A1466 820-3437 No Backlight

[Tony Tone]

Liquid damage around U7701. Replaced: U7701, C7714, R7741, R7715, R7731. Just a tiny bit of soy sauce got near this chip. Ultrasoniced.

C7714 5V
C7711 3.3V
R7731-1 8.57V
R7731-2 2.8V
R7704 1.46V
D7701-1 8.5V
D7701-2 21V
Lid 3.4V

New LCD assembly same.
PRAM Reseted
Image present

 

[PhilipC]

No boost occurring, check feedback line.

 

[Tony Tone]

21V, isn't that boosted?

 

[aprendiz]

Voltage on BKL_PWM???

 

[Tony Tone]

R7704 is 1.46

 

[Tony Tone]

So what should it be boosting to?
& what should BKL_PWM be?

 

[PhilipC]

it should be 25-27v. BKL_PWM should be 3.3v you can try taking a jumper from pp3v3_s0 to the BKL_PWM that will set it on full brightness permanently

 

[aprendiz]

After a PRAM reset should be about 3V on BKL_PWM (full brightness), on boosting I have 26V......... I say bad backlight driver, but also check connector.....

 

[Tony Tone]

Probably the U7701 then? I've been using pulled ones reballed with an iPhone stencil. I have some on order, but impatient. Thanx Philip, I'll try this jumper !

And thank you for clarifying the voltages ! ! !

 

[Tony Tone]

Thank you also Aprendiz.

 

[dukefawks]

Replace LP8550 before doing anything at this point.

 

[Tony Tone]

Rodger that. I actually found another LP8550 I'm going to try. Jumping PWM with 3V did nothing.

 

[Tony Tone]

3rd LP8550. Same results. All of these come off donor boards though. I may have to wait until the new ones come.

 

[jadao]

I had similar issue with another board, if iset or fset (I don't remember witch one) are open, I mean open/blown R7716 R7714, bad trace or bad soldering on B3 and B4 Lp8550 balls, bcklt output will be stuck (by default?) at something like 21v. Fset is what set the PWM frequency.it's different from one screen to another. Iset probably has to do with current setting depending on the led screen 11"13"15"
Check the values and trace continuity between R7716, R7714 and ground, R7716, R7714 and lp5880.