"3GS Backlight Circuit

Don't be surprised if just changing the 6R8 inductor (coil) doesn't repair the backlight fault.

The 109B MOSFET + schottky diode is part of the boost converter that, in combination with the 6R8 inductor (coil), boosts the voltage to a high level to run the 6 LEDs mounted in the screen. And the boost converter itself is managed by the 338S0768 power management IC. So, any of these parts can be responsible for the backlight not functioning.

Another fault common with the 3GS is that the copper tracks connecting the output of the boost converter IC (109B) to the LCD connector can become open-circuit. There is also an independent track from the 338S0768 PMIC to the LCD connector which is involved in backlight control, and this should be checked with a multimeter for continuity as well.

And, of course, the screen itself may be faulty."

Found this Tibbit on the Web.