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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Note: You may check out his previous repair article below:
LG 14 inch CRT Ultra slim television with shut down symptoms now repaired
December 12, 2016 at 6:25 pm
Very nicely done and explained! - Thanks.
Can you please explain this part more:
"I got a spark at the final anode cap"
December 13, 2016 at 1:48 pm
Thanks for your feedback, usually if the board is not installed to the tube, i usually apply power briefly to see if the HV will fire, this spark at the anode cap is good enough to confirm the tv is generating High voltage.
December 13, 2016 at 10:09 pm
Thank you for this wonderful report. It's very good to read your tip that the series number of the components mean they are related in the circuit.
Please kindly tell me how you check the HV for spark safely?
December 15, 2016 at 5:40 pm
Thanks for your feedback hongkongpom, to test the HV safely in circuit(tube) you can use HV tester but if the anode cap is not installed to the tube then you can apply power briefly and see if it sparks..
December 12, 2016 at 8:45 pm
Hello from Alabama, USA.
Thank you Humphrey.
What a great "thinking outside the box" and using the tools you have.
Never stop learning.
December 13, 2016 at 1:52 pm
Thanks for your feedback and glad to hear from Alabama, USA.
Albert van Bemmelen
December 12, 2016 at 10:05 pm
Nice repair Humprey.
The capacitor problems still keeps any electronics engineer busy repairing. I wonder when they ever will make E-caps or other caps that will last as long as coils do?
Thanks for the included schematic.
December 13, 2016 at 1:56 pm
Thanks for your feedback Albert, and you are "SPOT ON" regarding your remark on capacitors..though it is also a blessing in disguise for technician..these replacement of capacitors help technicians TO pay their bills on a light note...
December 12, 2016 at 11:42 pm
Great! I have never ventured to test the board without connecting it to a CRT so far. Normally, I check power supply by connecting a 100W Bulb Load. Your expertise helped you lay your hands on the 'culprit component!' Numbering following the prefix for connected circuits, is something new that I learnt today! Special thanks to you for the same!
December 13, 2016 at 1:59 pm
Thanks you for your feedback Parasuraman, Glad to note that you have learn something new today. keep learning and all the best from the desK of Humphrey Kimathi
December 13, 2016 at 2:52 am
Sir mr. Jestine n humphery
They both are indeed genius
No doubt about that
Stay blessed sirs
Sir i need replacement for 220v 330uf
For pc psu
Thanx n warm regards...
December 13, 2016 at 2:05 pm
Thank you for the blessing angelo, i have received them with both hands, sorry part B of your question on capacitor is not clear but you can use capacitor 330uF/400v.
December 13, 2016 at 3:09 am
Thanks for that comprehensive tactical way of finding the culprit Humphrey. You have, really, planted something in us all.
December 13, 2016 at 2:09 pm
Thanks you for your feedback and Passing by Tito.
Henrique Jorge Guimarães Ulbrich
December 13, 2016 at 4:55 am
Brilliant, Humphrey. Thanks.
December 13, 2016 at 2:10 pm
Thanks you Henrique,
December 14, 2016 at 12:50 am
Another good repair, and a great explanation too. Congrats.
December 14, 2016 at 8:12 am
Nicely done Humphrey. Well presented and well documented with good pictures including a section schematics. Good read too.
I would however appreciate a bit more explanation on the sentence “this capacitor [C408] can easily cause the HOT to short”. Why it that? This cap, as you have mentioned, is just a filtering component in the power supply line to the primary site of the drive transformer. The HOT is on the other, secondary site, so how is it so drastically affected by a failing filtering component.
December 15, 2016 at 2:00 pm
Thanks and welcome Humberto for your feedback.
December 15, 2016 at 2:18 pm
Thanks for your question Henry, The capacitor C408 is used to filter the supply to the drive circuit..Actually this is the circuit which drive the HOT transistor, kindly pay attention to the word "drive", and consider a scenario i have just explained where this driver is getting not well filtered voltage to drive it means the signal which will come out of this circuit which is being feed to the base of the HOT transistor via drive tranformer will also be affected and hence the HOT will become HOT and eventually get shorted...also note that the drive transistor is labeled Q401 and the HOT transistor is labelled Q402 and this means they are related components(same with C408) and what happens to one may affect the other in terms of delivering services(working)..hope i have answered your question Henry.
December 14, 2016 at 11:51 pm
Excellent work as usual Mr. Humphery... I really never worked on a CRT before but you have geat tips and inight. Thank you for sharing you experience. Happy Holidays to you and your family,to Mr. Jestine Yong and all the readers. Have a good one.
December 15, 2016 at 2:22 pm
Hey Andre, Long time and glad to see you here, wishing all the best to you and your family. Shalom!
December 15, 2016 at 4:28 pm
For wining the game managing exact replacement of the part is also difficult for us sometime.
sir, Thanks for sharing your experience.
December 15, 2016 at 6:04 pm
Nice repair and tips, Humphrey. Thanks.
February 5, 2018 at 8:49 pm
Thank you very much for sharing your tv problem. How did you find the lead on the harm of H.O.T. You know how good you are to find a piece that will follow the breakdown of H.O.T... More power to you Mister Humphrey.
March 30, 2020 at 5:12 am
I realy love the you explain the trouble in electronics