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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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Screen blank but chimes

I recently replaced the failing HDD in my iMac with an SSD. All was working fine, i started restoring the drive from my time machine and left the machine to do its thing. I came back to an error about the drive and i turned the machine off thinking i would sort it in the morning (i had created a new fusion drive so i assumed it was something to do with that).

the next morning i went to turn the machine on and it chimed but the screen stayed black… tried a few times and no change. I reset the pram etc but nothing worked. I thought maybe i didnt connect the screen up properly but all seemed fine… i also had a spare cable and that also didnt work.

since then ive noticed that led1 + 2 come on fine, but led3 is very dim… is this normal or is this a gpu issue? Its about 30% brightness of led1 and 2…

any help appreciated!

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What about LED 4? Is it on? I would think if LED 4 isn’t lit up then your LCD Panel isn’t being recognized. Is the PRAM reset actually working, you hear it chime multiple times? Do you have an external monitor to plug up and see if you can get external video?

I would pull power and try reseating the display cables. It’s worth a try.

Meaning of Diagnostic LEDs:

1st LED is the indication of a trickle charge.

2nd is an indication of power being received (machine turned on)

3rd indicates the GPU and computer are communicating

4th indicates the computer and LCD panel are communicating.

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No i cant get led4 to come on, ive even tried a brand new display data cable and still the same. It just makes no sense to me as the machine was on and working fine for at least 2 hours... ive reseated the cables at least 5-6 times now.

Is led3 supposed to be dim? Im wondering if its the pram battery at fault possibly?

Yes pram reset seems to be working fine, chimes twice (second one quieter than the first)

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I can't say 100% how bright it would be but it should't be really dim.

LED 4 not being on makes me think its the LCD Panel. You said you've tried display cables so that leaves the LCD panel. But, I'm not 100% certain if LED 4 is dependent on LED 3 being present. Someone else may be able to chime in and confirm.

Still curious to know if it can get video out to an external monitor.

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Ive just tried hooking up to an external monitor - it switches on when i turn the mac on but then quickly displays a no signal message and turns off. So its almost as if the mac is sending a blank signal through or something?

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@memphismac yes i think from my reading led 4 is dependent on led 3, led 3 is dependent on led 2, led is dependent on led 1. Im just very confused because ive done this procedure 4-5 times before and never had any issues. This was working fine before i went to bed and the issue started in the morning - just not making sense to me unless it is the pram battery becoming faulty? I just want to avoid changing that if possible as there is no easy way of changing it that i know of!

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Yeah, those kinds of failures are always a pain. I'm not sure about PRAM battery though.

I would normally swap the logic board at this point. Then send the old one in for a little $$ back.

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