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Changchong power fault (Help please).

Discussion in 'Troubleshooting and Repair' started by civildefence1, May 28, 2014.

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  1. civildefence1

    civildefence1

    13
    1
    Jul 31, 2013
    Hello house,

    I really appreciate what you people are doing here, the help you render can not be quantified in fact the influence you have on people like me who knows but little about electronics is tremendous, so I want to say a big thank you to you.

    This is the first time I will create a thread after being an observer for a very long time. let me go straight to the point. There is a changchong flat screen TV, with power issues that I have been trying to repair.

    The TV developed the fault when a power surge lead it into smoke coming out of the TV and then it stopped working, I unplugged it and plug it back and it did not come on, so I uncoupled it and found out that the varisitor and the fused were burnt. I replaced them and decided to run a test on the power panel. I start by checking the rectifier diode out, the reading on the meter seems good( it read one direction ) only, but whenever I connect it to source of light and measure the voltage on the rectifier the rectifier read both ways, and to my newbie way of thinking the meter reading the voltage both ways means something is not right.

    So I will not schooling from you guys, please do not let my late response to your upcoming replies deter you from helping a brother.

    Patiently waiting for your help.
     
    Last edited by a moderator: May 28, 2014
  2. Sadlercomfort

    Sadlercomfort Ash

    424
    53
    Feb 9, 2013
    Hello,

    Thanks for posting! To help further, would you upload a picture of the board? D


    Ash,
     
    civildefence1 likes this.
  3. civildefence1

    civildefence1

    13
    1
    Jul 31, 2013
    Sorry for the late reply, sadlercomfort. I have been away for the most part of the day. I have include an attachment as you require of me;
     

    Attached Files:

  4. Sadlercomfort

    Sadlercomfort Ash

    424
    53
    Feb 9, 2013
    Well, I'm not an expert on this. But that voltage spike may have broken down the junctions inside the rectifier diodes.

    A bridge rectifier normally uses 4 diodes, which I cant see clearly on the board. Remember that rectifiers can come in different packages like the images I've attatched.

    Check all the components for any signs of damage.

    Dsc06225.jpg index.jpeg 391468601_518.jpg
     
    civildefence1 likes this.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,298
    2,736
    Jan 21, 2010
    Hi CD (I hope you don't mind it if I call you CD) :) I'm may ask a lot of questions, please do your best to answer all of them.

    First note that fixing this will require working on parts of the circuit that are live (at mains potential) when the circuit is operating. The circuit also has capacitors that can store a potentially deadly charge. You need to be very careful. The use of the word "deadly" is not an exaggeration and it also applies to wiring at mains potential.

    Smoke coming out (assuming this is literal) is a pretty good indication of a major fault.

    If you're lucky, that was the varistors failing and the fuse subsequently blowing before the varistors exploded. Exploding varistors are only a problem because that means the fault (high input voltage) still exists and the rest of your circuit is now unprotected.

    You've recognised the varistors and replaced them, so I'm assuming you have some experience with electronics.

    (1) Can you tell us what the condition of the replaced varistors was? (2) And how did the fuse blow (if it was a glass fuse, was the inside blackened (suggestive of huge overload) or was it clean, but reading open (indicative of a smaller overload)?

    (3) Have you tried plugging the power supply in after replacing these components? (I'm NOT recommending that you do, but if you have, the results would be interesting -- presumably you would have told us if the fuse blew again.) A relatively safe way to power the board is to use a "dim bulb tester" technique involving a light globe in series with the power supply. The light bulb will limit fault current so you do less damage (or at least do it slower). It is an invaluable technique for testing power supplies like this.

    Your description of your testing of the rectifiers is somewhat unclear. You mention testing them (with a multimeter?), but then with a light? (4) Can you detail a little more clearly what you did?

    (5) What test equipment and tools do you have access to? A multimeter and a soldering iron are probably the basics here.
     
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  6. civildefence1

    civildefence1

    13
    1
    Jul 31, 2013
    Thanks all for your kind attention and helpful insight and I am sorry for the late reply.
     
  7. civildefence1

    civildefence1

    13
    1
    Jul 31, 2013
    @Sadlercomfort, your post has enlightened me the more, I mean I never knew rectifier exist in different shapes as seen in the pictures you posted above.

    I want to believe the diode in the power panel is the same thing as the one in the middle of the rectifiers pictures you posted earlier, I am not that experience in electronics repairs, the little I know about electronics repairs is from you guys the gurus in the house.

    I already bought one, the problem is the marking on the legs of the rectifier did not tally with the seemingly faulty rectifier, the positive(+) marking on the new rectifier is on the left legs, while the positive(+) marking on the seemingly faulty oneis on the right legs. So replacing it is the current issue. Before I do That I will also like to be sure that the rectifier is faulty

    I understand that if you want to test a rectifier while the power board is connected to power supply, you tune your meter to voltage in DC and measure the voltage in that particular rectifier. So the question I have been trying to ask is can the voltage read irrespective of the polarity?

    Thanks in anticipation of your insightful reply!
     
  8. civildefence1

    civildefence1

    13
    1
    Jul 31, 2013
    Hello steve, You can call me CD, I think is more convenient that way, you know the name is too long to type, thank you once again for stopping bye, I will try as much as possible to answer those questions.
    Nope I did not have any experience in electronics repairs, any thing you see me do ia from what I learnt from you guys and I hope to learn from you guys.

    1. The varisitor was burnt beyond recognition, it turn black as charcoal and it was split opened.
    2. The fuse is the glass type and inside the glass is a spray of some blackish stuff. Yes it was blackened.
    3. Yes I have tried that, the fused did not blow up.
    4. pardon me please, I am not a native speaker of the language English, you can see from my post I am just trying to converse in the language, I did not mean "with light" what I am trying ti communicate is " I plug it to power supply" the mains, and use multi meter to read the voltage passing through the rectifier. And this where I asked the question is the voltage supposed to read both ways, irrespective of the polarity.
    5. Soldering iron and multimeter.

    My problem here is how is the dc voltage reading of the rectifier going to be like, when the power board is connected to the mains? Can the reading stiil be in one direction?
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,298
    2,736
    Jan 21, 2010
    Thanks CD, you sound like you're doing well so far. However I would recommend that you stop trying to take measurements with the power turned on.

    For the moment, everything will need to be done with the power off.

    The first thing we'll need is a closer image of the top and bottom of the board. We need to be able to read the writing on the board.

    Here is a set of instructions that offers some tips.

    I'm most interested in the device on the bottom right corner of the board (as shown on your image above). If you can get a photo from an angle which shows the part number that would be great.

    And your English seems pretty good to me. Where are you from and what is your native language? (needless to say, I won't be able to speak it)
     
  10. civildefence1

    civildefence1

    13
    1
    Jul 31, 2013
    Thanks steve, you think I am improving? well I do not think so, anyway, thanks for the compliment and your advice is noted.

    I will post the picture as instructed in the link above, as soon as possible.

    I am from Nigeria and a native Igala language speaker. Thanks!
     
  11. civildefence1

    civildefence1

    13
    1
    Jul 31, 2013
    I was able to replace the rectifier with another one and the TV is now working, thank you all for your time!
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,298
    2,736
    Jan 21, 2010
    Congratulations.
     
  13. civildefence1

    civildefence1

    13
    1
    Jul 31, 2013
    :) I am happy you have all help me out!
     
  14. Sadlercomfort

    Sadlercomfort Ash

    424
    53
    Feb 9, 2013
    Thats fantastic! :) Much more capable than you think.
     
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