[SOLVED]820-3437 - No Backlight.

[gcain]

Liquid damaged all around the LED Driver.

I replaced it with a fresh LP8550, the old one looked like it had mud under it. Still no backlight.

Where should I be looking next?

R7780 is good.

Q7706 has 8v on 3 & 4 but nothing on 1,2,5 and 6.

 

[jadao]

Because pin 3 of Q7706 is high (8v) it will not allow 8v on pins 1,2,5 and 6. Pin 3 should be lower than pin 4.

In order to have pin 3 lower than pin 4 the voltage divider (R7788,R7789) must operate by providing it a path to gnd.

Check first voltages on EDP_BKLT_EN and BKLT_PLT_RST_L if they are present further troubleshoot must be done.

Assuming you checked that image is present.

 

[gcain]

BKLT_PLT_RST_L = 3.3
EDP_BKLT_EN = 3.3

R7788 & R7789 measure correct.

Image is present.

 

[SMMRepair]

What's your diode reading on backlight output? Might be a feedback problem or a poorly soldered LP8550. Obviously check the backlight fuse as well.

 

[gcain]

Fuse is good.

R7731::1 is about 50mV

Where should I measure backlight output?

Diode mode on D7701 = 0.507

 

[SMMRepair]

Looks like a bad feedback trace...run wire from FB ball @ LP8550 to repair the trace.

 

[gcain]

U7701::5 right?

Shouldn't R7731::1 have 3.3v on it?

 

[PhilipC]

did you check R7331 and R7715? you could check continuity from A5 on u7701 to D7701 pin 2 but you'd need to remove u7701 first obviously

 

[gcain]

R7715 is giving me low readings. I'll replace it to be sure.
R7731 is correct at 200k.

I have already replaced U7701. Should have thought to check while I was doing that... Maybe I should replace it again.
I'll try a few things first then do that if nothing else works.

 

[gcain]

Removed U7701 and it has continuity to D7701.

 

[gcain]

When doing a final test to return to customer I noticed that the backlight is out again.

In fact I have two of them now.

BKLT on LVDS is measuring 8.59v. Is this the LED Driver or feed back?
Both of these had the LED Driver replaced.