Friday, December 10, 2010

LG Television NO PICTURE- AUDIO OK blue background

This is a LG television made in Egypt (or is it made or assembled in Egypt) and the customer complain was No picture but audio is okay.

So I went down to his residence to check on it…remember most of my repairs are done on the site and therefore I normally do some preparation for the task ahead of the time.

In this assignment I had prepared myself with the basic equipment plus a circuit diagram of the LG television (any) because mostly TV circuit diagram for a particular model are very similar in terms of major circuit modules.

In this case from the symptoms given by the owner via the phone I was very sure the problem was likely to be on the CRT circuit board, so I went down to his place and found he was not around but the houseboy was around to assists me with what I wanted.

Asking him about the television he told me that there was another technician there and I wanted to know how far he went into the television even before I opened it and he confided with me the technician came and after a keen observation on the screen for about ten minutes without his eyes blinking he told them that the problem is new in the world and therefore he sees no hope on the television and therefore the best place for it is in the dustbin or make it a toy for his children.

My friend that is why it is very important always to get a second opinion even in the medical world. I mean if your doctor tell you that your leg need to be amputated (God forbid) please ask him/her for some times and try another doctor to hear his opinion.

For me I was very happy with this technician because he was honest with himself and never attempted to repair it because he could have done even more mess. So I opened the set and went straight to the small CRT circuit (RGB)


I did a voltage testing on the points highlighted and filled the figures (voltages) on the sheet of circuit diagram I had carried with me as shown Below.

 Now let us analyze the fact at hand…in electronics we deal with facts, in our case at hand the facts are voltage measurements and being voltages they are represented by figures and as you know figures never tell lies and therefore when need to deal with this truth.

From the figures and from what my electronics teacher told me about the RGB circuit I was not happy with the figures for the BLUE cathode at the CRT pins connector which was at 65 volts…as a rule of thumb the voltage at this point for the three cathode should be nearly the same with an margin of plus or minus 5 volts between the three cathodes and therefore the 65 volts on the blue cathode is first suspect.

So I tried to figure out why this voltage is very low compared to others , looking at the voltages on the B-C-E of the Blue transistor amplifier I noted that it has 4 Volts at the BASE and 4 volts at the EMITTER..

Again I am not happy with this because the Voltage drop across the base-emitter of a transistor should be between 0.3volts to 0.7 volts for silicon transistors but in this case was zero.

Did I say zero? Yes this means the voltage entering the base is 4 volts and coming out at emitter is 4 volts
again…how can it be?

The only reasoning is that the transistor is shorted base-emitter junction and hence behaving like a jumper across the base and emitter.

So I decided to remove this transistor out of the circuit completely and actually I was able to get a picture but missing the blue color and therefore the screen was yellowish..THE PRODUCT OF RED AND GREEN.

With that I concluded that the blue amplifier transistor is actually the culprit and having a few of these transistor at all times I rushed to the workshop and back with two pieces of the type and replaced it.

I replaced the transistor and the picture was back to normal.




Thank you guys

Let meet in the next class.

28 comments:

  1. HI MR KIMATHI
    in your first test
    when you turn on the tv the screen was black ?
    beh

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  2. i mean when you turn on the tv it was a black screen? or screen with no good quality picture
    lack of blue color

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  3. oh i hear you now Beh. the screen was all blue but no picture at all..
    Humphrey

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  4. ok but the mixture of the product of two other healthy electron guns red gun and green must be yellow but you said the screen was completely blue? is not that strange?

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  5. Correct Beh, please note this happened after i removed the blue transistor amplifier out of the circuit completely...is when i was able to see the picture which was yellowish...when the transistor was in the circuit...the screen was blue.

    Humphrey

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  6. OK this is your practical experience on this kind of LG sets it is valuable and very important for me
    thank you so much MR KIMATHI
    beh

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  7. Dear Humphrey,please can you post how to repair a dead phillips tv

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  8. I find it very interesting blog. In my blog journal, I put a link to this site. Hope is a hassle for you. The direction of my blog is http://alvarogomezcastro.over-blog.es.
    Greetings

    ReplyDelete
  9. Hi Castro, thank you for your fair comments...about linking my site in your blog its okay with me.
    regards humphrey

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  10. Hi sammstla, i have written an article on how to troubleshoot any television with dead symptoms. titled solving power supply problems(s.m.p.s.)on this blog..usually dead symptoms in all electronics equipment are caused by failure in power supply. also you can consider buying the book recommended on the same article for more info.
    Kind regards Humphrey

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  11. And for you Mr Beh. than you for your observation and your continued support to this blog..you are really a great source of my encouragement and God bless you.

    take care and more power....Humphrey

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  12. Excellent tutorial on RGB circuit Mr Kumar. keep up the good works. Humphrey

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  13. lg tv model no rt 29fc80vc my problem is no display but soung ok .when i shake the tv set the image is coming and suddenly black screen why it so

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  14. Hi Vikram, re-solder all pins on the CRT neck board circuit. the TV seems to have many dry joints. i am sure you will not need any spare parts..only the solder and the solder gun and there you go...
    kind regards Humphrey

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  15. Hi sir... Dis is Shourya Reddy.
    First of all i'd like to vsh u thanks fr ur kind solutions and feedbacks.
    Sir, Could u explain me how to check the functiinality of the IC vth a ddigital multimeter & the testing procedures of electronic basic components vth thediagrams pl....??

    Thank you...!!

    ReplyDelete
    Replies
    1. Hello Reddy, thank you for your feedback my friend..For more information on how to test ics see the link below.

      http://humphreykimathi.blogspot.com/2011/08/testing-integrated-circuit-ic.html

      and for more info on testing electronics components follow the link below.

      http://www.testingelectroniccomponents.com/

      Kind regards Humphrey

      Delete
  16. Wow! Humphrey I finally just enjoyed this piece you wrote and I must tell you you'd make a great Teacher! Extremely well written and so easy to understand and that's the way Teachers and people using their expertise can share with people in a simplistic way...love you brother. Songre

    ReplyDelete
    Replies
    1. Hi Songre, Thanks for your feedback..all the best my friend,
      Regards Humphrey

      Delete
  17. Sir my tv remote and tv button working condition but audio volume not control what broblem sir?

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  18. hai...I have lg flatron TV when I give a power to my TV the HOT will be burn in order to I disconnected the cap of yoke after that I give power the HOT will fine .....sir pls tell me is it yoke damage ??

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  19. Nice repair work sir, :) I have a question, I have LG 21" the problem was when I first turn it on, it works but after 20 minutes, it's automatically turning off and after a few minutes when i try to turn it on again, it works... then power off again, what might be the problem of this?

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    Replies
    1. Hi Ocampo, could it be something is overheating?
      Regards Humphrey

      Delete
  20. 21 inch China tv kit 1 minute working well then audio OK but video not come only blue screen showing. What a problem?

    ReplyDelete
    Replies
    1. This article is enough to help you solve that problem effortlessly.

      Regards Humphrey

      Delete
  21. Excellent article sir. Easy to understand that's great. I like ur teaching method

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    Replies
    1. Thanks for your feedback Joseph, Glad to hear you have found my blog useful.

      Regards Humphrey

      Delete