Backlight Troubleshooting

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PRAM RESET! Low boost can be the brightness being down.

Check voltage after backlight diode.

 If backlight is 45V or more there is no load on the backlight circuit.  Use a known good screen and check the connector.
 If backlight is 25V-40V the boost circuit is working.  Check the connector, cable, and LCD.
 If backlight is PPBus voltage your boost circuit isn't working.  Check the feedback trace, LCD driver, and enables.
 If backlight is 0 check for short then work your way back toward PPBus. The LCDBKLT_FET, fuse, and enable transistors are all suspect at this point.  Also check the voltage dividers.

checking SMC_LID (it shouldn't give an image either if it's missing)

To have boost on this circuit, the LCD must communicate. Be sure to use a known good LCD when troubleshooting.

Measuring before the inductor you should have PPBus, and after it you have some random rounded-square-ish (or triangle-ish) signal that a cheap multimeter won't know what to do with. Measure after the diode unless you are using an oscilloscope.

Missing LCD_BKLT_PWM: Inject voltage or jumper from PP3V3_S0 or similar.

Some great info from Louis Rossmann (User name - zzz) ifixit forum

Most "common" problem with no backlight is ball A5 on LP8550 or LCD connector, but guessing is BS and should not be done here.

put multimeter in diode mode, red probe on ground, black probe on pins 3/4 of LCD connector.

  • 0.0 or 0.007 means direct short to ground, usually inside LCD connector or LCD cable.
  • 0.200-0.300 is bad LED driver.
  • 0.459-0.511 is bad feedback via or corroded away feedback ball under LP8550
  • 0.565 is corroded solder ball or bad switch trace inside LP8550

Measure voltage along each point in the circuit. Before F9700, after F9700. At output.

At backlight output, measured on pin 3 or 4 of the LCD connector,

  • 0v means short to ground, blown fuse, or no LCD connected. Check that you see an image on the LCD, check backlight fuse, check board via from FET after fuse to boost coil.
  • 8v means no short to ground, good fuse, but no boosting. Check BKL_EN is 2.7 to 3v at voltage divider going to enable pin of WLED driver, check BKL_PWM signal exists.
  • 27v or higher means all good, but your LCD cable or LCD backlight itself is blown.

If you see power along the line, where is it? Where does it stop?

If you replace a fuse with a short to ground anywhere in the line you have wasted your time, and a precious little 0603 package. :(

Lastly if it worked for a few months and then died most likely your problem is on bad feedback via/ball on LP8550, pins 3/4 coming corroded on LCD connector, or BKL_EN voltage divider resistors in that order.

Have fun! Louis Rossmann