Power trans.

Discussion in 'General Electronics Chat' started by Bichrome, May 2, 2014.

  1. Bichrome

    Thread Starter New Member

    May 2, 2014
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    Hello!

    I recently got Denon PMA520 vintige stereo amp. for free, but it has a dead power transformer. I want to fix it, but i don't know what kind of tranformer i need. All i know that originaly it has torodial transformer with 37,2 V in second winding.
    Can I use laminated steel core transformer?
    Can I use transformer with 40V in second winding?

    I attaced schematics and some photos of the amp.

    I have only basic knowledge about eletronics and realy need your help to sort this out.

    Thanks!
    View attachment 67977

    View attachment 67978

    View attachment 67979
     
  2. #12

    Expert

    Nov 30, 2010
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    I told the moderators you were having difficulty getting your attachments to show up. Somebody will probably be along to help you with that.
     
  3. Georacer

    Moderator

    Nov 25, 2009
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    It seems that you have not uploaded the attachments successfully in the server, which is weird, since you have received attachment ID codes. Can you repeat the process one more time?

    Edit the message, use the "Go Advanced" button and re-upload your pictures.
     
  4. Bichrome

    Thread Starter New Member

    May 2, 2014
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    Realy could not find "Edit post" button, so I just added attachmens once more:

    Rest of the pbc and components looks undamaged (no burnt components).

    View attachment denon_pma520.pdf

    IMG_20140429_220111.jpg

    IMG_20140429_220016.jpg
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you are in N.A. there is Antek that makes Toroidal transformers.
    He also lists on ebay as Jonango.
    The reason for toroidal in audio equipment is the magnetic field is contained within the core, so hum problems are kept to a minimum.
    The diameter and height will indicate the Va required.
    Max.
     
  6. Georacer

    Moderator

    Nov 25, 2009
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    You probably couldn't edit your post because your post count is under 10 and you are thus classified as a New Member. I always forget class permissions.

    Sorry for the confusion.
     
  7. #12

    Expert

    Nov 30, 2010
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    I can tell you this: You're looking for a transformer that is probably rated as 75 volts, center tapped, or 37.5-0-37.5 give or take a volt or two. The power rating will be less than 460 watts which I derive from 230 VAC input and a 2 amp fuse. The power rating might be quite a lot less, like 200 watts.

    The manufacturer went to considerable expense to make this a toroid AND shield it with a metal can. Trying to use an open frame transformer is begging for trouble.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    How do you know the transformer is dead?
     
  9. Bichrome

    Thread Starter New Member

    May 2, 2014
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    Ok, it's the help what I was looking for!
    So it's ok to use 40,0-0-40,0 V torodial transformer?


    1) Transformer did not show any voltage in secondary winding when I connected transformer to 220V.
    2) Transformer's primary winding didn't show any resistence reading when i connected it to digital multimeter.
    3)Transformers primary winding does not conduct eletricity.

    Is there any other way to check transformer?
     
  10. Bichrome

    Thread Starter New Member

    May 2, 2014
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    I'm from Latvia so shipping would be expensive...

    And I realy can't get toroid out of it's metal case because it is filled with polyurethane putty, to get out toroid I have to destroy either toroid or metal case.
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Obviously you've checked for a blown fuse in series with the primary winding?
     
  12. studiot

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  13. Bichrome

    Thread Starter New Member

    May 2, 2014
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    Fuse was the first thing that I checked, and the fuse is ok. But how it comes, that there is dead toroid with intact fuse?


    Yup I have removed it.

    I have found toroid in my local electronics shop. 40-0-40 V 5 A in total for 220V. Those excess ~2,5 V should not do any harm....am I right?
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That depends how close other components are to their voltage/power ratings.
     
  15. Bichrome

    Thread Starter New Member

    May 2, 2014
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    Is there any way to tell it by looking in schematics? Rectifier and caps will hold 40V ....I think.
     
  16. studiot

    AAC Fanatic!

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    The simple way to deal with a slightly higher output from the transformer is to add one or more power rectifier diodes in series with the feed from the main rectifier bridge to each rail.

    This will lower the rail voltages to normal levels without huge power disipation or complicated circuitry.

    Is this enough information or do you need more detail?
     
  17. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Sometimes there is a thermal fuse inside potted transformers, which might have gone open.
     
  18. Bichrome

    Thread Starter New Member

    May 2, 2014
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    So all I must do is to put one or more power rectifier diodes after the original power rectifier diode bridge?

    I have attached png. file to show what i mean and photo with
    s4vb bridge rectifier.
    Rec_bridge.png

    IMG_20140504_171729.jpg
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you are absolutely sure it is the transformer, I would tackle it as you have nothing to lose.
    If it is hard epoxy it usually chips away.
    Extremely rare for a transformer to go open.
    Max.
     
  20. studiot

    AAC Fanatic!

    Nov 9, 2007
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