Chosen Solution
I have had display problems, so I changed the backlight fuse on my Macbook Air. After that, there is still no display. The computer works fine otherwise, I can turn it on and hear sounds. As of now I am wondering whether I soldered wrong, or whether the problem is with the backlight itself. I also noticed that there is no keyboard backlight. Even if I hit the keyboard backlight keys, it doesn’t light up. I was wondering whether that means that my soldering is wrong, or not. Is the backlight fuse also “protecting” the keyboard backlight ? EDIT: I did some measurements on the logic board. My interpretation of them is that everything seems fine, except the voltage divider for BKL_EN. For some reason, even though input voltage seems fine, and both resistors have correct resistance, divider output is slightly lower than it should be. The only weird thing I notice, that could explain this value, is the fact that resistance rises slowly when I measure it. It starts at half the final value, and rises logarithmically for about 30 seconds. Here are the measurements: Global values PPBUS_G3H : 8.3VSMC_LID : 3.4 V Driver output PPVOUT_SW_LCDBKLT : 8.43 VPPVOUT_SW_LCDBKLT_FB (at test point) : 8.4 VPPBUS_SW_LCDBKLT_PWR_SW : 8.46 VPPBUS_SW_BKL (before L9701 inductance) : 8.32 VOutput of inductanceWhen measured in DC : 8.32 VWhen measured in AC : 17.5 VFeedback line connectivity check:By putting multimeter in diode mode, placing red probe on ground, and black probe on PPVOUT_SW_LCDBKLT, multimeter displays 510/512/515A second measurement gave 530 BKL_EN circuit BKL_EN : 2.64 VPPBUS_SW_LCDBKLT_PWR : 8.43 Vmeasured both at test point, and on resistor closest to driverFuse : 8.43 VResistorsR9731 : 201 kOhmsR9715 : 101 kOhms=> theoretically BKL_EN should be : 2,72 Other driver inputs PP5V_S0_BKL : 5.17 VPP3V3_S0_BKL_VDDIO : 3.3 VBKL_PWM : 3.34 VLCD_BKL_PWM : 3.34 V LCD activation circuit PP3V3_SW_LCD : 3.25 V Other output LED_RETURN_1 : 0VBKL_ISEN_1 : 0VLED_RETURN_2 : 0VBKL_ISEN_2 : 0VLED_RETURN_3 : 0VBKL_ISEN_3 0VLED_RETURN_4 : 0VBKL_ISEN_4 0VLED_RETURN_5 : 0VBKL_ISEN_5 0VLED_RETURN_6 : 0.5 VBKL_ISEN_6 : 0.5V
If both the screen and keyboard aren’t lighting, the issue will be with the backlight boost circuit (most likely the backlight IC). You would require a schematic and board view for your particular board to diagnose this (board number etched on board, Google to find the files). You will also need a multimeter to measure the backlight output voltage (to see if this is the issue) and a hot air station (if it is a backlight IC problem, since it is a BGA chip). Get the files and equipment if you think you can do it, then let us know when you are ready to check this. If you feel it may make the situation worse, it may be best to send it to someone who fixes these on a daily basis.
