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Repair information and guides for the updated version of the 2015 Retina MacBook that was released in early 2016. Model A1534 / EMC 2991

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No backlight even after swapping screen & display cable

The backlight on a MacBook of mine has recently stopped working. No liquid or impact damage. I swapped out the display cable however it had no affect and also tried a different display entirely and it had the exact same issue with no backlight which suggests to me that perhaps it’s an issue with the connector on the logic board itself?

Anyone experienced similar before and might be able to point me in any directions to the cause/a fix?

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What does the external display show you? If it's not showing any image then the logic board has issues.

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Outputs fine to an external display.

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You’ve isolated the issue down to the backlight driver logic on the logic board. Its a Buck inverter (WLED driver) which can go bad and/or the fuse link blew.

Are you able to repair this logic board? If not you’ll need find someone with the needed skills, tools and access to the needed parts or you’ll need to replace it.

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Hi Dan! I've been trying to diagnose the issue on my board (820-00244), and originally thought the issue was the fuse F8400 however I'm getting 8.7V on PPBUS_S0_LCDBKLT_FUSED.

I think the issue is that LCDBKLT_EN_L is high (8.63V) which means PPBUS_SW_LCDBKLT_PWR (8.7V) is not getting past Q8406 as its output PPBUS_SW_BKL is reading at 0.82V.

On trying to look at LCDBKLT_EN_L I see it comes from U8401, passing through R8402. Here EDP_BKLT_EN/BKLT_EN_R is high (3.34V) and I2C_BKLT_SCL and I2C_BKLT_SDA are both present at ~4.6V.

I'm not sure where to go from here though/why LCDBKLT_EN_L is high given this though.

Apologies if I've made any rookie mistakes here, I decided to go down the rabbit hole for the first time somewhat and try to diagnose this myself. Any insights from you as to what could be going on would be much appreciated.

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U8401 ?? You mean U7701 right? TI - LP8548B1SQ-11 which is a custom part to Apple.

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Yep, LP8548B1SQ-11. It's labelled as U8401 on the 820-00244 schematic (for the 2016 model I believe — the one I've got) but as U7701 on the 820-00045 schematic (for the 2015 model).

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@danj Any thoughts on what could be going on here from your experience? I’m not sure what inputs on U8401/U7701 could cause it to disable the backlight boosting.

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How about jumping over to Louis Rossmann's YouTube's He does a lot more of these than me and can explain things more clearly that I can here.

Wayback, I tried fixing a few logic boards only to be stymied on not being able to get the needed parts as well as such a dense board its difficult to replace things without dislodging something in the process. This is not the best logic board to learn from.

.

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HI , if no liquid damage or other and you have replaced the screen with the right one ( it has o match model number and EMC number and yer ) , then it is logic board issue

regards

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also on startup plug the power AC

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i would also check the IPD Flex ribbon cable attached the right way

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Out of interest, how might the IPD flex ribbon cable for the trackpad being the wrong way cause the backlight issue?

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if you removed it completely , you may plug it the wrong way round , just in case !

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