Samsung NP300E5C-S01PT No Backlight

[sn4k3]

Problem: No Backlight, image OK

Description: A customer tried to install a lcd replacement, on connect LCD connector to motherboard he put connector in reverse order and also in normal order but with one column shift to the right. That result in no more backlight on PC.

Things i have tried before dig in:
- Connect in good order: No fix
- Test with other good known LCD: No fix

After research i find the datasheet from motherboard (Please see attachment).
The datasheet have "Backlight On" schemantic:



LCD3_BKLTON outputs 3.3V, Logic AND inputs are 3.3v

I probe all LCD pins and they have correct voltages.
In the end LCD have image, only doesn't have backlight...

Should LCD3_BKLTON be 0V or 3.3V to trigger backlight?

Last note: LCD is good, problem is on motherboard, i also tried to connect it shifting first pin col, and LCD backlight turns ON with PC OFF (Only conected to DC).
I also tried connect shifting the first col of pins, Backlight turns ON even with PC OFF (Only connected to DC or Bat)

[raajkumar]

Re: Samsung NP300E5C-S01PT No Backlight

LCD3_BKLTON Should be 3.3V to trigger backlight.

[mielta]

Re: Samsung NP300E5C-S01PT No Backlight

Hi
You must check lcd3_vdden and lcd_vdd3v and also vdd_led

[sn4k3]

Re: Samsung NP300E5C-S01PT No Backlight

LCD3_BKLTON Should be 3.3V to trigger backlight.

And it is, i write that on main post.

Hi
You must check lcd3_vdden and lcd_vdd3v and also vdd_led

vdd_led: 19V
lcd_vdd3v: 3.3V

lcd3_vdden not tested yet, but if LCD_VDD3V have correct voltage that doesn't means lcd3_vdden is working?

[mielta]

Re: Samsung NP300E5C-S01PT No Backlight

Hi
You must check this voltage at the LCD Direktly.
i think you have a Problem with the LCD Cable.
I Think the Voltages are all OK so that the LCD Must have brithness.

[sn4k3]

Re: Samsung NP300E5C-S01PT No Backlight

Hi
You must check this voltage at the LCD Direktly.
i think you have a Problem with the LCD Cable.
I Think the Voltages are all OK so that the LCD Must have brithness.

Cable is good, i checked for continuity, aprox 0ohm in every pin from motherboard to flex end. I didn't test voltage at lcd but i can do that.
Will post results latter

[sn4k3]

Re: Samsung NP300E5C-S01PT No Backlight

Hello again,

Voltages are correct at LCD connector.
Also as i said before if i shift connector 1 col to the right lcd will turn on the backlight, with PC off but connected to wall. see picture.



Because is bad connected BL_ON will recieve 19v, but that doesn't make sense.
Maybe BL require another line to work?

UPDATE 1: I manage to get backlight on again! After discarding obvious backlight circuit, problem was with "LCD3_BRIT" It was 0V... i think that something related with brightness so i test inject voltage there
First i tried to put 3.3V there, nothing happen, after i tried with 5V and for my surprise backlight is back!
Now if i power off pc completly, discharge and start again LCD will have backlight again without any help/boost
My question is: Is it possible that PC stored a value due bad connection to not let LCD gets brightness on and need this trick to reset?