No backlight + no keyboard backlight implies fuse problem?
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 : 8.3V
- SMC_LID : 3.4 V
Driver output
- PPVOUT_SW_LCDBKLT : 8.43 V
- PPVOUT_SW_LCDBKLT_FB (at test point) : 8.4 V
- PPBUS_SW_LCDBKLT_PWR_SW : 8.46 V
- PPBUS_SW_BKL (before L9701 inductance) : 8.32 V
- Output of inductance
- When measured in DC : 8.32 V
- When measured in AC : 17.5 V
- Feedback line connectivity check:
- By putting multimeter in diode mode, placing red probe on ground, and black probe on PPVOUT_SW_LCDBKLT, multimeter displays 510/512/515
- A second measurement gave 530
- By putting multimeter in diode mode, placing red probe on ground, and black probe on PPVOUT_SW_LCDBKLT, multimeter displays 510/512/515
BKL_EN circuit
- BKL_EN : 2.64 V
- PPBUS_SW_LCDBKLT_PWR : 8.43 V
- measured both at test point, and on resistor closest to driver
- Fuse : 8.43 V
- Resistors
- R9731 : 201 kOhms
- R9715 : 101 kOhms
- => theoretically BKL_EN should be : 2,72
Other driver inputs
- PP5V_S0_BKL : 5.17 V
- PP3V3_S0_BKL_VDDIO : 3.3 V
- BKL_PWM : 3.34 V
- LCD_BKL_PWM : 3.34 V
LCD activation circuit
- PP3V3_SW_LCD : 3.25 V
Other output
- LED_RETURN_1 : 0V
- BKL_ISEN_1 : 0V
- LED_RETURN_2 : 0V
- BKL_ISEN_2 : 0V
- LED_RETURN_3 : 0V
- BKL_ISEN_3 0V
- LED_RETURN_4 : 0V
- BKL_ISEN_4 0V
- LED_RETURN_5 : 0V
- BKL_ISEN_5 0V
- LED_RETURN_6 : 0.5 V
- BKL_ISEN_6 : 0.5V
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