Help needed to fix PC-TV Box

Discussion in 'General Electronics Chat' started by meijaz, Jun 2, 2010.

  1. meijaz

    Thread Starter Member

    Nov 20, 2009
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    I need to repair a TV Box which connects directly to a monitor using vga 15 pin port. Its model name is: "sunwin sm-230s". Main IC is "ARK7118". Datasheet of this IC is not available.

    The problem is that two transistors from the power supply located just after the rectifier are burned out. They burn out every time they are replaced. I want to know that if its possible to use an external 5 VDC adaptor instead of its internal power supply circuit?
     
  2. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    well either something connected to those transistors are causing them to burn out, or something before them is causing them to burn out.... if you could post some detailed, clear pictures of top and bottom of the circuit board, it would really help in guiding you... I can not just assume this will run on an external 5 volt power source without knowing some details....

    B. Morse
     
  3. meijaz

    Thread Starter Member

    Nov 20, 2009
    37
    0
    Well I managed to draw the circuit of it. The images are available at [​IMG] and and attached to this post

    The values of resistors and capacitors are not real. The transistors Q1 and Q3 keep on burning. your help required. thank you
     
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  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Ur circuit makes no sense. Post a picture of ur supply.
    this way I can verify it 4 u
     
  5. meijaz

    Thread Starter Member

    Nov 20, 2009
    37
    0
    Here are the pics of those tv devices. If more info is required, plz ask me.

    thanks
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    760
    can you mark the replaced transistors
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
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    I cant really see from those pics, but you want to be sure there are no solder bridges. This is when solder from one pad reaches over to another pad causing a short.
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    760
    [​IMG]

    Before replacing anything, try to find the fault.
    U need to check the bridge.
    and then give me the no.'s and values from 1 to 8

    Seeing this pic, I am assuming u have replaced those to small transistors.
    Am I right?
     
  9. meijaz

    Thread Starter Member

    Nov 20, 2009
    37
    0
    You are absolutely right. These are the ones and the last time I replaced the fuse went off too. I posted the other pics to show that all these devices use similar power supplies (One device uses power 22E IC instead of these transistors). Furthermore, soldering in the bridge is ok. Values are:

    1) IRF830B
    2) Resistor Brown Black Golden Golden (1 Ohm)
    3) A1015
    4) C1815
    5) two diodes (values not clear)
    6) Zener diode (value not clear)
    7) Resistor (value not clear)
    8) Resistor Yellow black brown Golden (400 Ohm)

    I shall try to tell you unclear values after desoldering these components.
    Can you please tell me what type of circuit is this and what type of waveform does this require?
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Wave forms are not yet important, it's a typical SMPS

    Fuse blew huh?
    Take out complete bridge and test or better yet replace with new identical diodes, check no. for replacement.
    Take out no. 1 , the switching transistor, it's probably shorted.
     
    meijaz likes this.
  11. meijaz

    Thread Starter Member

    Nov 20, 2009
    37
    0
    Thank you very much. You solved my problem. I shall give it a try.

    One more thing i want to ask. Its written on the main IC that it requires 5V. If so, can I use a 5V linear power supply instead, if not then can I use an ATX computer power supply instead, if yes then where should I connect anyone of them?
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Wait a minute.
    I really donno what happened.


    Is the supply working now? It's important that you update the thread telling everyone what you did.

    Don't just go off saying thank you, cause to me it is not enuf, you have to pay me.

    To pay me...
    you have to complete the thread so others can learn, this is why I am here, I want respect from the ones that I have helped. Teach others of their errors.
    Your information is more important than my methods.
    It's your work that others will see and learn from you.
    What you did is important to this thread and what you have learned too.

    Don't just leave. Cause I know you will come back.
    So leave something to come back to.

    By the way ....what 5V are u talking abt?
    and I have a request....update ur profile...like where are you from?
     
  13. meijaz

    Thread Starter Member

    Nov 20, 2009
    37
    0
    I am sorry to hear all this

    I am not leaving. This is my interest and hobby. If I am here to leave then why am I wasting time on this cheap Chinese TV device? why not buy a new one? you are right. I shall post every info about how it got right?

    The 5V written is shown on in the picture.

    The problem is the confusion that why are these devices using SMPS, can linear power supplies be used instead? If this is not possible then can the power supply of a computer (SMPS) be used instead?
     
  14. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    SMPS are far more efficient than linear ones. U should read the e books in AAC.

    As for running it from a PC supply, idea ain't bad, I always think of doing this if I cannot get a monitor supply working due burned components that cannot be deciphered.

    But there things to be taken into account for.
    I recommend doing this if the u cannot get that supply working.

    In ur case which is not difficult, but what you think is cheap and suits u will also be fine if you are willing to go for it.

    To learn new things is to start doing it when it seems impossible.

    I could have never got this good if I had stopped on this that I did not understand 10 years ago. I did not stop on things that cannot be done, infact if any one says it is impossible to fix it, I would spend day and night on it to see why the heck it is impossible. It was built by humans, then why cannot a human fix it.

    If you want to learn then you should try testing those components. It will take time, but if I am here, a TV is not a big deal to me, at least 99% of the time, I will find a the fault.
    Fixing the fault is another matter, knowing the exact cause is what matters to me.

    Tell me. how do you want to go about it.
     
  15. meijaz

    Thread Starter Member

    Nov 20, 2009
    37
    0
    Oh, I was just asking BTW about these alternatives.

    Another problem arose. The resistors near Q3 are partially turned black perhaps burned. I read these values as R8 = brown green (rest all black) and the other = Orange Orange black.

    Plz tell me how to find the values of them to be used for the replacement.
     
  16. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    We have to trace a schematic, to see where and why the resistor was used there.
    This ways atleast one of us will have an answer.
    We will need good close up of that area from both sides together with components that were their.
    Components need not be replaced but a note on which was where would suffice.
     
  17. meijaz

    Thread Starter Member

    Nov 20, 2009
    37
    0
    Sorry for such a late reply. I was studing some ebooks about SMPS. Now I know the efficiency, size and other advantages of smps over traditional supplies.

    I tried to repair the tv device but in vain. I replaced the bridge, transistors and resistors. I think the problem was in the main switching MOSFET which is mentioned above. The supply however worked fine. But there was a problem in the main circuit of the device, whose rectification is beyond my knowledge. Anyway, I call it a success.

    Moreover, I tried to connect this device with a conventional power supply (which I made, 5V regulated and 1A). It worked with it. Which concludes that these devices work on dc supply as contrary to a TV (which can not).
     
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