A1502 820-3536 no backlight - PPVOUT_S0_LCDBKLT=0V

[vincelx]

On this board I have no backlight, but I can see that an image is present when I put a strong light behind the lid.

On LCD connector:
PPVOUT_S0_LCDBKLT =0V - resistance to ground on this pin: 163ko
LCD_HPD_CONN =2.5V
PP5VR3V3_SW_LCD =5V
EDP_PANEL_PWR_OR_PSR_EN =3.3V

EDP_PANEL_PWR =3.3V
LCD_PSR_EN =0V
EDP_BKLT_EN =3.3V

EDP_BKLT_PSR_EN =3.3V
LCDBKLT_EN_L =12.5V

On U7700:
VDDD =5V
VDDA =5V
EN =3.3V
SD =12.4V
VSENSE_N =12.6V
VSENSE_P =12.6V

Any suggestion on how to continue troubleshooting?

[mon2]

Replace the backlight IC (U7700).

[reformatt]

Probably a good start. You can also refer to the wiki

https://logi.wiki/index.php/Backlight_Troubleshooting

Main issue I see is that Q7700 is not being turned on by U7700.

[vincelx]

Thanks for this information.
I did the PRAM reset, no change.
SMC_LID =3.4V
Tested R7732, R7731: values are ok, traces too.
Tested D7710: in diode mode: 0.3V in one way and 3V in the other.

I replaced U7700: still no backlight, the above measurements are the same.

What can be the next move? Replace Q7701?

[mon2]

Replace the diode D7710. The diode must block in one direction and conduct in the other direction.

If still not working after this replacement, then replace Q7701.

[reformatt]

From the 1st post, Q7700 was not being switched on. You stated that pin 11 (SD) was 12.4V in the original post. This needs to be 0V in order to pull the gate of Q7700 to 5.5V and turn on.

Refer to this part of the Wiki

BKLT_EN high but LCDBKLT_EN_L is not being pulled down

[vincelx]

The diode D7710 should be ok with these measures in diode mode: 0.3V in one way and 3V in the other. I confirmed on a working board.

About BKLT_EN high but LCDBKLT_EN_L is not being pulled down, indeed I2C_BKLT_SDA was not reaching the LCD, trace cut close to C8358, also damaged.

After repair, backlight is finally present. BKLT_SD is now at 6V, 0V is not even needed. Also confirmed on a working board. Then LCDBKLT_EN_L is at 8.8V

Thanks for the support! In future I will always check these communication lines first...

[mon2]

Q7701 is a P-channel mosfet so the gate pin must be lower than the voltage being passed through source / drain pins to enable. 0v; 6v are both low enough voltages to enable the P-channel mosfet. The datasheet for the part will offer details on the mininum negative (lower voltage) that is required to enable the mosfet.

For N-channel type, the gate pin must be a higher voltage than what is being passed through source / drain pins and the data sheet will reference a positive voltage to enable.