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27" iMac A1419 EMC 2834 5K retina display backlight connector

I've just replaced the broken screen in a 27" iMac A1419 and after wondering what to do with the old one for a while I decided to make a light panel out of it. There are numerous youtube videos showing how this can be done for laptop screens and TVs and I figured the same must apply to the iMac.

Being downright dangerous with a soldering iron I have avoided attempts to solder wires onto the connectors on the controller board and it seems to me that I ought to be able to simply put a voltage across the LED strip in the display to illuminate it.

The display panel has two 8-pin connectors as shown in this picture :

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and the cable that powers the backlight plugs into them using these :

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As the repair is complete and the customer has taken the computer back I can't measure the voltages on these wires. A google search didn;t turn up anything of use either.

So to my question : Does anyone know what the voltages are on these connectors in order to drive the backlight?

Thanks in advance.

Update (07/02/23)

Thanks everyone for the replies.

@oldturkey03 Are these what you requested?

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Contesta esta pregunta Yo también tengo este problema

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While you are only focused on the reuse of the display as a light table Let’s make sure we align this under the correct system.

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If you review Step 5 in this iMac Intel 27" Retina 5K Display Teardown we can see the two power feeds from the backlight bars are feed from a single cable from the logic board. I would remount the controller so you have a single cable connection and then viewing the iMac’s logic board schematics we can then ID the pinouts and the voltages going to the panel. As far as the LCD driver logic I can’t tell you how to de-energize that part but I don’t think that stops your use.

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@johnmckee we really would need to know the exact panel number that you are dealing with. Narrow down the iMac model where this comes out of. If you still have the logic board, add images of that as well.

Commonly, your LED backlight goes through a boost circuit and you should have between 25V-40V on that complete circuit. That is not to say, you can apply that voltage to the individual channels of your backlight. Again, we have to say common, because we do not yet know which setup you are having. Your backlight is divided into two bar's. Each one of those is for the different strips. Your image 2 shows the 6 connectors per bar, drives 3 sets of LED's. It looks something like this

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Anode being positive and cathode being negative.

Do not simply go and blast some voltages into those. It'll burn out your strips. Get a bench power supply and use it to drive one channel. Start a 3V (yes it's low, but it's safe) and increase the voltage in small increments. You are not going to achieve full brightness across the screen but only a partial illumination. Once you identify the max brightness for that channel, you know the voltage you will need for all the other channels as well. Ultimately, I'd opt for a commercially available, multiple channel LED driver. Scan place like ebay, alibaba, amazon etc. for some ideas. I am not endorsing any of those places but it's a start.

It will help to see where the connectors plugged in so we can identify pin 1 and the individual channels. Looking at the connector and a few different schematics, Black appears to be cathode and Grey the anode. That needs to be verified.

Just in case you really want to know how Apple drives the circuit :-) which is really irrelevant to your project but FYI only ¯\_(ツ)_/¯

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@oldturkey03 - Do you have the schematic and board view for the 2014/15 5k display assembly T-CON board that’s what’s needed here to ID the pinout of the cables and to reduce the cable harness to the one cable Apple uses. The T-CON logic is not Important just the physical connections for the back light power.

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@danj no I don't think it's needed. The only thing is need is the connector wiring diagram. WE can get that from teh panel end. We know what the backlight voltage is and we know how to drive the LEDs, we need to know anode and cathode. No worries about the cable apple is using since we are just going to bypass that..

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@oldturkey03 - You missed my point! It’s not a question of the power source it was the physical connections which you too where at a loss.

I was only looking at this as a plumbing issue nothing more, what you dump down the pipe is not the issue it’s the connections!

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@danj you know what, you are right I am missing your point again.

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If you read my comment, you can clearly see that I said "It’s not a question of the power source" and "The only thing is need is the connector wiring diagram"

We don't need anything else. No T-con board schematics needed since we will get that from the panel schematic. All the OP @johnmckee has to do, is to let us know what panel this is (looks like a LM270WQ1). If so, then the image of the LED connector in my answer, is the right one.

Use the same two cables that were originally on the backlight strips, and connect those to the new LED driver module.

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Thanks for the replies so far. The panel is LM270QQ1 SD B1 and the serial number of the iMac is C02RC5KCGG7L. I tried putting 3.3, 5 and 12 volts through it (both polarities) but notthing lit up.

As I mentioned before, the iMac is back together with a new screen and has gone back to the customer so I have nothing to plug it into.

The channel is 3.8mm wide so I guess the easy solution is to tear out Apple's LEDs and replace them with an LED strip. It seems like a waste but it's also awfully simple.

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