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iMac Intel 27" EMC 2309 (Late 2009, Core 2 Duo 3.06 or 3.33 GHz) ID iMac10,1, EMC 2374 (Late 2009, Core i5 2.66 GHz or Core i7 2.8 GHz) ID iMac11,1

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Late 2009 iMac - LCD backlight issue (intermittent)

Hi,

I have a late 2009 iMac (EMC2309) which is having an issue with the LCD, the systems backlight will suddenly cut off (used to work around the issue by sleeping and waking the system then the back light would work for a few second to hours at a time)

I have since opened the system and disconnected the V Sync cable and that has reduced the issue greatly (as long as I’m on full brightness)

Does anyone know if replacing the backlight inverter will help or is the LCD itself failing?

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I’ve often had to replace the LED Driver board which is what powers the backlight LED’s. Here’s the IFIXIT guide you’ll need to follow: iMac Intel 27" EMC 2309 and 2374 LED Driver Board Replacement and here’s the needed part LED Backlight inverter board

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I did think that originally, when I looked into it more the backlight inverter boards tend to fail and just not work rather than intermittent behavior, also that disconnecting the V-Sync cable could indicate a failing LCD panel, I’ll probably try the inverter board as it much cheaper than a replacement panel, thanks :)

I could be wrong about inverter boards usually failing completely

из

What I've seen happen is the caps break down which is why its intermittent.

из

That makes perfect sense, hmm, to replace all the capacitors myself or to buy a new inverter board...

Thanks will update once i get the money together to replace or repair :)

из

not had a chance to replace yet, now when turning on you get a chime, but stalls there, keyboard not resposive, mouse lights up and seems stuck, occasionally it will boot fime, if the backlight fails to come on will it still chime and then stall? dose that support backlight board theory?

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I changed my caps and its been working at full brightness now for a few hours. Previously it would go black (display still active if seen under strong external light source) after some time. Over time that period got shorter and I started to notice high screen brightness shortened the period. In the end, any brightness above very low would shut it off. I checked my caps for capacitance and they seemed ok. replaced anyway. Didn't have the correct values or sizes (2x35v 270uF 8mmx15mm, 2x63v 120uF 8mmx20mm) on hand so I used 1x35v1000uF and 1x63v 470uF. I knew the larger caps would not fit in situ due to other users trying that. Simply adding some extensions to the legs and some insulation on the legs so they dont short on things allowed me to tuck them a little further over where the imac is thicker. Obviously the right size caps is better but don't be afraid to try desoldering your bad old caps and testing some new ones! Its really easy. Even if this is your first go and taking components off the board. These big caps are the easiest to solder things you will find on a mac. Little surface mount resistors are far worse! Give it a crack people. A replacement inverter board is going to have old caps too. Repair for the win :-)

Worth also mentioning is that I have upgraded the GPU to K4100M. The machine had a failed GPU in the past. I'd had about 2 years of slowly increasing frequency of screen going black since the GPU upgrade and had thought it was related to that initially. External monitor has worked perfectly throughout and never any graphical issues otherwise. Ill report back if the issue arises again but I am pretty confident it was the caps that were failing.

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Still working flawlessly. Very pleased with my cap replacement.

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Exact same problem on a 27” 2009 iMac. I use the same workaround of keyboard strokes to force the display asleep and then wake with another keystroke.

The screen runs OK for very long periods at low light settings and is more prone to failure when set at mid to higher light settings. I replaced the inverter board with a used one but the problem got much worse!! I could hardly keep the display illuminated for a second before it went black at any light settings.

Electrolytic capacitors on the inverter board might be the culprit. Caps are notorious for failing with age. I see four large electrolytic caps on my board.

Perhaps I should acquire some spares and see if that fixes the problem. Anyone had success fixing this issue?

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I am debating the capacitor / inverter board approach, are you concidering caps on the logic board or the inverter board?

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I tested the caps for capacitance and ESR on the inverter board. All measured in specification, but they may still fail under load.

I plan to replace them and retest. If the display continues to malfunction, I would suspect the video cable / connector going to the logic board. The backlights on the iMac will go out if the video connector loses connection.

The surface mount video connector on the logic board is easily damaged during disassembly, which can cause an intermittent connection that will extinguish the backlight.

The computer I have began to malfunction after a hard drive replacement, which may have stressed the video connector / cable. I have noted that some people on the web indicate their display problems showed up after a hard drive replacement.

I have a couple other lines of investigation concerning the thermal sensors and the connections to the backlight LED strips.

I also wonder if the power supply board is failing to provide power to the inverter board? I have not considered the logic board caps.

из

In your case since the issue showed up after opening that would indicate mechanical damage as a lightly cause, I have no secret knowledge that replacing the logic board caps will help was just wondering to what extent you were planning to attempt to diagnose the caps, the inverter is another possibility, I don't have the money currently to buy the replacement parts first thing i will try is the inverter on mine though, please keep me informed of your progress :)

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I will keep you posted on the activity.

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I bought a new inverter board and a rebuild power unit for my late 2009 21.5 inch. The back light works again, but still has problems. Dont know if the sync cable might be the cause or the last high sierra update. Please help!

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It may also point to bad solder connections on the LED backlight strip. It was a common issue on the 27" Cinema LED Display, the solder seems to degrade over time...

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