[SOLVED]820-00165 no backlight

[MaxFix]

Good day gentlemen,

Today I have a 00165 board with no backlight. I didn't spot any signs of water damage (just a tiny spot of green kaka on C3232 and C3233 but that seems to be Thunderbolt related and I replaced them anyway) and I tested it with a new screen.
Following a sticky troubleshoot guide I have measured 8.5 Volts of on output voltage and 2.7V on EN pin of led driver. I have replaced the led driver but that did not solve the problem.
I am not sure how to proceed. PLease advice.

Thanks you.

 

[aprendiz]

Check feedback trace....

 

[dukefawks]

2.7V on enable is ok, is PWM present? Did PRAM reset? Image visible for sure?

 

[MaxFix]

PWM is 0.8V. Is that too low?

 

[aprendiz]

Check R7704, do a PRAM reset..

 

[MaxFix]

R7704 is good (33 Ohms). PRAM reset done! No improvements.

 

[dukefawks]

0.8V is too low. Remove R7704 and connect PWM directly to 3V and see if it works.

 

[MaxFix]

Any 3V? will C7711 do?

 

[dukefawks]

Yes fine.....

 

[MaxFix]

Done. 3.3V on PWM now but still no backlight.

 

[dukefawks]

Bad LP8550, bad soldering, feedback trace to ball A5, bad diode. Read sticky topic on this.

 

[MaxFix]

Thanks for doubting my solderging skills Duke. I've resoldered new LED driver and backlight suddenly decided to work!
When I desoldered my firstly soldered led driver I saw a few pads without solder meaning it has not soldered on those pads.
Many thanks for your help!