A1398 820-00426 no eDP missing LCD_BKLT_EN

[guzq]

Hey Guys,

Working on 820-00426 chime, fan spin but no display. Plugging HDMI or DP to external monitor screen works fine.
Sometimes at random time image appears on screen just for seconds then disappear.

Board pretty clean, no visual liquid damage.

J8300 pin 1 = 0v (PPVOUT_S0_LCDBKLT). Sometimes give 50v then drop to 0v
J8300 pin 28-30 = 5v (PP5VR3V3_SW_LCD)
Q7706 pin 1,2,5,6 = 0v / pin 3 = 12v / pin 4 = 12v, try replace it but issue still persist
Replacing U7701 but no good result.
Found that missing LCD_BKLT_EN = 0v (should be 3.3v), that sent it out from U9600. Should i replace this chip ?!

 

[2informaticos]

If sometime got 50V, means open LED strip.
You may have bad LCD connector.
Try first compatible LCD assembly.
Look if image is always present; check LCD_HPD and PP3V3_S0_EDP_SW.

 

[guzq]

Try first compatible LCD assembly.

Try with working LCD assembly with same model same year, same issue...no backlight no image

Look if image is always present; check LCD_HPD and PP3V3_S0_EDP_SW.

With LCD attached and LogicBoard powered i do get at J8300
pin 5 (PP3V3_S0_EDP_SW) = 3.3v
pin 8 (LCD_BKLT_PWM_R) = 2.575v
pin 9 (LCD_HPD_CONN) = 3.3v

R8300 reads as 0ohm as per schematic.
L8300 = 5v at both pins

 

[2informaticos]

In such case, you should have image, but is not easy to detect it.

Check for activity on BKLT_SCL/SDA lines.
Intermitent backlight can be caused by bad solders around U/Q/D7701.
Post D7701 voltages when no backlight present.

LCD_BKLT_EN should be high, when you get 50V at K/D7701.

 

[guzq]

Check for activity on BKLT_SCL/SDA lines.

R7757 : 0ohm (BKLT_SDA) = 4.789v
R7758 : 0ohm (BKLT_SCL) = 4.790v

Intermitent backlight can be caused by bad solders around U/Q/D7701.

Replaced U/Q/D7701 issue still persist.

Post D7701 voltages when no backlight present.

D7701, pin1 = 0v / pin2 = 0v

LCD_BKLT_EN should be high, when you get 50V at K/D7701.

LCD_BKLT_EN = 0v

 

[2informaticos]

"LCD_BKLT_EN = 0v"
You don't get 50V anymore at backlight output, to test it?

 

[guzq]

You don't get 50V anumore at backlight output, to test it?

Yes.

That's why i'm suspicious with U9600 that generate LCD_BKLT_EN, should i reflow it first ???
Or just replace it ??

 

[2informaticos]

You can try to reflow, but should be better to reball it.
If still the same, the change it.