LG 55" OLED TV - Backlight Driver Failure
LG OLED55C3PUA with no picture but audio working. LED driver circuit failure diagnosed and repaired with precision component replacement.
Symptoms
- Sound is working (can hear menus/channels)
- Screen is completely black
- Flashlight test reveals faint image on screen
- TV stays on, does not cycle power
Diagnosis
The 'Flashlight Test' confirmed the OLED panel was generating an image, but the backlight (or in this case, the high-voltage drive for the OLED stack) was missing. Checked the LED driver output voltage and found 0V. Input voltage to the driver IC was stable 12V. Further probing revealed a shorted ceramic capacitor in the feedback loop of the boost converter.
- Multimeter
- Hot Air Rework Station
- Fine-tip Soldering Iron
- Tweezers
- Magnifying Glass / Microscope
- Ceramic Capacitor (MLCC)10µF 50V X7R 1206Feedback filter capacitor
- Electrolytic Capacitor100µF 100V 105°COutput filter capacitor (preventative)
Repair Process
Access the Driver Board
On this model, the LED driver is integrated into the power supply board (PSU). Remove the back cover and locate the section of the PSU with the connector going to the panel LEDs.
Test Driver Output
With the TV on, carefully measure the voltage at the LED connector. BE CAREFUL - this can be 100V+. If it reads 0V or very low, the driver is not boosting.
Locate the Short
With power off, check resistance across the feedback capacitors near the driver IC. We found one reading 0.3Ω (dead short).
Replace the Capacitor
Use hot air or two soldering irons to remove the SMD capacitor. Clean the pads and solder in the replacement. Ensure the new capacitor is rated for at least 50V.
Verify Backlight
Power on the TV. You should see the LG logo immediately. Measure the output voltage again - it should now be stable around 60-90V depending on the model.
Conclusion
A single $0.10 capacitor caused a total backlight failure. Replacing it restored the TV to full functionality. We also replaced the output filter capacitors as a preventative measure since they are under high stress in these slim TVs.