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Replace old transformer with rectifier bridge

Discussion in 'Troubleshooting and Repair' started by pharaon, Jun 16, 2018.

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  1. pharaon

    pharaon

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    Oct 28, 2014
    i have this pc speaker with failure Transformer that i want to fix
    so how about replacing the Transformer with rectifier bridge that give me 12V DC
    is it possible and if it's what piece should i use

    [​IMG]

    [​IMG]

    [​IMG]
     
  2. Alec_t

    Alec_t

    2,927
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    Jul 7, 2015
    That board apparently already has a bridge rectifier (4 diodes) on it. A transformer is a completely different animal. If the one you show has failed you will need to replace it with a similarly (or more generously) rated one, or a suitably rated wall-wart (mains to low DC voltage adapter). Either way requires the transformer output voltage to be known or deduced.
    What are the markings on the 8-pin IC and the big fat capacitor?
     
  3. pharaon

    pharaon

    373
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    Oct 28, 2014
    [​IMG]

    and the capacitor is 2200uf , 16V

    so if it's not possible to get another transformer what can i do to fix it?
     
  4. kellys_eye

    kellys_eye

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    Jun 25, 2010
    You might find that there is a thermal fuse fitted against the transformer windings. Look for a 'bulge' under the tape and, working carefully so you don't cut any of the winding wires, you might be able to get the fuse out and replace it.
     
  5. Minder

    Minder

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    Apr 24, 2015
    If the transformer fix fails the 12v Wall-Wart method just needs the two 12v leads in and soldered at the bridge output.
    Also cheap.
    M.
     
  6. dave9

    dave9

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    Mar 5, 2017
    It might be time for a PC speaker upgrade with a new amp in them. I could see that burning up the transformer just trying to get more sound out of it since it is only capable of about 1W with a tail-wind. They say it's the first watt that counts but that one, THD high to get that far.
     
  7. pharaon

    pharaon

    373
    6
    Oct 28, 2014
    can you explain
     
  8. pharaon

    pharaon

    373
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    Oct 28, 2014
    the winding wires already cut
     
  9. Minder

    Minder

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    Apr 24, 2015
    The Wall Wart usually comes with a barrel connector, snip it off and solder the two ends, 12vdc, to the plus and minus output of the 4 diodes used as a bridge on the previous transformer output.
    M.
     
  10. pharaon

    pharaon

    373
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    Oct 28, 2014
    so there's no electronic piece can replace the transformer?
    what about the circuit that doesn't have any transformers and convert 220 VAC to 12 VDC???
     
  11. dave9

    dave9

    999
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    Mar 5, 2017
    ^ Finding exactly the right size replacement transformer that will solder onto the space on the PCB could take a very long time and you may never find it, and it may be very expensive including shipping, compared to the low value of the speaker amp.

    12VDC output AC/DC wall adapters on the other hand, are very common and inexpensive, and typically it is easy to find some rated for 1A which is probably higher current rating than that tiny original transformer was capable of, which is why I suspect it overheated and failed in the first place.

    There is no reasonable/safe way to convert 220VAC to 12VDC without a transformer. This is a pretty simple repair, why are you wasting time questioning people who realize how to fix this with minimal time and expense, since it is about the cheapest, crudest low end speaker amp possible? It really is not worth great time or expense to fix this.

    As Minder mentioned, you should solder the wires from an AC-DC 12V adapter to the PCB. The positive lead wire from the adapter would be soldered to the hole labeled "P9" on the PCB and the negative lead to the hole labeled "P10". If you prefer you could keep the barrel plug on the AC/DC adapter and buy a barrel socket to mount on the speaker cabinet, then solder wiring from P9 and P10 to the respective terminals on the barrel socket, but I question whether this amp is worth buying any more parts for than absolutely necessary.

    Note that you don't necessarily even need to use an AC/DC adapter. If it is a desktop computer so it has a source of 12VDC, you could just power this amp board from that, just get a $1 connector for the PSU 4 pin molex plug and solder that to the amp P9 and P10, maybe needing to add a piece of wire to extend the length.
     
    davenn likes this.
  12. davenn

    davenn Moderator

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    Sep 5, 2009

    pharaon, I'm going to repeat what Dave9 said so that hopefully it sinks in :) :)


    ""Finding exactly the right size replacement transformer that will solder onto the space on the PCB could take a very long time and you may never find it, and it may be very expensive including shipping, compared to the low value of the speaker amp.

    12VDC output AC/DC wall adapters on the other hand, are very common and inexpensive, and typically it is easy to find some rated for 1A which is probably higher current rating than that tiny original transformer was capable of, which is why I suspect it overheated and failed in the first place.

    There is no reasonable/safe way to convert 220VAC to 12VDC without a transformer. This is a pretty simple repair, why are you wasting time questioning people who realize how to fix this with minimal time and expense, since it is about the cheapest, crudest low end speaker amp possible? It really is not worth great time or expense to fix this.""

    Dave
     
  13. pharaon

    pharaon

    373
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    Oct 28, 2014
    i like to learn that's why i ask about other ways

    i think i can use
    X-Rated Capacitor to get 12 dc V
     
    Last edited: Jun 17, 2018
  14. dave9

    dave9

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    Mar 5, 2017
    ^ I recommend pursing your mad scientist experiments using a much lower voltage than 220VAC mains, but first fixing the amp the way we suggested so you have music like Franz Reichelt's video does:

     
  15. davenn

    davenn Moderator

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    PLEASE, DONT EVEN THINK ABOUT IT
     
  16. pharaon

    pharaon

    373
    6
    Oct 28, 2014
    why not
    what about this 230 v ac to 12vdc without transformer
     
  17. Alec_t

    Alec_t

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    Jul 7, 2015
    If you value your life, or the lives of others, DON'T USE IT!! It has no mains isolation, so anything powered by it could float at mains potential and be LETHAL.
     
  18. Minder

    Minder

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    Apr 24, 2015
    Technically speaking, equipment that does not use a transformer or safety earth ground conductor is constructed by being 'Double Insulated' for safety reasons.
    The reason is the circuit is at the same potential as the incoming mains service in spite of dropping the circuit voltage to 12v..
    Pretty much what others have said.
    M.
     
  19. davenn

    davenn Moderator

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    Sep 5, 2009
    on the wise advice from 3 of us

    the thread is closed
     
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