Did I Just Kill My Power Transformer?

Discussion in 'General Electronics Chat' started by vinylcafe, Dec 4, 2014.

  1. vinylcafe

    Thread Starter Member

    Dec 4, 2014
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    First of all, hello from Vancouver!

    This is my first post ... I just discovered this great site this morning.

    I am afraid of the answer to my own question here ...

    Working on rebuilding a vintage Sony power amp.

    During the process with the unit unplugged a SHIELDED lead from charged filter caps somehow touched a post on the power supply board.
    This post contained a lead from one of the transformer's secondary windings, supplying 85VAC.

    Huge spark and the accompanying noise. Also post melted a bit at point of contact, as did the metal sleave inside the insulated tip of the filter cap lead.

    These were 2 coupled 10000uf caps, so lots of juice.

    That winding now delivers 135VAC, rather than 85.

    The other two windings of 11 and 60VAC are fine.

    I would have thought that such a huge inrush of current would have just fried the windings, and they would be open - instead the output voltage seems to have changed.

    Does this make sense?

    Or does one particular windings in the circuit need to have a load to measure correctly, while others don't (which does not make sense either).

    This transformer is unobtanium.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Yes, normally a transformer would short (or open) from an overload and usually requires a longer overload period then the momentary spark you apparently generated. If it's outputting voltage, then I would think it's still okay. It may require a load to provide the stated voltage.
    Where does the value of 85Vac come from?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    First linear transformers are usually very rugged devices, so if the short was momentary I doubt anything happened to the TXFR itself.
    The winding would be either open (no voltage) or shorted, blow fuse and/or overheat.
    I would first suspect any rectifier or bridge, the AC voltage is normally going to be .707 x DC voltage, so 85vac seems OK, are you sure it delivered 135v before?
    Remove any load off of that particular winding and test with another bridge if possible and a known good electrolytic cap.
    Max.
     
  4. vinylcafe

    Thread Starter Member

    Dec 4, 2014
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    Thanks for the speedy reply.
    The 85 volts comes right off the power supply circuit board.
    I have attached a photo.
    All the other voltages printed on the board correspond with the output of the transformer's three secondary windings. Just not the one marked 85 VAC, it is being supplied with 135 VAC.
    Could there be some in-circuit influences here?
    Should I desolder the leads and test the voltages with the leads not connected to anything?
     
  5. vinylcafe

    Thread Starter Member

    Dec 4, 2014
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    Thanks for the speedy reply Max.

    I have attached a photo in a prior response, so you can see the configuration of the power supply board.
    Are you saying to completely remove the leads supposed to be supplying the 85VAC, and take another reading?
    PS: the reading should be 85 and I'm getting 135VAC.
    There is continuity on this winding, I am just wondering if an internal short could fuse some windings and somehow boost the voltage by 50 volts? My transformer theory is non existent (OK, maybe highschool).
     
    Last edited: Dec 4, 2014
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Are you sure the bottom four leads are not the AC into the board, they show ~85 (and ~60) which I would suspect indicates it is 85AC signal?
    Edit: I see you mean you are getting higher than normal?
    So remove them from the board and confirm the voltage, also check there is no continuity between secondary windings.
    Max.
     
  7. vinylcafe

    Thread Starter Member

    Dec 4, 2014
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    They are, yes.

    The lead marked 60 is getting 60Vac, the leads marked 85 Vac are getting 135 volts. That is what I cannot figure out.
    Wondering if I damaged the transformer to get this high by 50 volts reading?
    Also the leads at the top marked 11 Vac, are getting 11 volts.
    It is just the one winding which is out of spec.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Never ever seen that before? Especially if there is not other symptoms seen.
    I am guessing that the secondaries are all separate windings so as I mentioned, just make sure there is no continuity between windings, you may have to lift the 85v pair to confirm it. Measure voltage also when disconnected.
    If the transformer otherwise runs off load with not adverse affects, heating etc, then I would assume it is OK?
    Max.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    @vinylcafe

    Should you be allowed to use the screen name vinylcafe if you are not Stuart Mclean himself?
     
  10. vinylcafe

    Thread Starter Member

    Dec 4, 2014
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    Sounds like a good plan .. I will lift both 85volt leads and make sure there is no continuity between windings as a start. will report back.
     
  11. vinylcafe

    Thread Starter Member

    Dec 4, 2014
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    Don't know ... given it is the name of a show and not a person ... don't think it's an issue ....
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What are the blue wires marked ~, not C.T.'s by any chance?
    Max.
     
  13. vinylcafe

    Thread Starter Member

    Dec 4, 2014
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    Continuity between the 60 and 85 volt windings. NOT good.
    With leads lifted, still getting 135, where I should be getting 85.
    Looks like I need to find a new transformer, or get this one rewound (might be prohibitive re: cost).
     
  14. vinylcafe

    Thread Starter Member

    Dec 4, 2014
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    Those are a separate winding for the lamps in the amp. 11volts.

    They are fine and there is no continuity between that winding and the other two.
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What is not good? is there continuity, (read connection) between the 85v and others?
    Max.
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I would remove the insulation of the top layer and see if any thing can be seen if a winding blew across to another (85+60), it could be visible and possibly repaired?
    Max.
     
  17. vinylcafe

    Thread Starter Member

    Dec 4, 2014
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    Bad assumption on my part. Assumed that these windings should be discreet
    The 60 volt output with the leads lifted is reading 92 vac.

    Might be interesting to see if when reattached they come back down to 60, which is what I read off those posts before lifting the leads.
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Linear transformers rarely show the manuf voltage, especially off load, also depends on the primary rating, e.g. if 110v and you are running on 120v etc.
    Max.
     
  19. vinylcafe

    Thread Starter Member

    Dec 4, 2014
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    Will reattach leads and see if it brings the 92 back to 60.
    Are windings always discreet? I am presuming the continuity I am getting between the two is the issue here?
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Not always, depends on the application, but there could be some circuit board connection between them, either intentional, or in the case of a mishap, accidental!
    Max.
     
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