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LED TV - power circuit damaged after lightning strike

Discussion in 'Troubleshooting and Repair' started by michalw, Sep 6, 2014.

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  1. michalw

    michalw

    17
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    Sep 6, 2014
    Hi all,

    I'm repairing the LED TV damaged after the lightening strike.
    I've started with creating simple scheme (see below) and testing the components.

    [​IMG]

    Only the fuse was blown (NSFT 2.5A 250V).
    I replaced the fuse with the new one (<PS> E JET bel F2.5A 250V) and plugged the power, but the fuse immediately blew.

    I suspect the problem may be with the type of the fuse: F2.5A probably means fast-blow, but I'm not sure whether NSFT is the slow-blow.
    I've read that fuses used on the input are mostly the slow-blow, so I probably used the incorrect one, but I would like to be sure to use the slow-blow one.

    I've tried to google the phrase "NFST 2.5A 250V", but found nothing helpful.

    Does somebody know is "NSFT 2.5A" is rather slow-blow or fast-blow?

    Thanks in advance,
    Michał
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Assuming it's a switching power supply, which it almost certainly is, the usual problem is destruction of the switching device (usually a MOSFET) that drives the primary of the transformer, due to overvoltage from the surge. Other semiconductors on the primary side, and possibly also the secondary side, may have been destroyed as well. The driver IC usually survives.
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    A fast blowing mains fuse is usually a result of a failed bridge rectifier
    I have seen that so many times

    after checking the bridge rectifier ( may be 4 separate diodes or a single package)
    then move on to Kris's suggestions which would then be the next most likely choices :)

    Dave
     
    ADRT and KrisBlueNZ like this.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Ah yes, thanks Dave. The bridge rectifier is the best place to start.
     
  5. michalw

    michalw

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    0
    Sep 6, 2014
    I've checked the bridge rectifier and the diodes are working properly.

    There was also failed resistor (R2 on the scheme above, 1.2 Ohm ceramic) - I've replaced it with 1.5 Ohm (haven't had 1.2 Ohm, but I guess it shouldn't be a problem).

    As I understand you said that the problem was not related with the type of the fuse, am I right?
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Probably not. I guess the 1.2 ohm resistor was between the negative side of the bridge rectifier and the source of the MOSFET, right? In that case the MOSFET is almost certainly toast. What's the part number?
     
  7. michalw

    michalw

    17
    0
    Sep 6, 2014
    The 1.2 Ohm resistor is between the positive side of the bridge rectifier and the transformer.

    I found on the PCB that the fuse should be "T2.5AL" so it's definitely the slow blow fuse and I need to buy one.
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK, that still points to the MOSFET.

    When you're getting a new slow-blow fuse, get a pack of them :)
     
  9. michalw

    michalw

    17
    0
    Sep 6, 2014
    Hi,

    I've found the MOSFET near the transformer's primary, it's TK5A65D. The measured D-S resistance (in both directions) is ~0.6 Ohms without the gate voltage, according the datasheet it should be 1.2 Ohms with the gate voltage, so it seems to be dead.
    I've found also the short-circuit on the diode connected with the source of that transistor.

    I've examined (with the 'diode test' function of the multimeter) the bridge rectifier diodes and it shows the volatege ~0.550 V in both directions on every diode, so I guess the diodes should be also replaced.
    But as far as I remember they were not damaged when I was testing them on Saturday - maybe soldering slow-blow fuse and plugging the power cable killed them...
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yes, it's possible that replacing the fuse and powering it up with the MOSFET still short circuit could have forced enough current through the bridge rectifier diodes to take them out as well.

    Have you identified a suitable replacement for the MOSFET? If you want suggestions, add your location to your profile so we know what suppliers to look at.
     
  11. michalw

    michalw

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    Sep 6, 2014
    I haven't found the replacement yet, but as far as I know the most important parameters are:

    Drain-source voltage >= 650 V
    Gate-source voltage ~ ±30 V
    DC Current >= 5A
    Pulse Current >= 20A
    Drain power dissipation >= 40W
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Right. And a matching package (TO-220) and polarity (N-channel). Gate-source voltage is unlikely to be important. Aim for one that's not too over-rated for current - high-current MOSFETs switch more slowly.
     
  13. michalw

    michalw

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    Sep 6, 2014
    I've examined all diodes and found one more (probably) dead - it's RS1GJTB (fast recovery rectifier).
    The multimeter diode test shows the voltage ~0.300 V in both directions.

    I found a scheme for the TV set - link.
    On the page 10-3 there's detailed scheme of the power supply unit - it's D610 probably dead.
     
  14. Angelo

    Angelo

    30
    2
    Sep 10, 2014
    Seems the link is broken, almost for me.
    Yes, the diode seems damaged, but de-solder it to be sure and measure it off board, since there can be something in parallel.
    Also, for lightning issues, do a very well eye-check for broken tracks. As krisbluenz said, the mosfet driving the transformer on the primary side is one of the parts that often shorts.
    But, since you had a lightning, i would test quite all the semiconductors of the primary (quiet fast, tester and some experience, leaving them on board), and measure all the resistors also, since some of them can have an altered higher value, or can be open.
    Measuring on-board, keep always in mind that resistance that you measure can be lower of the expected value (something in parallel) but never greater.
    Then a brief check on secondary side rectifiers diodes.
    Then a prayer, and try to power on. Let us know.
     
  15. davenn

    davenn Moderator

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    Sep 5, 2009
    yes true ... its trying to go to a ftp site but doesn't load

    Dave
     
  16. michalw

    michalw

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    0
    Sep 6, 2014
    Last edited by a moderator: Sep 11, 2014
  17. michalw

    michalw

    17
    0
    Sep 6, 2014
    Little update:

    I've de-soldered the RS1GJTB diode and it turned out the diode is working.
    I've found few resisotrs dead, I'm going to replace them and hope that will be enough...

    Let you know.
     
  18. michalw

    michalw

    17
    0
    Sep 6, 2014
    I've replaced all damaged elements and still not working. I suspect the MOSFET is damaged again short circuit between Drain and Source.
    Could be the MOSFET destroyed during soldering / using multimeter ?
     
  19. HellasTechn

    HellasTechn

    1,495
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    Apr 14, 2013
    This problem almost sounds same as my psu problem.

    just for the information. Have you checked the varistors ? usually when they blow you can tell but some times you never know. Or look for inflated capacitors.
     
  20. michalw

    michalw

    17
    0
    Sep 6, 2014
    The capacitors are good.
    Regarding the varistor: how to test it properly? De-solder and measure the resistance?
     
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