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14Inch LG Television Dead Now Repaired
I got a message from my son that there is a neighbour who wanted her Television checked, I passed by her place and the faulty TV was brought to me. I connected it to the power outlet and applied power and indeed the TV was dead, I tried to listen carefully for any sign of life and I noticed that when power is first applied there is some click then dead.
I removed the cover and tested the voltage across the main capacitor and I got around 140vdc, in my country we use 240vac and therefore I was expecting around 300+ volts dc here. The voltage was also fluctuating, with that I decided to stop my initial testing since it was getting late and also the lighting in her house was not good enough for electronic troubleshooting.
So I requested the owner if I could carry the motherboard with me for further testing in my workshop and she had no problem with that. Back to my workbench I decided to start with the secondary side and the best place to start was checking the state of the horizontal output transistor (H.O.T).
Due to the close proximity between components around the HOT I decided to solder it out completely, HOT plus the heat sink. This TV was using Transistor 2SC6093 as the H.O.T.
I tested the HOT with my digital meter set to diode range and I got low reading both sides between Collector(C) and emitter (E)
Looking at the above picture of the HOT on the left side you will notice that I have included a diode between the C and E pins, this diode is usually installed internally and therefore when testing HOT transistor (C-E junction) always expect high and low if the hot is not shorted, by the way 90 percent of the HOT failure is shorting between these two pins(C-E) junction.
With the HOT out of circuit I decided to do B+ voltage test, I applied power with the load bulb across the Collector (middle) leg and the cold ground and I got 110Vdc. With this I concluded the primary side of the power supply was okay and therefore no need of troubleshooting and therefore I only need to concentrate on the secondary side.
Usually if you find the HOT is shorted the main cause is the snubber capacitor having dried up and therefore I did test this capacitor with my capacitance meter and also with analogue meter to see if it is shorted and both test failed meaning the capacitor is good.
So what could have caused this transistor to short, I decided to continue doing further testing to component related to HOT.
Tip: To know related component for any circuit just check out component labels on the circuit board, in my case here you can see the HOT has reference Q402, therefore all component starting with 4XX series is related component for this circuit (transistor)
First I scanned for dry joints around the drive transformer and drive transistor and whatever I was not happy with I re-soldered including the fly back transformer pins.
I paid special interest on the circled component and all of them passed the test but I was not happy with C408 (2.2uF/160V) with filter the supply to the drive circuit. When I tested this capacitor I noticed its ESR was quite high (16 ohms)
With that I concluded that this must the cause of the HOT getting shorted and therefore as I said before HOT rarely short without a cause and this capacitor can easily cause the HOT to short.
I went to the shop to buy these two components and I had no problem getting the transistor 2SC6093 but getting Capacitor 2.2uF/160V was not easy and the nearest I got was 3.3uF/160v for replacement.
You can see the actual capacitor in-circuit on the picture below.
I replaced the two components, applied power to the board without the tube briefly and I got a spark at the final anode cap and with that I concluded the TV was now okay.
I went back to the owner, re-installed the board and applied power and the TV came back to life smoothly and the owner was happy to see his TV back to life.
Humphrey Kimathi is from Nairobi Kenya and the author of CRT Troubleshooting guide, DVD Player repair guide, CRT Television repair course and Basic Electronics course. He is also a blogger at Electronicsrepairmadeasy.com
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