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LG 27 Inch UltraFine 5K IPS Monitor with macOS Compatibility

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Part of display/backlight won't dim - remains bright

Hi, I have exactly the same issue shown here:

https://www.youtube.com/watch?v=JY_OdRQs...

Would this be related to the Texas Instruments LP8860AQ1 LED Driver as shown in the ifixit page here: LG UltraFine 5K Teardown Or Is it likely to be something else?

This is a replacement motherboard from a second hand unit. The thunderbolt port on mine died hence why I replaced the motherboard. Something I have noticed with this new MB is that when you power it on the right hand side of the screen (exactly the area as shown in the video) flashes white - yet my original MB did not do that, hence I am convinced it is the motherboard (the same thing happens with the MB in the original unit (but the screen on that is cracked)).

The only other thing I can think of is it could be the connector for the backlight?

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@ebrooker definitely check the driver and check your wiring. It would be unusual for a driver to lose power to just part of the LCD but anything is possible.. Trace your backlight circuitry and check the components within that circuit and check the ribbon cables going to the LCD. Check for components that might heat up. A can of freeze spray may help with the diagnostics.

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Thanks @oldturkey03. I am almost 100% sure it is something to do with the motherboard as it does not happen with the original motherboard (I mean the LG monitor motherboard not that of my computer).

The fact that when you plug in the power it's exactly that side of the display that flashes white makes me wonder whether it's a short (maybe the plug that connects to the LCD) or component relating to the LCD that is at fault. I guess this would eliminate something heating up?

The other thing I noticed is that sometimes the screen flashes on/off, which is something I previously thought was due to a faulty thunderbolt port - could that also cause the behaviour seen in the video?

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@ebrooker it would be unusual for the main board to cause only a partially failure. Of course you can try a new board and I hope it'll work but it's unusual. Bad back lights can do this and so can the driver boards. check for any corrosion or debris in any of the cable connectors etc.

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Thanks again OT03. What do you mean by the "driver boards"?

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@ebrooker the boards that connect directly to the LCD. In the case of your LCD there may be only 1. That is the board that has the ribbon cables directly connected to the LCD and is not removable.

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I don't think it is that OT03 because when I put the original motherboard back in the screen does not flash on that side when it is powered on. It only happens with this motherboard fitted.

I also removed the two cables at the top (as per step 2 here: LG UltraFine 5K Teardown) and then powered it on and it still flashes on the right hand side. If I remove the connector to the backlight (which connects to the connector in step 13 of the same page) then it (perhaps obviously) does not flash when powered on.

So I am guessing whatever it is is part of the backlight circuit - either the connector (which looking at seems fine) or something that's part of that circuit. I guess there is no way to troubleshoot that? I have the old motherboard here so maybe I could desolder something from it and try it in the 'new' motherboard? (Unfortunately the old motherboard is wrecked - I tried to solder a new thunderbolt port to it but I messed it up, duh!)

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