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.