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toroidal transformer problem identification

Discussion in 'General Electronics Chat' started by johnintv, Feb 13, 2018 at 8:10 AM.

  1. johnintv

    johnintv New Member

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    Hi,
    Coming from a mackie powered subfoofer I suspect the transformer (part no. c1t51) to have a problem as it blows the fuse when I connect it.
    It has 2 primary lead pairs (115/230v) as specified on the label : black/red and blue/white
    When used for 230v, black and white are connected (under the circuit board plug) so mains is on blue/red.

    My resistance readings are as follows (black/white not connected to plug so just the wires coming out of the transformer)

    red-black = 2.4 ohm
    blue-white = 2.4 ohm
    They are equal which seems good

    but strangley enough as the following pairs don't form a winding according to the label
    black-blue = 2.8 ohm
    black-white = 1.6 ohm
    red-white=1.7 ohm
    red-blue = 2.9 ohm (red-blue measurement while connecting black/white remains 2.9 ohm ...????)

    Does this mean that there is a short circuit in the two primary windings or is there realy just one primary winding with 4 leads connected to it at different locations?
    I know that understanding what the problem is won't fix it but it would be satisfying to understand.

    thanks,
    John
     
  2. JimB

    JimB Super Moderator Most Helpful Member

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    Yes it looks a though that is what is happening.
    On a normal transformer there should be no internal connection between two 110v primary windings.

    What on earth does "mackie powered" mean?

    JimB
     
  3. johnintv

    johnintv New Member

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    mackie is the brand. it's a 500watt subwoofer of a PA system.
    thanks for your reply.
     
  4. dave

    Dave New Member

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  5. fourtytwo

    fourtytwo Member

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    I am sorry if I missed something but first of all it is not clear to me if you are doing these primary measurements with it completely disconnected (all 4 leads) from everything else especially some voltage selector plug/link. Frankly unless the transformer shows SEVERE signs of burnt insulation the chances of the two primaries being shorted is almost nill unless someone has again severely mechanically damaged it.
    The most likely reason this unit is blowing input fuses is a dead short in the secondary circuits, possibly the rectifier but more likely the power amplifier output stage.
    Whatever you do before further testing disconnect the speaker leads to protect the speaker from damage (if it is not already blown).
    I dont know what your experience of toroid size vs VA ratings are but the chances of this being a 500Wrms ampifier are slim indeed :)
     
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Why would you imagine that?, he's not mentioned the size of the toroid, and 500W PA subwoofers are quite normal, and also accurately rated.

    I agree the transformer isn't the most likely fault, although I would suspect rectifiers before the output transistors - as output transistors would usually blow the DC fuses.
     
  7. johnintv

    johnintv New Member

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    Thanks all for your time.
    This transformer comes out of a another powered (hi/mid) speaker (450 watt). I swapped it with the transformer of the 500 watt sub woofer and the hi/mid speaker worked on that just fine. The transformer in both the hi/mid cabinet as the sub woofer are identical with the same leads and connectors. So the rectifiers etc in this (sub woofer) unit were not the reason for the problem because this transformer comes from a different unit that works fine again now with the transformer from this unit. This unit worked fine before it was canabalised for it's transformer.
    In my original posting I had a link to the transformer. It is exactly that model.
    I did my measurements with all wires disconnected. I tested the unit with a fuse while the secondary cables were all disconnected and it blows the fuse instantly when I switch it on.

    So I am not saying that the seconday side is not the problem but I measure no short either there.
    for the record
    secondary side (18v-0-18v yellow/black/yellow) measures
    yellow - yellow 0.9 ohm
    yellow-black 0.75 ohm both
    red-red 0-100v 1.3 ohm
     
  8. fourtytwo

    fourtytwo Member

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    Sounds like you got yourself a transformer that was poorly made and the insulation has broken down in service, very unusual but possible if the lead-in insulation (sleeving) is not carried far enough into the winding. As you have done all the good tests there is no alternative than to replace it unless you feel skilled enough to repair it, alternatevly if you have a motor rewind shop around your way that could be a cheaper alternative than a new one. As for the power rateing it's immaterial to the problem except I have seen and heard many of these and similar units and I and many others don't beleive there figures :) One unit I forget from who'm claimed to be 500W with a class B output stage and a 60VA toroid lols
     
  9. fourtytwo

    fourtytwo Member

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    Whilst I did not need to say it I have seen many that are not and actually there is a photo of the transformer, I cannot honestly see the point of your counterpost!
     
  10. johnintv

    johnintv New Member

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    it has a epoxy or something core that seems hard as steel with the hole in it for the bolt that keeps it in place. So unwinding the thing is not an option.
    I was hoping that somebody could use the primary measurements to reverse engineer the failure.
     
  11. Beau Schwabe

    Beau Schwabe Member

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    Just an FYI .... transformer companies do get it wrong occasionally and there QC department doesn't catch it. I am working directly with a company right now because the Phase of the output windings were 180 Deg out of Phase on two transformers we found. Our company uses 100's of their torrid transformers so to find any problems at all are very odd, you would think they would have it down where these kind of errors just don't happen. They are being very cooperative with helping us out, so I won't mention any names, but because of our findings and the number of transformers we use, they are changing their QC policy to test 100% of their inventory before it goes out the door.
     
  12. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I see that the primary is not connected.
    BUT
    Is the secondary also not connected?
     
  13. Diver300

    Diver300 Well-Known Member

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    It's dead.

    Those transformers will have two isolated primary windings of 115 V each. They are connected in series for 230 V and in parallel for 115 V. That is a very common arrangement for toroidal transformers. Here is another example http://www.farnell.com/datasheets/2...663.217967748.1518680583-715415576.1512147136

    That arrangement relies on isolation between the two primaries, and it seems clear that your transformer has a short somewhere. It might be possible to use it at a reduced rating with a 125 V input with only one winding connected, but not on 230 V.

    It might be cheaper to replace the transformer with two separate ones. If you can work out how much power is needed on the 18-0-18 and the 0-100 V windings, you could buy an 18-0-18 transformer and a 50-0-50 transformer, both of which are easily available in a variety of power ratings.

    You could ask a company like this:- http://www.transformers.uk.com/about-us/ as they may be able to make one for you. I think it was them I used for a custom specified 3-phase transformer.
     
  14. unclejed613

    unclejed613 Well-Known Member

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    mackie toroids... i have one here i'm in the process of repairing, just like yours, a short in the primary. the two primary windings should be isolated from each other, but your measurement says they are shorted together. normally, with one of the primaries disconnected, it might not be an issue, but there is likely a shorted turn. the primaries are on the outside layer. if you think you can rebuild the primaries, it's not difficult (unless they filled the inside of the toroid with epoxy). the best bet would be to get new magnet wire, and those primaries are 18 or 16awg. you can also get mylar or polyamide tape to wrap the transformer back up when you are done. all you really have to do is count turns as you remove the primaries. then with new enameled wire, put the same number of turns back on. make it a point to put a couple of layers of mylar tape between the two primary windings. then attach the colored pvc insulated wires, duplicating the technique they used at the factory. then finish covering the transformer with mylar tape a few layers deep. it should work. on the one i'm rebuilding, i got lucky, the shorted turn was between the 20th and 21st turns as i was removing the wire. rather than continue taking it all apart, i covered the exposed copper with heat shrink tubing, and began winding it back up. all i need now is the mylar tape to finish the job...
     

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