LCD inverter repair


I found from a dumpster a Hyundai L17T LCD TV, pretty much a monitor with an analog TV tuner. I tried to turn it on, but the power indicator light was simply flickering slowly, indicating a short circuit somewhere. Nothing on the screen. The monitor was trying to power up, but then a fail safe switched it back off. And the cycle repeated.

Hyundai L17T. No image on screen. (Image source: PC Press)

I opened the monitor and saw that the circuit board had darkened around two components, identified as FDS8958A dual N & P-channel PowerTrench MOSFET's. A darkened circuit board should mean heat, and heat is an indicator of component failure. Solder around the components also looked a bit jagged, probably as a result of a cycle of melting and solidification. I could recognize that part of the circuit as the inverter power supply for the fluorescent backlight lamps. I scratched away the circuit trace that supplied power to the inverter, to see if the monitor would turn on without the backlight:

Darkened circuit board around inverter power transistors. The trace supplying power to the inverter scratched off to test whether the rest of the monitor was working fine

The monitor turned on without problems this time, and I could see a faint picture on the dark screen. In hope that it was just the power transistors that had failed, and that the rest of the inverter circuit was still OK, I looked for replacement components from my collection of electronics junk. Thanks to a hint from my brother Veli-Matti, I could find identically specified components from an old main board:

PC main board with similar-looking components (circled), some of which were suitable as replacement components for the burned ones in the inverter

The suitable components on the main board were AP5401 N and P-channel enhancement mode power MOSFET's. I carefully removed two of them from the main board, and replaced the inverter power transistors with them:

Power transistors of the inverter replaced with similar components from a PC main board

I didn't save on the solder, because I thought the transistors might benefit from a bit of heat sinking. After the repair, the monitor worked as good as new. Another piece of trash less in the world! I didn't have time to take a picture of the working monitor (or the broken one), so here is one fished from Internet to your satisfaction:

Hyundai L17T in working condition! (Image source: CompareStorePrices)

The following day the monitor left my disposal to be slowly on its way to Gambia where my friends at Apu-Paku Ry are going to build a music recording studio, among other great things that they will be doing.