rather than crack them open.

If you Google it, quite often the LCD has problems after the cables are disconnected to the logic board, and they are quite fragile.
 
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1tomcat1

macrumors newbie
Original poster
Mar 3, 2017
24
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Update: when I got the Imac the LCD was severely cracked and impact damaged at two points on the side edges of the screen. The LCD was taped to the Imac aluminum body and both LCD cables were disconnected from the logic board. There was no visible damage to interior components or to the aluminum body of the unit.

Last night I took a very close look at at both LCD cable ends. Under magnification, the LCD power cable end and the logic board connector appears to be intact and undamaged. The displayport LCD LVDS cable appears to have some pins missing or damaged on one side, the logic board connector appears to be intact and undamaged.

I tried leaving the LVDS cable unconnected with just the power cable connected. Still no video, just a black screen.

My question is does this make sense? Does both the power cable and the LVDS cable need to be connected for the video to work?
 

1tomcat1

macrumors newbie
Original poster
Mar 3, 2017
24
0
Thank You, you confirmed what I suspected! I had assumed the power cable ran the backlight and the other damaged cable carried the video signal to the LCD.

I have ordered the cable for the late 2012 Imac, A1418, 13.1, 21.5 inch LCD. If this fix works, I agree, I got off very cheap....LOL!
 

nambuccaheadsau

macrumors 68000
Oct 19, 2007
1,835
433
Nambucca Heads Australia
Look at your diagnosing this way. If its wasn't there no problems. When it is there it needs to be connected. If you Google it there are quite a few hits on this problem folk have when removing the screen. The plastic 'pudgie' or whatever one wishes to call them are great.
 

1tomcat1

macrumors newbie
Original poster
Mar 3, 2017
24
0
nambuccaheadsau: bad news and good news.....I replaced the damaged cable with the new cable received this morning in the mail. The screen remained black when I powered up and I scratched my head a bit! Then I thought maybe I should just try the new cable on the old cracked LED....and believe it or not....it lighted up and the apple appeared on the screen! So obviously there was a loose connection to the new screen or it is DOA. I will reinstall it again this afternoon and see what happens....I am pretty sure the result will be the same as before. This will probably get interesting since the RMA is to China....DUH!
[doublepost=1491068323][/doublepost]oops, must have been a loose connection....the new screen worked the second try! I'm a happy camper! Thanks nambuccaheadsau for your help!
 
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