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LCD inverter repair

Created by Olli Niemitalo on 2011-10-25, last modified 2012-12-05

2011-10-25

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.

8 Comments »

  1. I can’t even get the back off the monitor!!
    The power switch is very intermittent – now it won’t power up at all!?
    Thanks

    Comment by Alan — 2012-01-14 @ 08:42

  2. Alan, sorry, can’t remember exactly how to open it. Generally: Remove all screws, look for locking buttons and hidden screws, then pry (with screwdriver if necessary, it’s not an expensive monitor).

    Comment by Olli Niemitalo — 2012-01-14 @ 09:32

  3. I guess not. I have an monitor LG L1730S, was burnded same chip FDS… I changed it, it worked around 20 mins and then again black. actualy I see a picture first when I open it but in 1/10 of secont become black again. Caps seems ok. I cant find any other short circuit anywhere. and teh other same chip FDS… looks ok and acording with my multimeter is not shorted !
    What else can have?

    Comment by Baios ! — 2012-02-01 @ 01:45

  4. Baios, I don’t have too many ideas. But does the monitor switch off or is it just the lamps that turn off? If just the lamps then there should still be a very very faint picture on the monitor. If it’s the monitor that turns off then maybe try what I did, cut the trace that gives power to the inverter (you can repair the trace later). If the monitor then stays on then it’s probably still a short circuit in the inverter. Could be even your replacement transistor. There might be some useful info here for you: http://neoxy-yx.blogspot.com/2011/12/tips-and-tricks-on-repairing-lcd.html

    Comment by Olli Niemitalo — 2012-02-01 @ 12:47

  5. Hello

    I could you clarify a question I have? … Is that I see you’ve welded pins 1 and 2 each right?

    and 3 and 4 each component before welding to c1?

    thanks

    Comment by Toni — 2012-03-12 @ 10:41

  6. Toni, I made no new connections. Those pins were already connected together through the copper but I just added a lot of solder because I had damaged the copper somewhat when removing the burned chips.

    Comment by Olli Niemitalo — 2012-03-12 @ 11:03

  7. I wanted to ask why the sound is gone to me sometimes appears on the monitor. Hyundai L17T model

    Comment by Tomas — 2012-10-28 @ 12:58

  8. Sorry, no idea!

    Comment by Olli Niemitalo — 2012-10-31 @ 09:54

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