820-4924 no video or backlight

[ytesfay80]

This board was liquid damaged around the lcd connector area and the bottom side of the board by the lcd connector. I have replaced the lcd connector, lcd cable and the following components.
L8300, C8300/03, C8359, I also replaced a component also in that area(image attached) but I don't know what the component is for because i don't have a boardview for for 4924 so I have to use the one for the 820-3536.
There are some other caps/resistors and chips that don't look too good in that area but they are for Thunderbolt so I just left them.

There was no continuity from C8300/03 to the BKLT pin 1 of J8300 so I had to run a wire to connect those lines.
When I plug in the charger and screen I feel the CPU getting warm but even after 30 seconds of it being on I see no image on the screen even with a flashlight shined on it.
I also get 3.3v on Pins 6/7 of the connector. Pin 1 for BKLT gives me 0v
Q7700 voltages:
Pin 4: 12.56v
Pin 3: 12.46v
Pins 1/2/5/6: 0.05v

SMC_LID is a steady 3.4v

R7701/02 have the proper resistance measurements
R7742 has 0v on both sides so I think that may be an issue.

 

[2informaticos]

The circled piece is C7778, not critical...

Did you check external video?
Is there USB activity at least?
J8300 must get voltage at pins 5, 28-30 in the first seconds.
The voltage is maintained only if DP_INT_HPD goes high; which means LCD is correctly recognized.
No backlight before that, of course.

 

[ytesfay80]

Yes I get video when with hdmi output and screen plugged in. I do get USB activity.

Sorry I forgot to list voltages for pins 5, 28-30 in the earlier post but i took them
J8300 - Pin 5: 3.32v
Pins 28-30: steady 5.12v

 

[2informaticos]

Check DP_INT_HPD voltage; pin4/U8340.
Confirm 3V there.

Next, check all FL8300-3 coils and all C8320-9 caps, if present and good.

 

[ytesfay80]

Yes I get 3.3v on Pin 4 of U8340.
C8320 - C8327 all get a steady 0.45v on Pin 1 and on Pin 2 it starts around .13v and slowly goes down towards 0.03v.
C8328/29 have 0v on everything except for Pin 2 of C8328 which has around .04v
FL8300-3 look fine visually and it shows around .13v and slowly goes down towards .04v as well

How should I measure if FL8300-3 is good?
C8320-29 in diode mode all measure 0.324v on Pin 1 and OL on Pin 2.

 

[2informaticos]

"How should I measure if FL8300-3 is good?"
Check for continuity between their pins...

Be sure to have good conexions between J8300 and all related components.
Did you try other compatible LCD assembly?

 

[ytesfay80]

Yes there is continuity on all FL8300-3 pins.
I noticed C7766/7/9,C7717, C7776 were corroded and looked dark so I replaced those but still that didn't fix the issue.
All of the J8300 pins have a good connection and there is even continuity from J8300 to the connector on the LCD. I don't have another assembly to be able to test.

When I measure Pins 1/2/5/6 of Q7700 I keep getting -0.05v. Pin 3 has 12.46v and Pin 4 has 12.56. Do you know why it's not getting the correct voltage?
Does R7742 getting 0v an issue since that is the EN pin for U7700?

 

[2informaticos]

If you miss image and backlight at same time, is not from U7700 area.
Post U8330 voltages.
I still suppose corrosion issue near LCD connector; maybe on the other side.

 

[ytesfay80]

U8330
Pin 1 - 0v
Pin 2 - 0v
Pin 3 - 3.32v
Pin 4 - 0v
Pin 5 - 0v
Pin 6 - 3.32v
Pin 7 - 0v
Pin 8 - 3.32v

 

[2informaticos]

So you miss LCD_PSR_EN and EDP_BKLT_EN; both come out from PCH.
PCH recognizes LCD, because EDP_PANEL_PWR is kept high.
For some reason doesn't enables the other signals.
I suppose communication problem between LCD connector and PCH; corroded traces, or components.
Could also be bad LCD; you must confirm testing another assembly.

If liquid spill affected area from pin1 to pins 3, or 4, that could damaged LCD.
If affected pins 6, or 7, could damaged SMC.

 

[ytesfay80]

Okay I ultrasonic'd the board again, replaced the lcd cable and connector again and now I see an image on the screen but no backlight.
Now on Q7700 I get 8.6v on Pin 3 and 12.56v on pins 1/2/5/6.

On pin 1 of J8300 I get around 0.78v and it slowly keeps going down.

 

[2informaticos]

D7710 is open, or you have broken trace from it to J8300.
Post volatges for D7710; it could also be a problem between Q770 and D7710, maybe L7710...

 

[ytesfay80]

Pin 1 of D7710 is 12.55v and Pin 2 gets around 52v.

 

[2informaticos]

So broken trace to J8300.
Backlight circuit works now; you get 52V because of open circuit...

 

[ytesfay80]

Okay I ran some wires and its working fine now.

Thanks!