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A2115/2019/ Procesador desde 3.0 GHz de 6 núcleos core i5, hasta 3.6 GHz de 8 núcleos i9. Lanzado el 19 de marzo 2019.

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iMac starts, chimes, but no backlight, no video. Light #1 off.

2019 27" Retina iMac with i9. Working fine until I left it off for 3 weeks. I get a startup chime, but no backlight. No video on screen (checked with flashlight), nor on external monitor. The fan runs. I replaced the 1632 battery, which was down to 1.9V, but that hasn't helped. Also tried swapping RAM; no joy. LED #1 never comes on. LEDs #2, #3, and #5 come on, but #4 does not [The display is connected]. I've reviewed prior posts by people with similar problems (no LED #1) but haven't seen a solution. How should I proceed?

Update (05/09/24)


I'll have to borrow another mac with Sonoma. My spare computer is a 2012 , still running Catalina. Here's a picture of the LEDs:

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A further question: per the serial number, this computer is an A2115/EMC3194, made in 2019. Apple says this model doesn't have a T2 chip. What's up with that? Should I give up on DFU?

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since this is a T2 model, it is quite possible that DFU Revive or Restore might help with troubleshooting it or even fix the device.

as a host mac you will need a Sonoma device. once you put imac to DFU mode, use finder to perform a restore (erases data) or revive. If you open console app after that, you will see a very detailed and verbose log, which helps me and many other technicians to lokalise problem on hard cases. for example, there is a "update firmware" stage which effectively checks all programmed components on board to retieve their FW version.
i already posted a vague instruction above on how to find boardview and schematics for this machine.
post log here and we will see if there is any clue.
running fan indicates

also this machine has quite a lot of LEDs, could you please post a picture or tell what LED do you mean as a "first"

EDIT:
I misread machine description, this one is Pre-T2 device, so method above wont work. I assumed it should be T2 since on PRE-T2 Intel machines LED1 is always a S5 indicator which should be always on.
Schematics is available in proprietary programm called XZZ only in windows version. unfortunately it is subscription-based, however you can use 7 day trial license.

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this model has pretty perverse LEDs.

left to right on your picture:

LED3 GPU_OK

LED1 S5

LED2 CPU CORE Voltages OK (S0)

LED4 VIDEO_ON (Signal from TCON indicating it has link, LCD was reset and ready to render and backlight can be enabled) Is off because GPU did not initialized the LCD

LED7 MEM_OK

so this basically points into an issue with the GPU unfortunately

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That’s the most useful info so far. Are gpu issues repairable, or should I be thinking about a new board?

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GPU is replacable if you find skilled repairshop which is capable of big BGA jobs. Expect it to be around 400-600$. I would recommend first testing with another CPU/RAM, since GPU init it somewhat later than CPU, I've seen bad CPU causing no GPU led as well.

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Besides what you’ve done recently to the system what other alterations have you preformed to the system?

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No alternations since buying the computer 6 months ago, other than OS updates. Previous owner had removed the original RAM and replaced it with 64GB from OWC. I've tried swapping the original RAM back in.

Before leaving on vacation, I made a Time Machine backup and shut the system down. When I returned 3 weeks later, it wouldn't start.

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@jeffreyzurkow - Just bad luck then. If I were to guess a power surge did you in. The issue is a common to neutral spike that the power supply didn’t get hit, instead the logic board did. But that’s a guess! A good reason to have a good surge suppressor and replaced every few years if you are in a heavy lightning area or have poor power.

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Actually, the computer is powered by a CyberPower UPS, which should have suppressed any surges. Are you convinced the logic board is dead? Any suggestions on finding a replacement? Or is it cheaper to buy another used iMac?

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@jeffreyzurkow - Well your better than most! Having a UPS should have caught the transient.


I’m not leaning on logic board replacement yet, nor jumping to a new system either.


Where do you live? Country and nearest large city. Let’s see if we can aim you to someone with the deeper skills to repair your logic board.

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Portland, Maine -- about 100 miles.

Lots of experience here, and test equipment, but I can't do much more without schematics.

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