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Lexuco 29" CRT TV (blowing horizontal output transistor)

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    #1

    Lexuco 29" CRT TV (blowing horizontal output transistor)

    Hi all,
    This one is definitely for the veterans out there that still work on old CRT type TV's.
    Firstly i cannot get a diagram for this TV, its a Chinese chassis number T08-T37A. I have attached photo's for you to look.
    The symptoms were PSU no secondary outputs due to short circuit on +B line to HOT.
    Replaced line output transistor D5036 with new one, replaced supply resistor 1.8 ohm 1W. Checked around line stage for faults saw a burnt coil marked 37uH. Tested it measuring 45uH so i changed it out with a scrap part which i measured with my LCR meter to be exactly 37uH. Just not sure of the polarity of this coil since the original wrapping had burnt off and i know that they are polarity specific. I matched up the coils vertical line on the side with the dot on the board. Found E/W transistor shorted 2SD2012. Replaced it.
    Found a fat diode shorted on the line stage close to the HOT, RU4DS, don't have something like that so i used a RU4C, lower spec, hope it is compatible, i just thought that the RU4DS was perhaps over specced.
    Checked all non polarized caps on line stage, i think they were 470nf 800V, 9.1nF 2KV and 8.2nF 2KV, all checked good. I then decide to power up the set. It started up but i noticed that there was a frame fold over on the right hand side vertically. Was still looking to see what it could be when D5036 HOT shorted out again. Excessive heat perhaps being driven wrong??
    Checked pre driver transformer but i don't have a number like that (Y119)
    I checked scan coil for shorts by removing it from the tube neck. All checked good.
    Now my issue is what can be a suspect for causing this frame fold over, LOP
    T? I cant test that? Also i cannot trust the D5036 since it could be fake spares.
    I could upload a circuit diagram of a similar chassis if necessary just to show what we are dealing with. I have a oscilloscope to measure the HOT waveform, what i can do is remove the line HOT ans scope it's drive,BTW the+B is 132V DC which is correct on these 29" sets.
    Any help anyone?
    Attached Files
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    #2
    Re: Lexuco 29" CRT TV (blowing horizontal output transistor)

    There could be a electrolytic cap on the supply side for the horzontal drive transformer, replace it. It is a small value (10-47µf) maybe 100v?
    I might also try replacing the old horz. lin coil, even though it burnt it might still be ok and your replacement could be causing the foldover.
    Upload the schematic.

    Comment

      #3
      Re: Lexuco 29" CRT TV (blowing horizontal output transistor)

      I checked the electrolytic, measures about 2 ohm.
      Replacing H- Lin coil made no difference to the fold over.
      Will send you the link to a similar schematic since i am now using my phone to chat to you and it cant upload a picture.

      Comment

        #4
        Re: Lexuco 29" CRT TV (blowing horizontal output transistor)

        Since the schematic is different, and you have a scope, check that the supply to T431, C454 is clean. I have seen that cap go bad and cause the hot to short.
        Then check the capacity of the poly caps in the horz circuit, like C435, C436, C437. an open cap can cause width and over scan problems, then check that you have drive signal to E/W transistor, I would not rely on a esr meter, caps like C305 and C444 could be bad but check good, either replace them or at least solder another one across to see if it makes a difference.

        Comment

          #5
          Re: Lexuco 29" CRT TV (blowing horizontal output transistor)

          Blowing the Horizontal Output Transistor is usually caused by a defective FlyBack Transformer.

          Comment

            #6
            Re: Lexuco 29" CRT TV (blowing horizontal output transistor)

            Originally posted by Andrew F. Ali View Post
            Blowing the Horizontal Output Transistor is usually caused by a defective FlyBack Transformer.
            So the LOPT can be the problem here. Cool i will check and get bqck to you

            Comment

              #7
              Re: Lexuco 29" CRT TV (blowing horizontal output transistor)

              UPDATE: TV Repaired.
              After pondering on what Andrew said in the previous post i started by changing out the cap that smooths out the voltage going to the pre drive coil. Then for good measure i removed the D5036 transistor and went to my scraps and looked for a 2SC5296 and stuck it in. Problem solved, no more frame fold over.
              Dealing with crap supplied by our local suppliers who source this from China and all their illegal chop shops.
              Unfortunately RS components does not keep stock of tv related transistors, pity.
              Those who have Farnell, Mouser etc, you are very fortunate.
              Dead trade.......
              Check the pic for the fake part.......
              Attached Files

              Comment

                #8
                Re: Lexuco 29" CRT TV (blowing horizontal output transistor)

                Great that you got your TV fixed. Thanks for the feedback.

                Comment

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