Backlight 820-00165

[arnonabuurs]

Hello fellows, I have a 820-00165 with a backlight issue. Liquid spil on U7701. Replaced that one and when the IC was removed checked for shorts on 2 feedback lines but they were OK. Voltage on pin 3/4 is 20v but still no backlight. Checked BKL_PWM it has 1.47v and BKL_EN is moving in a range from 0.3-0.8v so too low. No short on these lines and the resistors have the correct values. I have tried to replace another U7701 but still the same, any ideas?

 

[2informaticos]

PWM seems to be good, is a medium DC value. There is a square wave as control signal.
Check R7731 and its traces. Does it beep with L7701?

 

[arnonabuurs]

Yes there is continuity between pin 1 from R7731 to pin 1 of L7701. I have only checked pin 5, 6 and 7 for continuity before, should i remove U7701 to check more? I get 0.576 on pin 1 from R7715 when measuring resistance to ground so I assume its safe to say its connected with pin 3 of U7t01, otherwise it would be shorted.Is that a correct assumption?

 

[2informaticos]

If you get 20V (at least) at backlight, R7731 should get PPBUS_G3H, as L7701 too.
Check if resistor is OK.
20V is no suffcient for backlight of course.

 

[dukefawks]

No way you have 20V on the backlight if BKL_EN is 0.8V.
1.47V on PWM is way too low, there is the issue. Remove R7704 and connect pin 2 directly to 3V somewhere, if there is backlight now you know the PWM is the issue.
I assume you did a PRAM reset!