Spotty backlight pattern on 820-2936 after liquid damage

[spakhnyuk]

820-2936 was originally liquid damaged. There was corrosion around the LVDS connector. I cleaned the board and swapped the LVDS connector. Backlight works but it seems that not all the LEDs are working. I'm getting a spotty LED backlight pattern (see photos). It's not the display because I tried several other displays with the same exact result. Any ideas?

 

[larossmann]

This is an issue with the LED returns. See page with U9701 or J9000 - there are LED return lines from the LCD connector to the LED driver. Inbetween the LCD connector and the LED driver, there are resistors, on the LED return lines. If any one of those return lines is broken, you get those shadows.

So either

1) The LCD connector pin is corroded.
2) The pathway between the LCD connector and the LED return resistor is corroded.
3) The LED return resistor is open and bad.
4) The pathway between the LED return resistor and the LED driver, U9701 is bad.
5) U9701 is bad or has a corroded ball underneath it.

Which of the 5 it is will be your adventure to measure, but it should be obvious if you look at the board.

 

[XooP]

Had the same issue a couple of weeks ago and like Louis says it was one of the lines from the U9701. In my case it got worse when i dimmed the backlight.

 

[spakhnyuk]

Awesome! I will try this and let you guys know.

 

[1centchips]

Hi spakhnyuk, does the connector lock the cable? Check if the actual grounding/locking plate works on the board.

 

[spakhnyuk]

Yes I believe it does. I currently don't have the schematic for this board. As soon as I get the schematic I'll double check what larossmann mentioned and get back to you guys.

 

[dukefawks]

Don't need a schematic to fix this. There are six 0 ohm resistors around the backlight chip in the LED return lines. I bet one of them is open or the trace to the LCD connector is corroded.

 

[JorgePayar]

I had the same problem in a 820 3115b till yesterday, then I checked and it was because a resistor, over that area that Duke and Louis said. Now it is working perfect!!

 

[spakhnyuk]

I'm still trying to work mine out, but I'm glad this thread helped you!

 

[spakhnyuk]

None of 6 LED_RETURN resistors are open.


When I wiggle the LVDS connector, the backlight issues seems to correct itself which leads me to a bad LVDS connector or a bad LVDS cable.
Still diagnosing.

 

[JorgePayar]

I don´t think is the cable. Not in my case at least. Could you post a picture to watch carefully the u9701 area?

 

[dukefawks]

Bad soldering on the new connector seems likely.

 

[Tony Tone]

Got that once from an LCD repair only on the left side. I thought I didn't put that sliver bar in right. Thanks, if I see it again, I know its something I have to fix.

 

[leonattpcs]

What are led returns for, do they tell the computer the led screen is on?

 

[dukefawks]

There is one line that feeds all of the leds together. There are 6 return lines that are distributed over a few the LEDS. Not really sure why there are 6 return lines, in theory it should be possible to tie them all together and just make it one. I guess it must have something to do with the design of the LED driver chip or LCD.