Help with Plasma TV Power Supply

Discussion in 'General Electronics Chat' started by Coefficient, May 25, 2015.

  1. Coefficient

    Thread Starter Member

    Sep 5, 2012
    37
    1
    Hello everyone,
    i wish to thank everyone here for help you've been given me over the years,i am grateful.
    i have been working on Plasma TV power board to get it fixed for over 48hrs but to no avail.i decided to bring the problem here to seek advice and/help from experts,
    i keep getting fuse blown up when i thought i have sorted out the problem...i look forward to getting any help soon.below are the pictures of the power board of plasma tv.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    I see no images at all.
    You can upload images to the forum using the "upload a file" button.

    Bertus
     
  3. Coefficient

    Thread Starter Member

    Sep 5, 2012
    37
    1
    20150525_135904.jpg 20150525_135958.jpg
     
    Last edited by a moderator: May 25, 2015
  4. Coefficient

    Thread Starter Member

    Sep 5, 2012
    37
    1
    TV MODEL:AKAI
    MODEL NUMBER:LK4400057A
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Make and model, but no manufacturer.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Take care when you are posting pictures.
    You can adapt pictures with the gimp.
    I have taken the essential parts of both pictures and tried to enhance the contrast and brightness of the second picture:

    coefficient1.jpg

    coefficient2.jpg

    Bertus
     
  7. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
    156
    Boot up the power supply without anything hooked up and does it still blow the fuse ??
    Connect one thing up at a time to help pin point the problem..
    I need the model number of the TV cause that number is wrong one or missing something cause I am not coming up with any info..
     
  8. Coefficient

    Thread Starter Member

    Sep 5, 2012
    37
    1
    TV MODEL:AKAI
    MODEL NUMBER:LJ4400057A
    Sorry for the wrong information.it blow fuse only when connected to power,tried tracing the fault one after the other but no avail......
    @bertus......I AM GRATEFUL
     
  9. DickCappels

    Moderator

    Aug 21, 2008
    2,651
    632
    AKAI is the name of a television manufacturer.
     
  10. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
    156
    Try starting the power supply up by it self and see it still blows..
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    783
    It could well blow up if started with no load, an old trick of the trade back in the CRT days was to use a 60W lightbulb to load the main supply rail, but last flat screen I looked inside had loads of rails.

    The first thing to do is check whether the PSU has a PFC input stage, This is pretty much just a flyback boost converter, you can short the MOSFET source to gate to stop it turning on, but its far better to remove it and test it. With the PFC disabled, the rectified mains passes through the DC resistance of the PFC flyback inductor - this will feed the main PSU switcher with a lower voltage than it was expecting, but still capable of operating.

    If it still blows the fuse; suspect the main SMPSU MOSFETs (there's usually 2) If they're not shorted its most likely a regulation fault causing core saturation (which, incidentally can also blow the MOSFETS). If it turns out to be a regulation fault, you can put a 60W lightbulb in series with the pre-regulator DC, hopefully this will limit anything expensive happening and glow to indicate the current draw.
    Hopefully this will get the regulation control sub-panel operating so you can take measurements.
     
  12. Coefficient

    Thread Starter Member

    Sep 5, 2012
    37
    1
    y
    Yesterday,i did a little experiment,i was able to get an exact power board of the TV from a friend who has the same type with the one i am working on,happily,the TV powered up...My headache now is how to get this faulty to work now
     
  13. Coefficient

    Thread Starter Member

    Sep 5, 2012
    37
    1
    i will check the two MOSFETs for shorts.
    in the second picture,you can see those heatsinks,four MOSFETs were connected just before the two power capacitors,and two MOSFETs were connected to the heatsink just after the two power capacitors.......i don't mind changing them if it is where the problem arises......
    i appreciate everyone
     
  14. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    I'd look for a replacement board, now that you know that this will fix the TV. Fixing the bad board could be nearly impossible for even a skilled technician, unless a schematic can be found. But these power boards typically come out of China and schematics are not available.

    About the the only thing I'd try without a schematic is to replace all the capacitors, or test them for ESR if you can, and replace the bad ones. Bad capacitors are a very common reason for power supply failure.
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    783
    Almost any fault can make a switcher go a very expensive bang, so its as good a plan as any to remove and test any and all power semiconductors.

    When testing MOSFETs, don't forget that a floating gate can give a false leakage reading drain to source. On thing you can do is solder a wire from gate to source, with the gate definitely at 0V - any drain leakage whatsoever means a faulty MOSFET.

    The tester I use is basically an A23 12V battery, a pair of indicator LEDs and a thin-film PTC thermistor to limit the current to a few mA, I don't think you can get those anymore, but you can do close enough for MOSFET testing with the battery, a 10k resistor and a LED - preferably high efficiency type that only needs a few mA.

    For quick MOSFET testing I use the voltage from the continuity tester to charge the MOSFET gate capacitance, making the gate positive WRT source should switch the MOSFET on, making the gate negative WRT source should switch it off - completely off, any leakage means a dud. Obviously the polarities stated are for N-channel MOSFETs, reverse the polarities for P-channel.

    The gate capacitance should also have extremely low leakage, that is to say if the gate charge leaks away in the time it takes you to move the prods from the G/S pins to the S/D pins, I'd regard the part as suspect - put it back in at your own risk!
     
  16. bertus

    Administrator

    Apr 5, 2008
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