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Modelo A1312 / Mediados de 2011 / Procesador Core i5 de 2.7 & 3.1 GHz o Core i7 de 3.4 GHz

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Display light randomly shuts down

Hi!

I'm gonna write down everything what happened so you can get a clear picture about the problem.

One day I've been working on the iMac and the backlight suddenly shut down. I've tried everything, but it hasn't came back. So we took it apart and checked every cable etc. concerning the LCD screen and GPU...then checked if the 4 LEDs on the motherboard are lit (yes), and anyway I've been able to see the image on the screen with a lamp pointing on it.

So I've measured what comes out from the PSU to the inverter board, and it was ok (12V), then moved to the output of the inverter board and only measures a few mVs, which seemed to me not ok. So later I went hunting for an other inverter board, and I found one at a local repair shop, but it wasn't the exact same type that's on my iMac. The one I bought was actually inside the same year model of iMacs, but I thought I have to try it out anyway.

The type of inverter my iMac had is the: Sumida 612-0094 (4 pin)

and the one I put in instead is the: V267-604HF (4 pin)

So i changed the inverter board and voila the backlight worked again. And after a while suddenly it shut down again, but i was able to wake it with a keyboard shortcut. And it happened again and again after I found out that it's not shutting off when the brightness is turned only until 5. I clean installed a new Mac OS, and it worked for 2 hours straight then it started all over again...

I took the iMac to the repair shop to check the LCD screen. They put inside an other (used) LCD screen which seemed to solve the problem. The backlight worked for 3 weeks, then it started again...

My question is: Is it possible that the inverter board is causing this issue? Or can it be, that the new (used) LCD screen is also bad? I think it's strange and I assume it's not the LCD screen itself, but maybe I'm just really unlucky? What else can cause such a behavior if it's not the inverter board?

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It does sound like the Inverter is your issue! I would inspect it looking for staining/burnt marks around the caps and the Power FET's

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Thanks Dan!

I think I'll try to find the same type of inverter that was in my iMac and change it. But it's only possible to find used ones :)

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@transistorgirl - Thats all you'll find is used as no one is making new.

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If you still have your original inverter card check the electrolytic capacitors values (capacitance and ESR). This is an old card so it's possible that theses capacitors could be out of range.

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@lemerise - Thanks! I did that before, but I don't have a proper ESR meter, just a very cheap one. We changed two caps that were seemingly damaged, but after soldering them out they seemed good. Anyway we put back new ones, hoping for the best and the problem wasn't the caps :( I've tried to find schematics for the inverter board but, yeah...no luck of course...I checked some other components that were easy to check like diodes, fuse, some random resistors, nothing...so I found it easier to get an other one than troubleshooting such a complex circuit. I'm not an engineer sadly, just a hobby DIY-er.

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Last Friday we changed the inverter board to the original part number one (it took a long time for it to arrive :D) and so far so good. But I don't want to be too early with the verdict, so I'll wait another 2 weeks maybe. But to me it seemed like the backlight got better after the swapping, so I'm really really hopeful now :)

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