Vostro 1520 UMA (LA-4596P) No LVDS (crt working)

[r ez a]

hello my friends
i have 4 new motherboards with same problem
crt wotking fine but there is no LVDS
matrix turns on but its just backlight and black screen
i tried another cpu,ram,lcd and lcd cable tested
flashing bios not help
another motherboard work good in this setup

+LCDVDD= 3.3V
LCD_TST=3.3V
LCD_CBL_DET#=0V
LID_SW#=3.3V

any idea?