Alienware M18x R2 No backlight repair

Problem description
Troubleshooting Alienware M18x R2 missing backlight. This machine mounts Compal LA-8321P board. This post applies to other machines, with different motherboard, but using similar backlight control.

Symptoms

 * No backlight on LCD

Solution
Make sure the problem is no backlight and not no image!

See Fig. 1.

Very likely we find +INVPWR_B+ is not present. We must assure no short to ground w/ and w/ LCD connected.

Next will check LCD_BKL_EN signal. If present (3V), then check R423 voltage on both pins. Getting 0V at pin 2 assures Q44 works correctly. 19V there suppose bad Q44; need to change it. If we get 0V at pin 2, but 19V at pin 1 (PWR_SRC_ON), 99% Q45 is bad; replace it. Is still possible (very low chance) to have bad C346 (corroded), or R423 (open).