LCD Backlight not working = solved

Hi,

I have an iMac 2009, 21,5" which has been working well until last week. First there was some flickering and sometimes a black display. Since yesterday the display seems not to work anymore.

I have found the apple technicians guide for this iMac online and tried some of the solutions described there. I did a SMC/PVRAM Reset, but this did not solve the problem.

To determine if it's the gpu I connected an external monitor -> everyhing working fine on the external monitor.

It seems that the LED backlight isn't working anymore, because if you use a flashlight you can see everything on the internal screen.

I asked at a local repair store but they told me (without looking at the device...) that it was either the LCD panel, which would cost almost 500€/650$ to replace or the SATA/Board (Logic board?) which would even be more expensive.

My question is whether it's possible to determine if it is the logic board, ta cable connection, the Backlight controller or the LCD-Panel.

Thanks fpr your answers,

Max

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Thanks for all your comments! Can anyone explain how to check the Pin voltages on the board? I have a Multimeter and I would just check them. If there is some low voltage would that mean that my power suppy is not producing enough power anymore or what would be the implication?

I just don't want to spend a large amount on something (LCD-Panel, power supply or inverter board without beeing sure if it really is this part...

Thanks,

Max

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Hi Max, sorry this is 3 years late, but you should get an a/c voltage at the pins, 0.012 volts.

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Jack, it may be three years late for Max, but right on time for me. Thanks.

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Deck the Halls
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4 Answers

Chosen Solution

Max Beck, here is what Apple suggest in how to test the components:

Quick Check

Symptoms

Backlight Issue/No Backlight

• No image

• Partial backlight failure

• Backlight flickers

• Display dim

Quick Check

1. Check for Software Update.

2. Verify that System Preferences > Displays >Brightness control setting is above minimum.

3. If no image on built-in display, connect external display to verify if iMac video circuitry is functioning. If no image visible on external display, go to No Video symptom flow.

Deep Dive

Check Result Action Code

1. Verify boot chime present and fans running when computer powered ON. (Reset SMC and clear PRAM if necessary for proper boot up.)

Yes Power on self test OK.

Go to step 2.

No Go to Won’t Start Up

symptom flow.

2. Verify if image is visible on built-in LCD panel, with adjustable backlight level.

Yes Video present. Verify system functionality and return to user or go to appropriate

troubleshooting flow.

No Go to step 3.

3. Connect supported external display. Verify if image appears on external display when system is booted.

Yes External display detected by system. Video circuitry on logic board functional.

Go to step 4.

No Go to No Video symptom flow.

4. Verify if LCD backlight is ON by looking for faint glow from display when viewed in darkened room with brightness set at maximum.

Yes LED backlight is functioning.

Go to step 7.

No

Go to step 5.

5. Shine bright (low heat)flashlight onto front of LCD. With computer powered ON verify if a faint image is visible.

Yes LCD panel functional but backlight is missing or dim. Remove LCD panel and inspect and reseat the following cable connections:

-Output cable between lower end of LED backlight board and lower end of LCD panel.

-Vertical sync cable between upper end of LED backlight board and LCD panel.

-DC power cable between upper right of LED backlight board and power supply.

Replace any damaged cable.

Reinstall LCD panel and go to step 6.

No

No image or backlight.

Go to step 7.

6. Power ON system. Verify if image is now visible on LCD panel with correct backlight level.

Yes Issue resolved.

No Replace LED backlight board.

Retest. If issue persists, go to step 7.

7. Inspect and reseat LVDS cable between LCD panel and logic board. Power ON system. Verify if image is visible on built-in LCD display.

Yes Issue resolved.

No Replace LVDS cable.

If issue persists, replace LCD

panel. Retest.

You can also check on Logic Board Test Point pin 6 to see what voltages you get there. Pin 6 tests Backlight Control pulse width modulated signal (from logic board to LED backlight board, to adjust the backlight level setting according to user setup). Use a multimeter on the DC scale for that. To find the proper backlight board, it would help to get the last three digits of your computers serial number. Here is the guide showing you how to replace the backlight board. Hope this helps, good luck

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OT - is my research on this one faulty? http://www.ebay.com/itm/Apple-iMac-A1311...

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@mayer, no Sir. Just trying to answer this part "is it possible to determine the reason?" of the question :-)

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I would suggest never going back to the repair shop that did not even open the iMac and told you it was the LCD or the MLB. I am an Apple Consultant and also repair only Apple computers, so I have been in quite a few iMacs. Usually ends up being the backlight board but always check the cables because I have had 2 that I can remember where another repair shop and even the Apple Store had done a repair not related to video issues but messed up the v-sync cable. The v-sync cable is very delicate and you can easily mess the pins up or crimp the cable causing intermittent or total loss of backlight. I know you have solved your issues but thought I would add this for anyone else searching on how to fix this problem.

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Hello, I've got this problem : my iMac 27" mid 2011 sometimes gives me a dark alone in the lower left corner of the screen. Then it fix by himself and then gets dark again. Apple Repair shop told me to change all the display panel. What should I do? Can you please me give me a help ? Thank You

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Replace the inverter. The working external monitor clears both the logic board and the GPU. Look on the board for Apple part # 612-0073, possibly 661-5537.

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Thanks, but couldn't it also be a unsufficient power supply or damaged cable connection then?

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@Max Beck, no. Not the power supply, a damaged cable possibly, but unlikely.

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@mayer

I have this exact problem. REPLACED the inveter with a new one and nothing worked.

Do you think could it be the cables that connects the INVERTER to the power supply?

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What year? Mine was the ccfl tubes blown, got replacement tubes from a supplier in Poland, simple swap out, no soldering.

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CCFL tubes. Never heard of that. Where is it located? How did you find it was the problem?

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Problem solved: Bought used LED Board and replaced the old one following the online guide. Seems that this was the reason for the black display ;)

Thanks to all the helpful members who helped me a lot with finding out which part was broken!

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Have the same problem here with a mid 2010 imac EMC 2389 - A1311z I bought a new led panel with new inverter board.. but still got black screen. Important: diagnostics led 1,2,3 are ON and Led 4 OFF and don't blink on startup.

Please help me guys!!!

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Does it have ccfl backlights or led? My 2009 has ccfl that are gone in the screen. Can you still see the images on the screen when it's on if you shine a torch on the screen? If you have replaced the screen and inverter and you can't see anything, try the vsync cable

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Cristiano an solve your problem?

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@volatileloser34

It doesnt make any sense!

@Danual ????

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Sorry, just realised it's LED backlights not CCFl tubes, mine was early 2009 and an easy fix as the tubes were blown. If you've change the inverter for new and still no working, check the supply power if you can.

Mine was12v

Gnd-ground

En-3.3

Dim-0-3.3v (dependant on brightness setting)

Gnd- ground

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Max Beck will be eternally grateful.
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