plasma tv power board repair

Discussion in 'Technical Repair' started by me217, Apr 26, 2016.

  1. me217

    Thread Starter New Member

    Apr 23, 2016
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    hey
    i was gifted a 50" plasma tv today, the catch is it doesn't turn on. when the power button is pushed, the light flashes green then back to red again.
    after a bit of research, i'm pretty certain it is the power board. a common issue is the caps fail. i looked at these and none of them have leaked or bulged. i was wondering if there is a way to test caps without removing them all from the board. also if there is any other thing i should be checking.

    any help would be appreciated.
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The two times I have run into this it turned out to be a component failure somewhere else in the set. The power supply started up, detected an over-current condition on an output, and shut down.

    Without a schematic it is impossible to say how the capacitors might behave when connected to a tester while in circuit.

    ak
     
  3. SLK001

    Well-Known Member

    Nov 29, 2011
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    Where are the photos?
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Caps can be tested on board with an ESR tester. As caps age, the ESR is the first thing to go. The cap can look completely normal and even test for rated capacity (out of circuit), but has enough internal resistance to cause big problems in some applications, particularly switch-mode power supplies.

    The ESR tester applies a high frequency (~100kHz) signal at ~0.6V. This voltage is not enough to turn on nearby transistors or diodes, so testing on board can be meaningful. Any cap that tests bad can be removed for more scrutiny. I built my own tester and can share that if you're interested. But you can just buy one. They're not terrible expensive.

    Another approach is to simply replace them all. They're all aging. If one has failed, others may be getting closer to old age. It's not terribly hard to remove and replace them.

    Heed AK's warning though. I wouldn't assume there's a power supply problem without a genuine diagnosis.
     
  5. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Telling us the brand and model would be a big help towards getting usable advice too. :rolleyes:

    The first test I would do is to disconnect all of the outgoing higher DC power leads and see if it will power up and supply the correct output voltages. If it's built like most plasma TV's there will be two higher voltage outputs in the 50 - 100 volt range and the rest should be under 12 - 15 volts.
     
  6. dannyf

    Well-Known Member

    Sep 13, 2015
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    Good looking caps can go bad in terms of ESR. Get yourself an ESR meter.

    Otherwise, try to parallel a good cap to a suspect cap and see if it works
     
  7. bwilliams60

    Active Member

    Nov 18, 2012
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    Agree with tcmtech but I would check all your power supply voltages before you do anything. Depending on what set you have, there may be two or three connectors to outlying boards, all labelled with voltage values. Check these and see if all your voltage values are good first. Post the results as well as the required values labelled beside it.
    As mentioned, it would be a good idea to give us the brand and model number, located usually on a tag on the back. Take really high quality pictures and post them here. Good chance I either have or can get a schematic for your set.
     
  8. SLK001

    Well-Known Member

    Nov 29, 2011
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    Do you have special powers to get schematics (not a joke - I'm serious)? I've used quite a bit of my internet "mojo" and there are some sets that I literally can't find squat in technical information. I would even be happy to pay for schematics, if I only had a source.
     
  9. bwilliams60

    Active Member

    Nov 18, 2012
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    I have downloaded quite a few from different sources. No special powers other than lots of time searching. What are you looking for, anything in particular?
     
  10. SLK001

    Well-Known Member

    Nov 29, 2011
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    At the present moment, I am looking for a Westinghouse EW50T5KW 50" LED TV. I haven't been able to find any Westinghouse schematics. Also, Seiki and RCA documents are as scarce as hen's teeth.
     
  11. me217

    Thread Starter New Member

    Apr 23, 2016
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    its a LG 50PG20D

    i'll have another look tonight when i get a chance.
     
  12. me217

    Thread Starter New Member

    Apr 23, 2016
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    got round to getting some pictures.
    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    i just checked the fuses on board they are still intact. one had 240 ish volts and the other had 270 volts, so there must be a step up transformer in there, not really important, i did notice that the coax plug on the main board is bent down slightly. i'm not sure if that could casue a short though. i'll check voltages now and get back to you guys
     
  13. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I'm going to wager a GA vote that the problem is a short in the screen or in the X axis sustain boards on the left-hand side of the unit as viewed in the pictures.

    I say disconnect that far left board assy with the four heatsinks from the bigger board with three heatsinks and see if it powers up and stays on.

    The screen will be blank of course but if the remaining boards stay powered up that's where your problem is.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    You can buy an in circuit ESR tester, but they're not cheap - the Peak Atlas instrument I have is somewhere in the general direction of GB£80.

    Some years back I bought the DSE instrument from Australia for around £50. The schematic is online - but it uses a pre-programmed Zilog MPU that comes from DSE.

    My own DIY first attempt was a simple measuring bridge running at 200kHz - its simply a matter of making Xc very small compared to ESR.

    Bulged isn't the only sign of failure; if they vent electrolyte through the bung, you get a vacuum inside when they cool - that makes the top appear concave.

    If you do remove any electrolytics - replace any that feel light as a feather.

    In a SMPSU; ESR is also known as loss factor - that dissipates heat. If the PSU gets going at all; you can find out which ones run hot - but avoid the primary side of the PSU where the aluminium cans are likely to be live!
     
  15. me217

    Thread Starter New Member

    Apr 23, 2016
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    0
    i did try that last night actually. the only thing i had plugged in was the main board to the power board. as the power switch runs through the power board. and it still just flashes green then goes back to red again
     
  16. me217

    Thread Starter New Member

    Apr 23, 2016
    13
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    i might see if i can borrow the laser thermometer from work tomorrow see if i can see any hot ones.
     
  17. bwilliams60

    Active Member

    Nov 18, 2012
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    What are the voltages on the power board connectors? This is your starting point. You can randomly guess at things or we can do this logically.
     
  18. wayneh

    Expert

    Sep 9, 2010
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    You need a service manual and schematic. I had a CRT TV with the identical symptoms. I had the schematic and service manual and still ended up giving up. No chance without those aids.
     
  19. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I have a 50" Hitachi plasma screen I have been working on with the same issue. With that unit all boards that have the LV DC power (5, 12, 15 ) have to be connected in order for the unit as a whole to power up otherwise it does just like yours does.

    It can power up and run with the two HV DC outputs disconnected from the main power supply but it can't run with any board unplugged from any other board except the X sustain boards like what your has on the left side.

    That's what I am getting at with my thoughts on disconnecting the HV DC output from the main power supply board first and doing a powerup test from there. If it still can't give you a stable HV DC output then yes the problem is in the power supply board.

    I got my TV from my brother who went through the exact same procedure as you thinking it was power supply or capacitor related problem and spent a fair amount of money replacing boards that were not the problem simply due to not doing a proper voltage output channel isolation test correctly.
     
  20. ian field

    Distinguished Member

    Oct 27, 2012
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    The latest incarnation of that is an IR camera in a similar style - but they make an ESR meter look like loose change.
     
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