Apple MacBook A2337 - No backlight

[tommy97]

Hi guys, I've got MacBook Air a2337 with no backlight. Once while testing it displayed with the backlight, however after restart problem come back.

The laptop was tested with fully working screen.

PP5v_BKLT_A/B 5V
PP5v_BKLT_D 5V
PP5V_S2 5V
PP5V_SW_LCD 0.4V
P5VS2_PWR_EN 1.8V
P5VS2_BST_R 8V
P5VS2_SW 5V
PPBUS_5VS2_VIN 5V
PPVOUT_LCDBKLT - Doesn't short 0V
FP800 - Both sides 12v
BKLT_EN_R & BL_PWR_EN - around 1.8v

LP800 0.014V On both sides (should be 12? / no short)
UC300 & UP800 Replaced with tested one - no difference.
Just to clarify, no backlight - I can see display very dim.

[tommy97]

Any ideas?

[mon2]

What are the voltage to ground measurements for:

I2C_BKLT_SCL
I2C_BKLT_SDA

Share the voltage to ground of the mosfet @ QP800:

source
gate ; LCDBKLT_EN_L
drain

[tommy97]

I2C_BKLT_SCL 5.1V
I2C_BKLT_SDA 5.1V

QP800 - LCDBKLT_EN_L (3) 12V, (leg 1,2,6,5 - 0.4v PPVIN_LCDBKLT_Q), PPVIN_LCDBKLT_R (4) 12v.
During test again I've got display, however after restart same thing with no backlight, while backlight were on Each side had 12v on the QP800

[mon2]

Take a piece of wire and short pin # 3 on QP800 to ground. This action will force enable the P-channel mosfet @ QP800. Be careful to touch only pin # 3 on QP800.

[tommy97]

After shorting pin 3 same problem, rest of the pins 12v

[mon2]

The thread below this one is with a similar fault.

Check the FB pin on the backlight IC. Confirm the FB resistors are present. Confirm the diode @ DB800 is not defective.

What is the voltage to ground of IPD_LID_OPEN_1V8?

[tommy97]

IPD_LID_OPEN_1V8 - 1.8v
DP800 - 0.350 One way, checked same reading on the spare board
PPVOUT_LCDBKLT_SW 0.3V
PPVOUT_LCDBKLT - 0.15V

[mon2]

Is PP5V_SW_LCD voltage to ground stable?

Measure the LCDBKLT_HS_ISENSE signal on the board. This signal is a gauge of the current sense of the display assembly.

Confirm the details of the mosfet @ QP801. Test with the meter in resistance mode.

source / drain
source / gate
gate / drain

A low resistance means the mosfet is defective.

Suggest to flux and reflow the replaced known good backlight IC. Test again.