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Short on a psu LS1904006A

Discussion in 'Troubleshooting and Repair' started by markm6164, Feb 18, 2012.

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  1. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    Hi guys

    I just bought a faulty LCD tv for learning purposes. The PSU has a short circuit as it blows the onboard fuse. I can't seem to find a circuit diagram for this psu so i can start to diagnose the problem. Can anyone point me in the direction of a diagram?

    Thanks
     
  2. davenn

    davenn Moderator

    13,865
    1,956
    Sep 5, 2009
    have you googled that model # ?
    did it give you any manufacturer info ?
    If so have you tried to contact them ?

    you havent given us much to go on ;)

    A large portion of stand alone power supplies are installed into equip as modules, manufactured by some 3rd party supplier and a cct diag maybe next to impossible to obtain.

    soo.... how about some close up, SHARP and easy to read pics of both sides of the PSU board and us guys may be able to help you to navigate through repairing the PSU you have :)

    which onboard fuse blows ?
    one of the DC output rails fuses ?
    the main AC input fuse ?

    is an output fuse then unplug all the output connecters from the PSU board replace the fuse and see if it still blows ?
    YES ? then fault is elsewhere in the TV
    NO ? then the fault is likely to be on the output side of the PSU secondary of transformer, rectifiers etc

    Is it the AC fuse that blows ?
    Yes ? highly likely that the input bridge rectifier has blown or some other component in the primary side switching circuitry

    cheers
    Dave
     
  3. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    Thanks very much for the reply Dave.

    I am an auto electrician and just starting in the hobby of electronics. The Tv is of no importance and is soley for educational purposes.

    The Tv is a Ventura 19'' LCD TV. I purchased it as a project and have opened it up and found someone has been in before, with various connectors disconnected and one point damaged from psu board. Anyways on first inspection i found the main AC input fuse on the psu blown. I removed the psu and replaced the fused. I visually checked all components and can't see any obvious burn marks or blackening. I powered the psu on its own with nothing else connected and the fuse blows. I have removed the bridge rectifier and bench tested. All internal diodes check ok with no short across the ac pins. I wanted to use a diagram to check my work.

    On an automotive circuit finding a short would be easy for me. I would connect a bulb in series on the shorting circuit and track down the short on the brightness of the bulb. Being new to PCB work i'm not sure on the method of doing this on the board.

    Here are some pics

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    The coil i removed on one side reads 5kohms and other side reads 0.6ohms. The coils look identical both side with same number of turns.??
     
    Last edited: Feb 19, 2012
  4. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    [​IMG]

    [​IMG]

    Coil number is LCL-1608WH
     
  5. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    I just soldered in another coil from another psu i have and the fuse now doesn't blow. Tv now turns on fine!!! Result!!
     
    Last edited: Feb 19, 2012
  6. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    I have been looking around for a replacement coil and its not looking good. Can i rewind this coil i have?
     
  7. davenn

    davenn Moderator

    13,865
    1,956
    Sep 5, 2009
    Excellent result Mark !! :)

    That coil isnt essential to the operation of the PSU. But it is part of a filter network circuitry in the input AC supply. It, along with those two yellow capacitors and the other similar looking coil, stop all the hi freq switching crud from the PSU from going back out the onto the mains line and being radiated far and wide.

    The windings each should be pretty low resistance 0.6 - 1 Ohm sounds about right

    That higher resistance is indicating a fault with that coil. I would also look for a resistance measurement between the 2 windings. there shouldnt be anything but in your case there may be and thats what is blowing the fuse. There looks to be a black burn/smoke mark on the pcb between the windings --- near the edge of the board

    below is a quick drawing I did to show you the basics of input Mains filtering
    in your unit. I havent bothered with those other coils you see to the left of the yellow cap, the resistors, the PTC etc

    [​IMG]

    the caps from each input leg to ground are the 2 blue caps in your pic, the bridge rectifier is the rectangular black item above the yellow cap with the red dot, and the DC filter/smoothing cap is that large black electrolytic cap above the bridge rectif.

    cheers
    Dave
     

    Attached Files:

    Last edited: Feb 19, 2012
  8. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    Thanks again for the reply Dave. I checked that the coils may of shorted together but they haven't. The only fault i can tell is the high resistance over one of the windings. Also i was unsure about that black spot because its not like soot. Its more like the paint you see outlining the board plus there is nothing on the coils. I still need to fix this coil or find a suitable replacement because the one i salvaged from a donar psu has pins closer together so i soldered wires from each pin to the pads on the board. I've had the TV on for test for about 2 hours and everything seems fine.

    Thanks for taking the time to draw a diagram. It helps me understand things better.
     
  9. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    Holy crap, I am so gonna start repairing broken electronics with fuses that blow. One thing I have learned, it doesn't seem to be all that difficult.

    Nice work on that fix! dare I ask what you paid for the tv?
     
    Last edited: Feb 19, 2012
  10. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    You can rewind the filter choke coil. Desolder copper wire of defective ( high resistance ) and unwind the winding from plastic bobbin. . Remember or record winding direction ( clockwise or counter-clockwise ) and count number of turns. Buy same awg no. size of enameled copper wire. That's it. I'm sure you can rewind it back.
     
  11. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    Only £15
     
  12. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    I have unwound the coil and found it broken in various places which was shorting to adjacent coils. I measure it as 0.45mm with 77 turns so i think i'm just going to rewind it with some new enamelled copper wire. :D Might aswell do other side too for good measure.
     
  13. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    I'm just puzzled, how come its the filter choke was only damaged and not other psu parts ., such as pwm ic, diodes and power transistor. Your really a lucky man.
     
  14. davenn

    davenn Moderator

    13,865
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    Sep 5, 2009
    who knows ....
    could have been some moisture ingress that caused shorted turns after a bit of long term corrosion of the enamel insulation ???

    removing the coil winding really carefully could quite possibly reveal the cause.

    other thought ... it may have been a manufacturing fault, maybe the enamel insulation was a bit thin in places and resulted in an arc-over between turns due to the lack of insulation resistance.
    This may have failed under long term use or possibly a voltage surge that took out the inductor and fuse before anything else

    cheers
    Dave
     
  15. davenn

    davenn Moderator

    13,865
    1,956
    Sep 5, 2009
    Mark,
    You will find those inductors is virtually EVERY SMPS find other old TV's etc to scrounge parts from :)

    I wouldnt leave it out permanently

    Dave
     
  16. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    Cheers guys. I am also puzzled how only one side of the coil being shorted to itself and not the other side caused the fuse to blow. If someone could explain it to me that would be good.

    I haven't left the filter choke out. I have fitted a temporary one from another psu. The pins didn't line up with holes so I used jumper wires and secured the filter to the tv case. This is only for testing anyway, I have ordered some enamelled copper wire and I will rewind original choke.
     
  17. davenn

    davenn Moderator

    13,865
    1,956
    Sep 5, 2009
    its possible that the blowing of the filter winding didnt blow the fuse
    They may have both blown at the same time as a result of a voltage surge

    weird and wonderful failures happen under adverse conditions

    As I stated above unwinding the faulty section of the coil carefully may reveal more info on what happened

    Dave
     
    Last edited: Feb 20, 2012
  18. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    I've also noticed that there's no MOV in the psu pcb to protect it from over voltage or transients.
     
  19. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    There is a thermistor in circuit.
     
  20. davenn

    davenn Moderator

    13,865
    1,956
    Sep 5, 2009
    That green coloured disc device just below the word "caution" will be a MOV or thermistor
    some times they dont always operate fast enough or depending on the fault cause, wont operate at all.

    Dave
     
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