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Black Screen - Computer Works Otherwise

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StockDC2

macrumors regular
Original poster
May 16, 2011
212
12
Hello, I currently have an issue with my iMac. For some reason, the screen does not turn on. I know it's not the backlight because nothing is visible on the screen. Other than the screen, the computer works fine.

When I removed the screen to see if the problem was a loose cable, everything showed up just fine on my external monitor. I know the computer detects the screen because when I turn on the computer with the external hooked up, the desktop is expanded across both monitors. Also, all 4 LEDs are illuminated on the logic board.

Can it be something as simple as the cable needing to be replaced? I believe the LED board can be ruled out since the screen doesn't turn on at all. Is this a fair assessment? Thanks in advance for any help :D.
 
M

Macintush

macrumors newbie
Jan 16, 2015
4
0
What model iMac is it?
As for the backlight vs a full screen failure. I can tell you that they are 2 different circuits and the backlight can most certainly go out but the screen remain working. I had this exact problem once.
You can actually shoot a flashlight at various angles into the screen from the front to see if it is functioning. Be sure to play something like a video with motion because it's a very faint image using a front light.
You said you have a 2nd monitor so you an actually control it, right?
 
S

StockDC2

macrumors regular
Original poster
May 16, 2011
212
12
Thanks for the reply.

I've already used a flashlight and nothing is visible on the screen.
 
M

Macintush

macrumors newbie
Jan 16, 2015
4
0
Well, if you are into adventure. I've done this a couple times but you need some extra hardware.
Depending how old your machine is it will have 2 connectors to the screen. One LVDS that has the signal and the other for the backlight. I dismantled an old 19" screen I had and carefully plugged the iMac LVDS connector into the 19" screen. but had to keep the 19" backlight plug connected since those didn't match. Also LVDS seem to come in 20 pin and 30 pin and laptops use a smaller connector that won't work for you.
So, running the 19" with iMac LVDS 30 pin signal and the original 19" powered up for the backlight. The hardest part is that the cables are super short and manipulating thing to make it work.
Not for the faint of heart. Maybe not for a NEW machine either. On an older machine there's a lot less to lose
 
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StockDC2

macrumors regular
Original poster
May 16, 2011
212
12
It seems that I'm an idiot. The backlight is bad. I took an LED flashlight (not the one from the my phone which is what I used before) and I can clearly see my desktop. I'd say that it's the LED board correct?
 
S

StockDC2

macrumors regular
Original poster
May 16, 2011
212
12
hfg said:
Not to simply overlook the obvious...
I assume you tried the "brightness" keyboard controls to be sure it wasn't turned off? :)
Click to expand...

I removed my HDD in preparation for an upgrade to an SSD and didn't think about playing with the brightness controls :(. For what it's worth though, I did the PRAM reset which should have reset the brightness back to default levels. I ordered the LED board though. Crossing my fingers that it works!
 
redheeler

redheeler

macrumors G3
Oct 17, 2014
8,419
8,841
Colorado, USA
hfg said:
Not to simply overlook the obvious...
I assume you tried the "brightness" keyboard controls to be sure it wasn't turned off? :)
Click to expand...

I know the OP has already ruled this out, but the brightness on the iMac doesn't go down to 0 like the rMBPs, the backlight is still on at the lowest setting.
 
S

StockDC2

macrumors regular
Original poster
May 16, 2011
212
12
Hey guys, so I received a new LED board but the screen still does not turn on. Could it be because the cable is bad? If I took it to the Apple store, would they be able to pinpoint the exact issue? Also, I should mention that the replacement LED board is slightly different than the one that was in my system; the capacitors are arranged differently although the plugs are the exact same. Could this be the issue as well?
 
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