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  1. #1
    Join Date
    Dec 2007
    Location
    Adelaide
    Posts
    620

    Default Transformer Rewind

    Not really metalwork, but transformers are an electrical item and this my interest some. Moderators feel free to chastise me if posts like this don't belong in this forum


    Got a job to rewind an output transformer for a 1960's vintage Goldentone all valve guitar amp, the amp's owner wanted it kept original so unless an original drop in replacement could be located the only option was a rewind.


    The transformer primary winding was split in two with a centre tap and one side was indicating an open circuit on the ohm meter, on removing the bell ends I could smell and see signs of an overload, possibly caused by running the amp without a speaker load which is strictly a no-no for a valve amp. Removing a few layers of insulation the extent of the flash-over could be seen, one side of the primary winding had been burnt right back and only a few mm protruded from the winding, while it would have been possible to solder a new length of wire onto this, to salvage the original windings I decided to continue with the rewind as the winding may have been stressed somewhere internally that couldn't be seen which could lead to failure down the track. As it turned out when that particular winding was unwound to count the turns the wire broke very easily in two places - maybe signs of stressed wire.


    I use my mill and machine up a mandrel from wood upon which to mount the windings on my homemade winder which will count the turns as I pull the wire off by hand, while it may seem like an overkill to use a mill to make the mandrel, as after all it is just a simple square section with a hole down the middle, a 5 min job with a handsaw and drill, it does need to be made with some care so the winding run true to the winding machine spindle so the wire is not skewed at an angle as it goes on.


    I photograph and document as I pull the wire off so there are no mistakes regarding turns, winding direction etc. the wire is measured with calipers, primary is 0.3mm and secondary is 0.64mm.
    The primary winding is split into 4 equal windings, each winding has 4 layers, the windings are connected in series and the secondary has 2 windings connected in parallel, older transformers like this weren't wound on a plastic bobbin but on a rectangular tube, the wire is not wound up to the edge of the tube but with a margin, to prevent the wire from falling off the edge of the tube and to provide a separation distance between each layer of the winding, each layer of the winding is separated from the previous one with thin paper insulation, this provides a nice even flat surface for the next layer, without this flat surface it would be hard to get a close wound layer of wire and instead of 106 turns I may only get 98 turns, do this a few times and the turns ratio between primary and secondary would be upset.


    Armed with all of the winding data, layout etc I write up a plan with a drawing of the winding tube showing where each winding will start and finish - I break the job into a series of steps which I document on the page - if I get called away during the job it's much easier to pick up where I left off. Each winding is finished off with 2 turns of kapton insulation, I cut these and the thin paper between each layer to width on an A3 paper guillotine, my home made winder has a "stop at the end of each layer switch" which does just that - at the end of each layer of wire the spindle stops, to allow me to insert a the thin paper that goes between each wire layer, at the end of each winding I label the wires which will help me make the final connections once all windings have been done. The process continues primary - secondary - primary - primary - secondary - primary, at which point I finish off with a few layers of kapton insulation paper.


    Next step is the make temporary connections between the windings, insert the transformer laminations and power it up with around 50V ac through the primary and measure the secondary voltage to check that I have the correct turns ratio, if this is all good I test the primary/secondary insulation at 1KV - the limit of my insulation tester. With all of these tests passed I can attach the leadout wires, immerse the whole winding in varnish, for an hour or so or until bubbles top rising and leave it to dry, once the varnish has hardened enough I use my workshop toaster oven to accelerate the varnish curing process, the varnish helps to stabilise the windings and leadout wires.

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  2. #2
    Join Date
    Sep 2012
    Location
    York, North Yorkshire UK
    Posts
    6,816

    Default

    Hi Ozzie,

    Very nicely documented Thanks.
    Best Regards:
    Baron J.

  3. #3
    Join Date
    Oct 2010
    Location
    NSW
    Posts
    216

    Default

    Clever guy familyguy!

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