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TV PSU Repair - Diode replacement

Discussion in 'Troubleshooting and Repair' started by Griffinballs, Jul 25, 2015.

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  1. Griffinballs

    Griffinballs

    24
    5
    Jul 23, 2015
    Hi all
    Got on great with my other TV repair that you guys helped with with the blown caps, many thanks for that.

    So I then remembered, we had a TV that just stopped working one day and I thought, what the hell, its probably something easy again.
    Whipped the back cover off was expecting the caps to be gone again, not this time. But I do know what the problem is.

    The TV is a Digihome 23" LCD and inside it the PSU is a Vestel 17PW80 v2. I've included a picture with a circle around the bad diodes which are causing the dead TV.
    How I came to my diagnosis was I looked on ebay for the PSU and found loads of ads offering a repair kit for this model, all picturing these diodes. I then looked up how to test diodes with my multimeter, and tested all diodes on the board starting with those three and the three circled all appear to be bad to me.

    My diode test on my meter reads 0v in both directions, this I'm lead to believe is open circuit and so...bad. I have read that the test can be different when not in circuit so I should remove them all and test individually off the board?

    Also, going by their markings, they look like they say sb5100 2099. I've also read that Vestel designed these bad and the 5V rating on them is un-necessary so I can get away with 3A? Reason being, Maplin don't stock these diodes but 3A ones seem to be possible, so can I use an alternative that Maplin sell?

    Any help muchly appreciated.
     
  2. Griffinballs

    Griffinballs

    24
    5
    Jul 23, 2015
    A pic of the board
     

    Attached Files:

  3. Martaine2005

    Martaine2005

    2,741
    739
    May 12, 2015
    Hi Griffinballs,
    Welcome back.
    First, you don't check diodes with V. You use the continuity or Diode function on the meter.
    The SB5100 diode is a shottky diode. 5A 100v or similar.

    Martin
     
  4. Griffinballs

    Griffinballs

    24
    5
    Jul 23, 2015
    Hi Martin

    Thanks, yeah sorry, I knew how to test the diode with the right function, just didn't know how to write my reading lol so chucked v after it, lol.

    Quick update, googled about a bit and found that the favourite is usually the bottom diode. So I desoldered one leg, retested the diodes, sure enough. Thats my only bad one too.
    Put the TV back together (only to test) and sure enough, power led is now back on. Didn't start it up, just plugged in to check it worked.

    So now I need to replace that bottom diode.
    As you say, its a shottky 5A 100V. Worst case I'll order it off ebay but Maplin are open tomorrow, if I can find a suitable replacement on their site, then I can get it straight away.
    Is this one any good? http://www.maplin.co.uk/p/1n5401-3a-silicon-rectifier-ql82d
    Its not a shottky, but since I know sweet FA really about electronics, I don't know if thats any good or not? Its a 3A, not 5 but I read on another thread about the PSU that that part doesn't really matter in this application. The thread in question is here http://www.badcaps.net/forum/showthread.php?t=28778

    Thanks again for your help Martin, I'm really enjoying this new hobby. Gonna start having a look for some half decent dead TV's on gumtree, see if I can get a nice cheap upgrade :D
     
  5. Martaine2005

    Martaine2005

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    May 12, 2015
    To be honest, if a shottky diode was in the original design, there could be other reasons.
    Faster switching is one reason.
    The other is lower Vf, forward voltage.
    It might work perfectly well, but I would personally put a direct replacement.

    Martin
     
  6. Griffinballs

    Griffinballs

    24
    5
    Jul 23, 2015
    Thanks dude. I'll order one off eBay now
     
    Martaine2005 likes this.
  7. Martaine2005

    Martaine2005

    2,741
    739
    May 12, 2015
    I really like people like you!!!
    You get stuck in and ask the right questions.
    Cudos to you. And happy electronic tinkering.
    Just be careful playing with TV's..High Voltage, Don't become complacent.

    Martin
     
  8. Griffinballs

    Griffinballs

    24
    5
    Jul 23, 2015
    Same goes to you. No superiority complex like a lot of people on many forums.
    Straight up answers, can't thank you enough mate.
     
    Martaine2005 likes this.
  9. Martaine2005

    Martaine2005

    2,741
    739
    May 12, 2015
    Ha' have you met the others?
    Just joking. Best site on the net this.

    All the best
    Martin
     
    chopnhack likes this.
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