Need help activating LCD backlight in marlin

Hi, Yesterday I connected my 2004a I2C lcd screen in my mini kossel. In marlin uncommented #define LCD_I2C_SAINSMART_YWROBOT.

After upload the blank screen with blue backlight turned dark. I can see the text with a lot of light... but its almost invisible. Simply the backlight is off.

I looked around in code for hours trying to pass the backlight.init() and other similar function even tryed different screen option and different liquidcrystal_I2C librairies but nothing. I tryed removing the jumper at the back of the screen, still nothing.

I think its about BL_pin_position. maybe in ULTRA_LCD_IMPLEMENTATION_HITACHI_HD44780_H. I even tryed looking in the liquidCrystal_I2C librairies to try if I could fin a way to simply let the backlight on all the time.

Sadly im not an expert at all in this language and I jsute started fidling in marlin code since 2 days. If someone could help me with that i would be really grateful.

When I power up the printer le lcd backlight turn on for a second before turning dark juste before the text appear. I've included pictures of the lcd front and back and the connection to the ramp 1.4

Please help ! (Leeloo Dallas) ^^

I don´t know if it helps, but the backlight LED pins are labeled "A" and "K" on top of the display.

For LEDs : Anode is GND and Kathode is + with a matching resistor in series.
-Olaf
PS: Did you try the potmeter on the I2C board?

Edited 1 time(s). Last edit at 05/19/2015 03:04PM by o_lampe.

Thanks I do not have any spare electronic parts. yes I tryed the potentiometer and its for the brightness of the letter only. they go from dark to tranparent without the backlight.