No backlight even after swapping screen & display cable

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.

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

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).

@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.

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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