Macbook A2289 - backlight problems - have checked a lot already - out of ideas

[survivalbloke]

This is A2289, board 820-01987-A. It came in with the screen not working. We could not see any image on the screen, so we ruled out a backlight issue and instead replaced the panel. The display now comes up, but no backlight.

I didn't see any signs of liquid damage, but there was some odd looking spots around U8400. F8400 has voltage on either side of it, and I have 5v present on R8444 and R8445. I did not have 3.2v at R8442 so I figured U8400 was bad, and I replaced it. After replacement, a brief probe to R8442 showed 3.2v so I assumed I had fixed the device. After reassembly however, there was no backlight. So I pulled the board again and when I checked R8442 there was no voltage. I probed around a bit more and came back to R8442 and then there WAS 3.2v. What the heck? I removed R8442 from the board and checked in resistance mode and the numbers were all over the place - highly irregular. I pulled one of these resistors from another board and installed it, but I'm getting the same thing - intermittent 3.2v on R8442.

What on earth am I missing?

[mon2]

Re: Macbook A2289 - backlight problems - have checked a lot already - out of ideas

Flux and remove R8442. Power up again and check if the enable voltage is ok @ R8442, pin # 1.

What is the voltage to ground of EDP_BKLT_EN? (without R8442)

The replacement device @ U8400 may need to be fluxed and reflowed to possibly correct solder shorts that are not visible.

[survivalbloke]

Re: Macbook A2289 - backlight problems - have checked a lot already - out of ideas

Flux and remove R8442. Power up again and check if the enable voltage is ok @ R8442, pin # 1.

What is the voltage to ground of EDP_BKLT_EN? (without R8442)

The replacement device @ U8400 may need to be fluxed and reflowed to possibly correct solder shorts that are not visible.

With R8442 out, I get varying voltage outputs on EDP_BKLT_EN. Mostly 0 volts, but sometime .2 volts. Just to be completely sure, I removed the U8400, wicked all pads, cleaned and prepped pads - then replaced the U8400. 375c is no too hot for installing IC's like those is it?