Home Forums Electronics Restoration Audio Transformer Rewinding.

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    Ed Kraushar
    CVRS Member

    In an earlier "Show and Tell" I commented on rewinding an open audio interstage
    transformer. There was a request for more information that I did not reply to as I did not
    have photos to demonstrate my procedure. I am currently restoring a Stromberg Carlson
    601A radio of mid 1920’s vintage and this gave me the opportunity to photograph the

    This is my "homebrew" method of rewinding 1920’s audio transformers and those with
    sophisticated coil winding equipment or more experience may have other

    My standard rewind is a primary of 4,000 turns and a secondary of 12,000 turns, both
    with number 40 wire. I find this an easy size wire to work with. Most factory
    transformers were originally wound with slightly lighter wire. The SC used wire
    approximately 0.0025 inch diameter while my number 40 is about 0.003 inch. In the
    future I may try lighter wire.

    For winding I use a mini lathe as it is available and very convenient. Almost any other
    method of turning the coil form could be used.

    The Stromberg Carlson (SC) radio has three audio transformers and two have both the
    primary and secondary coils open.

    My first step is to make a wood plug that fits tightly inside the old coil. This will hold
    the coil form. The hole in the center of the plug is to hold the plug on the winding jig.

    The original transformer was wound with a layer of paper between each coil layer, the
    winding was wax impregnated and did not go to the edge of the paper layer. For the
    rewind I make a paper coil form from cardboard with cheeks to contain the wire as I wind
    out to the edge of the form. The cheeks are much larger than the desired coil to facilitate
    winding at high speeds. They also protect the wire from the transformer steel.

    The wood plug is fit into the coil form.

    I use a winding jig made from 1/4 inch thick plastic with a 5/16 inch threaded rod and
    nuts. The plastic was cut with a fly cutter in a drill press. The bevel edge from the fly
    cutter is placed inwards to help guide the wire when winding close to the coil edge.

    The coil form and plug are mounted in the winding jig.

    The jig is mounted in the lathe for winding. A power drill would work also. Here is a
    partly wound coil. The white is masking tape that I use to tape down the coil form
    cheeks to the winding jig so that they do not flap around. The tape and the bevel help me
    keep the wire inside the coil form. I wind the coil at 700 rpm so that precautions must be
    taken to prevent the wire snagging or being broken by the coil form. It helps to have
    small pieces of precut tape ready to hold the wire if you need to pause the winding.

    I place the wire spool on its side down near the floor and let the wire come off over its
    edge. It is higher here to show the spool.

    I use a layer of hockey tape over the bottom of the coil form and in between the primary
    and secondary layers. This is a strong fabric tape similar to the old friction tape but with
    a good adhesive. I like to periodically paint a bit of coil dope or shellac on the windings
    as they are being wound. This helps to stick the coil together. I find that shellac will
    give odd resistance readings on the coil until it totally dries out. The beginning and end
    of each winding is soldered to a heavier flexible wire that is brought out along the edge of
    the coil. The solder joint is covered with tape and doped.

    When finished I add another layer of tape to the coil outside and add another piece of
    cardboard to the outside of the coil cheek. The oversize coil cheeks are trimmed down
    and the coil is ready for transformer assembly.

    Finished Coil.

    Finished transformer.

    It takes me less than an hour to rewind a transformer after the coil form has been made
    and the equipment set up.

    Now a word about counting turns when winding. I had a Radio Shack counter that I
    added a Radio Shack window security alarm switch and a reset switch to. The alarm
    switch contains a reed switch that is activated by a rare earth magnet stuck on the lathe
    chuck. I clamp this unit on when needed to keep it out of the way of other operations on
    the lathe.

    This is another homebrew transformer. I used a telephone ringer coil mated to
    transformer steel that was cut to fit. I die cast the frame from scrap zinc die cast items
    using the castings from an Acme transformer as a pattern to make the moulds. I need to
    break out the Brother P-Touch to make a proper label. The next step is a totally
    homebrew transformer.

    Sorry to be so long winded.


    Rogers flipdial
    Forum Participant

    That is a great presentation Ed – you couldn’t have described the process better !!

    Forum Participant

    Thank you for posting this. Great detail. I always thought that it would get messy without a mechanism to feed the wire back and forth to distribute it evenly. After reading your post, well, you proved it is possible without a fancy coil winding setup. Nice work !


    Ed Kraushar
    CVRS Member

    phil wrote:

    I always thought that it would get messy without a mechanism to feed the wire back and forth to distribute it evenly.

    I had originally thought of using the gear driven lathe carriage to level wind evenly but then realized that although it would work, stopping and reversing would take a lot of time when you are dealing with 16,000 turns. I would estimate that this transformer if mechanically level wound would only get 330 turns per layer and have to be done at a much lower rpm. It would be no problem with a commercial coil winding machine but beyond my budget.

    I guide the wire with my fingers and try to get it as even as possible. I just try to do the best I can and live with the results. This may be critical on Hi Fi audio transformers but these old 1920’s sets seem to be forgiving as noticed by the many different transformers that have been used sucessfully as replacements.


    Gerry O’Hara

    Excellent work and article Ed.

    How do you find that Mini Lathe for actual metalworking? – I was thinking of buying one to make small parts/turning knobs etc (obviously comes in handy for winding transormers also!).


    Ed Kraushar
    CVRS Member

    Gerry O’Hara wrote:

    Excellent work and article Ed.

    How do you find that Mini Lathe for actual metalworking? – I was thinking of buying one to make small parts/turning knobs etc (obviously comes in handy for winding transormers also!).


    I find them ok if you respect their size and take easier cuts. I do not do a lot of metal working though, but have turned new pulleys for AK’s that had pot metal disease. Also some other radio parts. I do some plastic parts on it. Last week did some parts to replace the pot metal gear on a Stromberg Carlson 601A with a plastic pulley and heavy belt. It works very well, brass replacement pulleys alone cost more than the radio is worth.

    Mine is an honest 7 X 14 from Micro Mark. Many of these small lathes are made by Seig in China but have some differences. Some will not handle parts as long as they claim. I turned down Busy Bee and Princess Auto as I found them deficient compared to this one. Some are calibrated in metric only complicating threading.

    https://www.micromark.com/MICROLUX-7X14- … ,8176.html

    They also have a related milling machine. Their service is excellent.

    Soon I would like to try the carriage feed to space and wind a replacement toroidal coil for an early Magnavox radio.


    Forum Participant

    I have seen printing press rollers that were designed to oscillate on their own while turning. the mechanism inside is quite simple and you might be able to make something like it if you do a lot of them.

    If you picture a really coarse thread that is cut from left to right and back to left again. there is a dog that follows this track. the dog has a spring that holds just a bit of tension on it . The dog then goes from left to right, turns slighty, following the "thread" where it then goes back to the left and so on.

    I am interested in lathes too. we have 4 of them between my two brothers and my dad. I don’t actually own one or have one at home but the odds are good that I will one day 🙂 one is a south bend , one a Hurcus , and two are boxford. These are all south bend clones so most of the parts are interchangeable.

    If you look for one I would advise looking for one of these old machines used. They tend to come from high school shops and places like that and if you watch you will likely find one for around 1000- 1500 which is comparable to one of these newer ones, but much more sturdy. watch the auctions and such , the do come up.
    If you buy one used, try to get the tools, cutters, knurling tools, three and four jaw chucks, dividing head, toolpost grinder, boring bar, large drill bits with morse tapered shanks, etc.

    All the accessories are expensive so if you can get them with the original purchase you are ahead of the game.
    This machine Ed has is not a toy it is still very useful and as he says if you don’t put too much of a load on it, it will do nice work. where you will wish it is larger is when you want to remove a lot of material. you will sometimes want o make a part that is such a shape that more material removal is needed. sometimes the stock you find around your shop is quite a bit larger than your intended project. Whatever you end up with will be very handy, even just to make the odd little bushing or washer a certain size. you’ll turn to your lathe rather than driving all over trying to buy one. I don’t mean to knock ed’s machie because I know that someone who is precise and patient can create beutiful work, you just need to compensate. likewise if you find on e that is big and heavy but quite worn, there are workarounds and you can still usually do some fairly precision work if you get to know how to work around any shortcomings.

    The great big huge ( but old) monsters can sometimes be even cheaper as no one has room for them ( like ones with a 12 foot bed) if you happen to have that kind of space..

    CNC is the way of industry so these old machines are sometimes getting pushed aside, but you don’t need CNC for home projects, repairs etc.

    I think a milling machine would be just as useful. of course one of each would be nice.. and there are some lathes with milling attachments. My brother set his up by making a milling attachment which is basicly a third axis to move the workpiece vertically ( the workpiece being mounted on the carriage , and the cutter in the chuck. he makes all kinds of parts with that setup.


    Ed Kraushar
    CVRS Member

    Hi Phil,

    I agree with your comments on the larger lathes. In my case I don’t intend to get into larger projects so the price and size of the mini lathe appealed to me. The plus it has for coil winding is a DC drive that has continously variable speed control that allows it to speed up or slow down very rapidly, necessary if you need to stop winding in a hurry.

    I did look at larger lathes in local auctions etc but price and unkown condition worried me. Some looked pretty tired and may be very inaccurate or require extensive repair.

    Some good information on mini lathes is available at–



    Gerry O’Hara

    Been checking the MicroMark site out as well as info from local suppliers (Busy Bee and Princess Auto) and I would agree that the MicroMark lathe looks a better spec. The price in US$ is not bad – did you have them ship it to a Canadian address or bring in over the border yourself. If they shipped, any significant brokerage/customs costs on top of their quoted shipping rate?

    Ed Kraushar
    CVRS Member

    Hi Gerry,

    I had it shipped up by UPS so there was the UPS brokerage charge and the Canadian taxes. I believe the UPS site lists brokerage fees. Shipping and delivery was very fast. Micro Mark has a customer service agent that takes care of the Canadian orders by phone.

    One advantage of Micro Mark is the lathe is already cleaned and ready for minor assembly. Some of the others sell it packed in grease.


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