820-00165 flickering backlight

[vsa100]

Hi!

i have this 820-00165 board that has no water damage, but suddenly the backlight started flickering.
I?ve repaired a lot of not working backlights on these machine but never seen before a flickering one.

Where do you suggest to start?

thanks!

 

[JohnB8812]

Tried new screen? Inspect LED Driver area for corrosion as well as DC-in board/flex as they control the SMC_LID signal (Hall sensor)

 

[2informaticos]

Probably bad solder at A5 ball of backlight chip; replace it.

 

[vsa100]

Found slight water damage under U7701...so i've changed:

U7701
R7715
R7731
R7714
R7704
C7704
R7716
C7711

Nothing changed, no backlight!...what can i check now?

 

[2informaticos]

Did you check XW7720???
You have a sticky post about this problem.
https://boards.rossmanngroup.com/for...shooting-guide
Did you really check all the info from the forum?
Hundreds of threads with same issue, even pictures a lot.
https://boards.rossmanngroup.com/sea...22xw7720%22%7D

 

[vsa100]

Yes, connections on XW7220 are perfect!
Voltages on D7701: 7.7v on both sides

(Yes, i'm trying to use that threads too, thanks!)

 

[2informaticos]

Did you check continuity to A5 ball?
Any corrosion sign when chip lifted?

Post BKL_EN/PWM.
Try PRAM reset.

Did you try known good full LCD assembly?
Flickers more on low backlight, or viceversa?

"Voltages on D7701: 7.7v on both sides"
Cannot flickers, should be always black then...

BTW, use only charger; confirm if PPBUS_G3H is 8.5V aprox.

 

[vsa100]

U7701:
12 - BKL_FLTR 9.8k
3 - BKL_EN 75k
8 - BKL_ISET 16k
4 - BKL_PWM .47
19 - BKL_SDA 10.5k
9 - BKL_FSET .19

Yeah, after changing U7701 and surrounding components blacklight is always off now.

You mean pin5 of U7701? If yes, under scope is perfectly soldered,
and had no corrosion on pads when chip lifted.
Tried with another know good full lcd assembly.

PPBUS_G3H is 8.58V

 

[Greg]

Check Continuity A5 ball(pin 5 U7701) to D7701

 

[2informaticos]

"Post BKL_EN/PWM"
I mean voltage...

 

[vsa100]

Continuity from U7701 A5 ball and pin2 of D7701 fine!

Sorry, i was really tired after a lot of non-stop work!
Here are the voltages:

3 BKL_EN 2.8v
4 BKL_PWM 0v
(voltages taken with magsafe, lcd connected)
thanks!

 

[2informaticos]

"BKL_PWM 0v"
No way to get backlight then.

Post diode mode to grounde for BKL_PWM and EDP_BKLT_PWM.
Check R7704 integrity.
If all good, do PRAM reset few times.

 

[vsa100]

R7704 looks good, perfectly soldered and has a value of 47ohm.

BKL_PWM 0.815
EDP_BKLT_PWM 0.026

tried PRAM reset few times, nothing changes

 

[2informaticos]

"BKL_PWM 0.815
EDP_BKLT_PWM 0.026"
Impossible with 47 ohm resistor in the middle; no matters 33, or 47.
Check again please.

If you really have low value at PCH side, it is probably bad.
Still possible to froce BKL_PWM high, after R7704 removed.
Detailed instructions in the mentioned threads.

 

[KC Micro Tronix]

Could this be a analog fail? Is the flickering random?

I had a back light that was killing me. Under the scope the resistors looked ok. Nice solder blobs present. But I later discovered that it got under the resistor and released it from the pad even though it looked OK on top! First time I saw that and it ate my lunch.

Grab a known working board and start comparing resistance and diode checks.

 

[vsa100]

I have backlight!
First tried with the jumper method and worked, then resoldered R7704 and now working perfectly.
Pads underneath was perfect by looking at it, but scraping it showed a layer of oxidation.

Thanks a lot guys!