Macbook Air, no backlight 820-3024

[ginobarabasi]

Hello everybody, this is my first post of this very interesting forum. I hope I can give my help if I can.

I am having a problem with Macbook air, board 820-3024: there is no backlight.
I measured some voltages and found the following:
PPBUS_SW_LCDBKLT_PWR = 8V
PPBUS_SW_LCDBKLT_PWR_SW = 7.8V
PPVOUT_SW_LCDBKLT = 7.6V (I think it should be more high)

I tested:
D9701 seems OK
C 9796 seems OK
C 9797 seems OK
C 9799 seems OK
L9701 seems OK
F9700 seems OK

at this point I suppose it could be the U9701 damaged, but before trying to change it, I ask all of you if you have any advice.

Thank you everybody

[mon2]

Re: Macbook Air, no backlight 820-3024

PPVOUT_SW_LCDBKLT = 7.6V (I think it should be more high)

Correct. It should be. In the range of 25-50v when enabled.

measure the voltage readings for:

BKL_SCL
BKL_SDA
BKL_EN


Post the measurements.

The backlight IC is the LP8550 from TI and is a very common part that does get damaged. The intensity and function of the component is configured using the 2 wire I2C line (SCL & SDA).

Using an I2C bus analyzer, you can monitor the traffic to learn more or review the datasheet from TI's (not the rapper) website.

[piernov]

Re: Macbook Air, no backlight 820-3024

Bkl_pwm

[ginobarabasi]

Re: Macbook Air, no backlight 820-3024

thank you for the help. I think I should view these signals with an oscilloscope, because the mean value measured with a multimeter is not useful, right?

[mon2]

Re: Macbook Air, no backlight 820-3024

A voltage meter check should be fine for the I2C and enable line check.

The scope will help for the PWM line.

I2C is an open drain interface that will feature pull-up resistors on each, the SCL & SDA lines. We are checking for stuck logic levels on the I2C bus which will cause faults.

Often the exercise results in the replacement of the LP8550 device which is a WLCSP packaged component. With care, it is not difficult to replace.