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repairing switch power supply in samsung plasma

Discussion in 'Repairing Electronics' started by Repair-me, Aug 3, 2009.

  1. Repair-me

    Repair-me New Member

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    hey all., how are you,

    my situation is this, i have a samsung plasma PS42s5h and the power supply has no vs voltage. it however has vd.

    once i taken the power supply out, i soon relised tracing the problem with the 206v dc vs output, as i traced the parts, both diode transistors where short. then from the diodes, the tracks lead to the switching transformer. prior to the transformer, the switching transistor was also short. and a 5amp fuse blew. now im not to worried about the diodes, but the switching transistor is what im worried about, the original one is a stw26nm50, but i could only get my hands on a stw26nm60, im not to sure if it will work ok. also on the other side of the board, it has the same transformer and a nm60 as well. i just wonder why one side of the board has a nm50 and the other is a nm60.

    anyway my quesition is can i put in a stw26nm60 instead of a stw26nm50?

    http://www.stmicroelectronics.cn/stonline/products/literature/ds/8292/stw26nm60.pdf

    http://www.stmicroelectronics.cn/stonline/products/literature/ds/8292/stw26nm50.pdf

    the powersupply is PSPF501B01A
     
  2. sheldonstv

    sheldonstv New Member

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    the problem you most probably have could be sustain board failure and/or driver failure which would destroy yr powersupply....best to check for faults with yr sustain boards first because due to the price of these boards depending which sus board has failed and if the driver has also failed it may scrap the set unless you have very deep pockets or you like throwing money at the set to repair it.........
     
    Last edited: Aug 3, 2009
  3. Repair-me

    Repair-me New Member

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    see i was thinking why the diodes have short, since from there it pretty much goes into another board. but there are fuses on the other board where the vs ends up and they didn't blow even though they run on the vs line, so im lost as to why the diodes died in the first place.
     
  4. dave

    Dave New Member

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  5. bammbammfran

    bammbammfran New Member

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    Hi Repair-me
    Did you ever get that board going ?
    I am playing with one of my own also from a 42 Samsung .
    As far as the diodes, the 60 can be used to replace the 50. The 60 just has a higher rating so it should be better.
    As far as the vs power, I believe the vs side transformer may not activate till another board allows it too after initial power up. I've worked on several monitor boards that worked that way (they only show dc voltages in certain spots then once the power is turned on via a switch then the other part of the power board will activate)
    Fran email bammbamm@ptd.net
     
  6. unclejed613

    unclejed613 Well-Known Member

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    the doides fail all on their own, especially in samsung power supplies. the -60 is a 600V transistor and the-50 is a 500V transistor, there shouldn't be any problem replacing it with the higher voltage device. the diodes should be replaced with a faster diode. if the originals were 200nS diodes, replace them with a faster diode. diodes in switching supplies usually fail from internal heat. the heat is caused by the diode having too long a reverse recovery time. the heating can be worsened by dried out electrolytic caps. i would recommend checking the ESR of the caps in the secondary side, or just replace them if you don't have any way of checking ESR. if you can find a source for them, there are Silicon Carbide diodes on the market now that don't have reverse recovery problems like standard and even high speed silicon diodes do. replacing all of the secondary diodes with these will male the whole power supply run cooler and more efficiently. less heat translates to a lower failure rate....
     
  7. Anneke

    Anneke New Member

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    Lightning vs samsung plasma now the hd sucks

    OK,

    This doesn't exactly relate to the topic but its the closest thing I have found that might help me.

    I recently had my tv repaired by a local guy- its a Samsung HP4264 and it was meant to be protected by a Monster Surge protector. While after a thunder storm and subsequent lightning the area or maybe even house was hit, surge protector failed and tv started to go weird. Turning itself off and on until pop pop pop and it doesn't turn on anymore.

    So we had it repaired at local place but since having it return to us, I almost feel that the HD on it is not so good. The images aren't that clear or atleast I don't thing they are.
    The guy just replaced the powerboard, so my question is could this have affected the my tv in anyway.

    Any help or information in this topic would be great
     
  8. unclejed613

    unclejed613 Well-Known Member

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    did they change the resolution settings? was your antenna damaged in the T-storm?

    other things related to the resolution settings that may have been changed:
    color depth
    interlaced or progressive scan
    sharpness settings

    these are all adjustable and can have an effect on picture quality
     
  9. Anneke

    Anneke New Member

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    Hmmm

    Antenna? I'm not too sure I even have an antenna. We have everything through Verizon and Fios, whether or not that means I have an antenna- I'm not sure.
    I will have a play around with the other settings too, but thanks so much for helping me out with this.

    I'll let you know if I see any changes.

    Thanks
     
  10. sheldonstv

    sheldonstv New Member

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    before you get too deeply involved-if you are able to access service mode all the settings can easily be checked but you need the sm for the set otherwise you can cause a lot of problems if you change settings that should be left alone if you dont know what you are doing........
     
  11. bountyhunter

    bountyhunter Well-Known Member

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    Amen. The elec caps in switchers are the prime failers. Switching current flows in and out of the cap on every switch cycle (like 50,000 per second) which creates heat as it passes through the cap's internal resistance. Heat + elec cap = short life. With switchers, always get switching grade caps which have very low ESR and make sure the ESR is specified at 20kHz or higher. A standard elec cap has the ESR specified at 120Hz, and they can not be used with a switcher.
     
  12. tunedwolf

    tunedwolf Well-Known Member

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    If the power supply has failed on the VS, for whatever reason, I would doubt that the epm module on the Ysus board survived, if it has, it's life will certainly be short(no pun intended). The Ydrive board may also be damaged by a flaky VS. I hear the rustle of money...:)

    rgds
     
  13. user_88

    user_88 Member

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    If your AC wall outlet was not properly grounded .... it does not have a specific wire going to an earth ground ... then any repair to the TV may be susceptible to a repeat performance of the surge protector failure.

    ... Suggest that you verify that your receptacle is in fact grounded ...and that all electrical wiring connections going to it are clean, and solidly fastened together.
     
  14. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I think you may be a little confused?, ESR is always specified at 100KHz, which is also the same frequency all ESR meters run at, for obvious reasons.

    But you're certainly correct that you need to fit capacitors designed for switch-mode use. The reason they fail is that too cheap capacitors were fitted originally, replacing them with high quality low-ESR switch-mode versions will provide a permanent cure, and a MUCH longer life for them.
     
  15. unclejed613

    unclejed613 Well-Known Member

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    well, the better ones run at 100khz. the cheap ones run at 50khz. the laboratory and industrial capacitance bridges run at 1Mhz. a lot of the newer switching supplies are switching at between 1 and 5Mhz, and capacitor ESR is a bigger obstacle to proper operation of these higher frequency supplies.
     
  16. bountyhunter

    bountyhunter Well-Known Member

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    Right, I'm confused.... I only designed power supplies for the last 25 years. What I said is 100% correct: capacitor manufacturers have specified ESR at various frequencies, depending on service type. generic grade aluminum electrolytics typically have NO high frequency spec for ESR, and it would be a big mistake to use them in switchers. For a long time, the "computer grade" caps only had a 120 Hz spec. You know how I know? I explained that to more customers than you could count who bought our switcher products and hung junk capacitors on the output and couldn't figure out why they didn't work.

    As for "all meters run at 100 kHz"? I don't know where that came from, but cap manufacturers do not always specify ESR at 100 kHz. Some do, some don't. Obviously, the higher the better. Bottom line, if a cap has an ESR spec at a higher frequency like 20kHz or above, it is probably intended for switching applications. If not, then stay away from it.

    BTW: many junk caps have no spec at all for ESR, it is specified as "tan theta" which can be related back to ESR but that generally means you don't want to use those.

    here is an example of a major manufacturer (Cornell Dublier) specifying their switcher caps at 20 kHz:

    http://www.electro-tech-online.com/custompdfs/2009/09/380-382.pdf
     

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    Last edited: Sep 7, 2009
  17. bountyhunter

    bountyhunter Well-Known Member

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    Actually, internal resonance frequency is the biggest problem which is indirectly related to ESR. At frequencies above 1 MHz, ESL (equivalent series inductance) is the real killer. The frequency where the impedance of the ESR and ESL are the same is resonant and it makes the "capacitor" look like a resistor. Above the resonant frequency, a capacitor is useless. Tantalums usually have a resonance frequency of a few hundred Kilo Hertz. We had people putting them on the output of a 1.6 MHz switcher and wondering why they didn't work. Above 1MHz, ceramic caps are your friend.
     
  18. Preher TV

    Preher TV Member

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    Impedence of a capacitor would be the square root of the ESR squared plus the total reactance (ESL reactance minus the capacitive reactance of the capacitor) squared.


    The resonant frequency would be where ESL reactance and the capacitive reactance of the capacitor are equal Where Z of the capacitor = ESR

    frequencies above resonant frequency cause ESL reactance to be high, increasing impedence of the capacitor leading to overheating and failure

    frequencies below resonant frequency cause the capacitive reactance of the capacitor to be high,increasing impedence, causing overheating and failure.

    And of course we all know increases in ESR increase impedence and cause the same failure as well.
     
    Last edited: Sep 7, 2009
  19. bountyhunter

    bountyhunter Well-Known Member

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    The problem is when the cap makers specify impedance and not ESR. It is ESR we cared more about for a couple of reasons:

    1) The switching ripple current passsing in and out through the ESR is what creates the heat. The cap makers do specify maximum ripple current which tells where the temp gets too high, but the actual ESR value is needed because:

    2) The ESR is a critical design spec in the loop compensation of the switcher (and some linears as well) so we need to know it, not just the impedance.
     
    Last edited: Sep 7, 2009
  20. Preher TV

    Preher TV Member

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    I would not think ESR actually changes at different frequencies but that reactance of the capacitor and ESL reactance change causing impedence to change.ESR appears to change when measuring it at different frequencies but not when you account for the reactance, then ESR is the same at all frequencies,caused by real resistances within the capacitor.

    I think it's just easier for most to put the ESL the ESL reactance and Capacitive reactance in with ESR and think of them as part of each other, especially since ESR meters for the most part don't really measure ESR alone by accounting for reactance, none of mine account for ESL reactance and the reactance of the capacitor, they just give you a single measurement which includes the ESR and the reactance together thats why most give errounous readings on small caps beacuse as the caps get smaller the reactance at the test frequency goes up and the meter does not account for it, It just shows a combination of the 2 making ESR appear high when it really is not, same for ESL.
     
  21. Preher TV

    Preher TV Member

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    I agree I think people interchange Z of a capacitor and ESR freely and or only think of the capacitave reactance of a capacitor with respect to the capacitors impedence and not it's ESR and ESL reactance.


    But yes ESR is the most important, and I find increase in ESR to be the most common failure of electrlytics.

    still good to understand how the reactance within a capacitor can effect ESR readings though, not just capacitive reactance but ESL reactance.
     
    Last edited: Sep 7, 2009

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