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  1. #1
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    iMac 27" (Late 2009) Intermittent Loss of Backlight

    Somewhat new to circuit and component level repairs, so hopefully someone can shed some light (no pun intended) on an issue I've been seeing more and more in my line of work as an Apple service technician.


    We're seeing a lot of 27-inch iMacs of the Late 2009 and Mid 2010 series (A1312, EMC 2309 and 2390 respectively) come in with intermittent backlight issues. The behaviour is always the same. The machine works for a while, then during use the display backlight suddenly turns off. An image is still visible when viewing the panel with a secondary light source.

    If I put the machine to sleep and wake it, the backlight functionality is restored momentarily. Sometimes it will restore for an hour, sometimes only a few seconds.

    The backlight board and display still exhibit the same issues when swapped into another machine, isolating the fault to these two components. If I replace the backlight board with another part, the issue still occurs. If I replace the LCD, then the issue is fixed. So I'm confident the fault is within the display. Unfortunately I don't have another machine that exhibits this particular issue at the moment so I haven't been able to troubleshoot further than this myself yet.


    I've been searching the forums since this morning and noted that the 27-inch panel has a common defect with the backlight connector that can cause half of the display to dim. I'm wondering whether this intermittent loss of backlight entirely is a related issue that manifests with a different set of symptoms, or whether the loss of backlight is due to the failure of another component inside the display somewhere.

    As we're seeing more of these machines come through it would be nice to know the actual solution rather than the Apple official solution of replacing the entire display, at a rather exorbitant cost as well. That is if we could still source these panels - Apple has since discontinued the supply of replacement parts for this model.


    Cheers,
    ~ M.
    Last edited by iMic; 06-20-2016 at 12:41 PM.

  2. #2
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    Re: iMac 27" (Late 2009) Intermittent Loss of Backlight

    yes just remove the lcd and wiggle that two connectors at corners of the back of the lcd. and you will see that they are loose. fix it and here you go

  3. #3
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    Re: iMac 27" (Late 2009) Intermittent Loss of Backlight

    I think it does not have anything to do with loose LED strip connectors.
    Have multiple panels with rock solid corner connectors but intermittent backlight death.
    It's always the LCD.
    Only solution is to find a broken LCD with good backlight.

  4. #4
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    Re: iMac 27" (Late 2009) Intermittent Loss of Backlight

    Quote Originally Posted by widerstandproject [Only Registered and Activated Users Can See Links. Click Here To Register...]
    I think it does not have anything to do with loose LED strip connectors.
    Have multiple panels with rock solid corner connectors but intermittent backlight death.
    It's always the LCD.
    Only solution is to find a broken LCD with good backlight.
    ok i think i have the same problem with the 21.5" imac you telling here about. have you tried to remove backlight and resolder each led? well this must be simple enough - either it is a soldering fault or a led that is faulty, right?

  5. #5
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    Re: iMac 27" (Late 2009) Intermittent Loss of Backlight

    I'm still attempting to source a panel I can disassemble to inspect the backlight circuit. Whatever causes the intermittent loss of backlight causes all the LEDs to cease functioning at once, both on the left and right sides of the display, almost as though it was causing some kind of protection on the backlight board to trip and stop supplying power to the display backlights entirely. Sleep and wake the machine, and it reinitialises and the lights function again - at least for a moment until it falls over again.

    What I'm hoping to find out is whether the backlight connectors in each corner feed directly into the LED array, or whether there are some passive or active components in between - an IC, resistors, capacitors, whatever - that could potentially fail open circuit, short to ground or somewhere in between that would cause this type of failure.

    Not overly concerned about whether it's a viable or cost effective repair at the moment (although if my own iMac develops this issue, you can bet I'll be repairing the panel rather than shelling out some exorbitant amount for a new one), just interested in finding out the why and how of the issue at the moment.

  6. #6
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    Re: iMac 27" (Late 2009) Intermittent Loss of Backlight

    Managed to find a damaged 27-inch panel to disassemble and take some photos. These are the two backlight LED strips mounted in the base of the panel. It looks like they're regular old LED strips, with no logic or components in between to control them. So in other words, not all that promising.

    These strips must get hot though. The entire backside of the LED strip is a metal plate with a green compound - likely a thermal pad - to draw heat away into the LCD frame. When the LEDs are installed in the panel, a reflective foil is stuck behind them to direct the light around, and that foil looked slightly brown and discoloured behind some of the LEDs.

    The good news however is once you remove the frame that secures the panel to the rear housing, remove the foil, remove the LCD frame and unclip the black plastic frame inside the display, it would be possible to solder down the LED connectors that are prone to breaking with the strips still mounted in the display. It's a pain of a job to get to them, but it would be a cleaner repair than attempting to solder 6 wires in place through the tiny cut-out in the back of the display.

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    Re: iMac 27" (Late 2009) Intermittent Loss of Backlight

    My research also shows this to be a very common problem.

    My backlight (mid 2010 iMac) also drops out.

    It drops out on brightness settings above 3. It will work premaritally on the lowest 1-3 settings.

    It the period of time it would work on full brightness deteriorated over time.

    I replaced the inverter but the problem continued. Interestingly with the replacement inverter the time the screen would work in full brightens did increase.

    I used a refurbished inverter so it may also have a fault.

    I’m thinking the LCD Temp Sensor may actually be the fault. This part likely comes with a new replacement screen so it might appear that replacing the whole screen will fix the problem when in reality the only fault is the temp sensor. Just a thought.

    The other common problem is when a part of the screen backlight fails. I believe is a different fault to the intermittent loss of the entire backlight. Permanent backlight failure in one part of the screen seems to be caused by failed solder joints due to overheating. A different problem altogether.

    Whatever the fault is, it's cutting out all the backlights. My fear is that if the backlights could be replaced, it would be a big job only to find the fault continue.

  9. #8
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    Re: iMac 27" (Late 2009) Intermittent Loss of Backlight

    I have now tried replacing the LCD Temp Sensor but this has not fixed the problem.

  10. #9
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    Re: iMac 27" (Late 2009) Intermittent Loss of Backlight

    I have a EMC 2309 iMac 27'' on the bench, exhibiting the same symptoms. The backlight will turn off after a minute, or even after only few seconds. This seems to be more often when the iMac warms up, but it happened even when it was stone cold, after 10 seconds when I turned it on. You can restore the backlight by putting the iMac to sleep and then waking it. Sadly, I do not have any similar computers here currently, so I can't swap panels and components to check for sure.

    I am not sure, which thing is telling the LED board to turn off LEDs, and my best guess would be that it detects abnormal current to LEDs and it then shuts down. The sensor data looks okay, so I highly doubt that it would be the SMC that is telling the LED driver to turn off. The probably most definite answer would be to monitor the current to the LED strips, and to check the voltage drop across each LED and compare it with the others. Could be that one or more LEDs are "leaky" and are drawing too much current. If the LCD panel is actually bad, it would be interesting to compare the LED current to the known good panel.

    However, if I disconnect the VSYNC cable, the backlight remains on. I had it on only for about half an hour, but it held up. The only drawback is the whine from the LED driver board. But I do not like to leave it like that.

  11. #10
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    Re: iMac 27" (Late 2009) Intermittent Loss of Backlight

    The VSYNC cable did something, but still after an hour the backlight turned off. So dead end.

    I traced the enable pin on the LED driver board and it goes from 2.90V to 2.70V when the backlight shuts down. Even when I used a weak 3V cell (about 2.95V) to force it on, it still turned off. So, the logic board and the LVDS part of the LCD is most certainly not the culprit, and the LED driver is shutting down by itself. But I can turn the backlight on with shorting the enable pin to the ground momentarily (I used a 100 ohm resistor).

    The LCD panel is a LM270WQ1 LG part. I got the datasheet and tested the voltages on the pins at the LED driver output. The brightness was about 75%. The odd pins are anodes of the 6 seperate LED channels, and the even pins are cathodes.

    Pin1: 43V
    Pin2: 2.4V
    Pin3: 43V
    Pin4: 7.9V
    Pin5: 43
    Pin6: 4.7V
    Pin7: 43
    Pin8: 8.5V
    Pin9: 43
    Pin10: 6.4V
    Pin11: 43
    Pin12: 8.1V

    It looks to me, that the cathode voltages differ way too much from each other. If I lower the brightness to 0% (the lowest), it will stay up, with the 35.2V on the anodes and 6.1V on the cathodes, with almost none differences between LED channels. However, I do not have a known working iMac 27'' to compare with.
    Attached Images Attached Images
    Last edited by mitchW; 12-16-2016 at 11:24 PM.

 

 

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