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Model A1419 / EMC 2806 / Late 2014 or Mid 2015. 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (ID iMac15,1); EMC 2834 late 2015 / 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (iMac17,1) All with Retina 5K displays

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820-00134-A iMac 27" Late 2015 - No display, backlight works

Hello
I received 27" iMac, Late 2015 model that's broken. I don't know what exactly happened but it appears to be unopened and there are no marks indicating prior repair attempts.
Initially it didn't display anything on the screen and backlight didn't work. Only 2 diagnostic LEDs would light up and 3rd was very faint. I opened it and found F4400 fuse near eDP connector to be blown. Replaced it with a suitable part and backlight came back to life. Unfortunately it still doesn't display anything on the screen. Now I get 4 green diagnostic LEDs.

I can hear the chime and also I can adjust brightness using keyboard buttons.

What I've tried so far:

  • replaced F4400 fuse
  • tried removing coin battery
  • tried booting it without Blade SSD
  • tried booting it with audio jack PCB disconnected
  • tried reflashing EFI with one found online, also tried cleaning ME region in the original EFI
  • checked eDP connector both on the logic board and TCON
  • checked main fuse and all filters on TCON module
  • I get 0.9 Ohms reading on GPU coils but I assume it's okay for a GPU


I know the next logical step will be to try my iMac with an external display but I haven't received the adapter yet. Ultimately I wanted to convert it into 5K display but I thought it would be nice to fix it first to make sure the LCD is fine.

Do you have any other clues?

My iMac has 820-00134-A logic board.

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@webhdx post some good pictures of what you refer to as the T-con module. Voeg afbeeldingen toe aan een bestaande vraag

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By TCON I mean display driver in the panel itself. This is the module where other end of eDP cable connects to. Sorry I have no pictures for now as the iMac is in the workshop. The module looked fine, no visible damage. Also all flex cables connecting it with the panel looked good.

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Yes, there is 12V on the eDP connector and it was restored by replacing F4400 fuse. This resulted in backlight coming back to life. Sadly still no image.

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@webhdx I guess you really need to wait until you have your external display adapter. this sounds like a video signal more than anything else. Any idea what happened that made the fuse blow? You said it's a late 2015 with a 820-00134 It should be a 5K display already. Do you have the number for the display?

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Unfortunately I don't know the history of this iMac. I got it in this state and it appeared to be unopened. The display is LM270QQ1(SD)(B1) model. It's Retina 5K panel.

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Maybe someone will have the same issue and comes across this thread via google.

My screen was completely dead - no backlight or image at all. I tried flashing directly at the screen with a flashlight and got nothing. The diagnostic LEDs only showed 2 green lights.

I noticed F4400 fuse on the logic board was blown. It's 32V 2A fast acting fuse. Mine was welded to the board and I had to use a lot of force to remove it...

This only recovered the backlight but I still couldn't see anything on the screen. The iMac was displaying image via thunderbolt port and I got 4 diagnostic LEDs after replacing the fuse.

It indicated there are issues with the panel itself. I decided to check the electronics in the panel next. I hooked up 12V to VIN on the board and the board was drawing 2A - something was definitely shorted. What's more - the main voltage regulator TPS65168 was getting extremely hot. I thought it's blown and actually replaced it but it didn't fix the issue and I also learned this IC is programmable and you can't use factory settings, it has to be programmed via I2C for correct voltages. After more checking I noticed a capacitor near BUF16821 on the left side of the board that was also getting hot. It's directly connected to VIN line so if there was a voltage spike it's understandable why it blew. After desoldering it my LCD started working. Unfortunately in my case I got thin black bar on the screen from heating up the area near TPS65168 And I was extremely careful to avoid that :/ I was very close to getting this screen fixed but broke it in the process.


In case someone needs to program TPS65168 or check the voltages:

OUT1 3.5V
OUT2 1.9V
OUT3 7V
SWO 14V
VGH 24V

Find the datasheet on google and you'll get pin numbers.

Working LCD panel when connected to the bench power supply should draw around 250mA.

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webhdx zal eeuwig dankbaar zijn.
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