820-4924 no backlight boost.

Yoel Ren

Member
820-4924 no backlight.
liquid damage at q7701. shorted inside the board at PPVIN_SW_LCDBKLT_SW, so i dig out the short, now resistance is good at 0.4kohm
fuse is good 12.6v at both side
ppvout is 12.6v at pin 1 of lvds connector.
i have bklt_en_R 3.3v
I use known good must be working 2015 lcd.
I have tried replacing q7701 q7700
need some pointer at why the output doesnt boost. any help appreciated, thank you..
 

2informaticos

Administrator
Staff member
Just to confirm, do you get image, but no backlight?

Are you sure no more damage in the burnt area?
You may need to change LCD connector.
 

Yoel Ren

Member
Yep absolutely sure theres an apple logo there, i get usb activity too.
The liquid damage i found is only that short i dig out.
Im a bit concern maybe theres some line didnt reach it destination maybe cut out by the traces i dig?
I will try to change lvds, even thought it looks good
 

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Yoel Ren

Member
i have changed lvds connector, still same.
diode mode at lvds :
1 : 0.400
2 : nc
3 : 0.464
4 : 0.464
5 : 0.525
6 : 0.640
7: 0.610
8: 0.510
9: 0.730
10: 0.430

u7700 have continuity from their pin to their surrounding component.
scl and sda line from u7700 have continuity to lvds connector.
what voltages supposed to be on pin 1 of u7700 ? its cycling with 1.2v at peak.
 

2informaticos

Administrator
Staff member
Pins 1/2 and 4 controls Q7701.
Be sure R7731-33 are all good, as their traces too.
Backlight level is adjusted through I2C bus; check voltage and diode mode to ground at SCL/SDA pins.
 

Yoel Ren

Member
R7731-33 is good resistance all according to schematic.
Diode mode both scl and sda is same, so its good 0.451,
Any chance q7701 and q7700 kill each other? Should i replace at once ?
*update : q7701 and q7700 replaced at once still not boosting.
 
Last edited:

2informaticos

Administrator
Staff member
Check R7744/45; U7700 is correctlt powered up?
Just in case, replace D7710 and L7710.

BTW, a big coil is pure short in ohm scale as normal.
 

Yoel Ren

Member
r7744/45 is 0 ohm resistance , normal
u7700 is powered up 5v by vddd and vdda normally too.
d7710 and l7710 replaced. no backlight still.
 

2informaticos

Administrator
Staff member
53V, why???
Correct voltage is around 39V.
Higher volatge will mean no connection with LED strip.

I hope you've cheked all above with LCD connected.
 

Yoel Ren

Member
no.. i know 53v is without load, let me revise my question, what component turn that 12.3v to 39v ?
i have checked all my measurement with lcd connected.
 

2informaticos

Administrator
Staff member
Backlight level is adjusted through I2C bus, is not just an enable signal required.
You must be sure to test with compatible (known good) screen.
PCH sends adjust signal (EDP_BKLT_PWM) to LCD and this one will resend adjust to U7700 via I2C bus.
 
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