Hallicrafters SX-117 audio output transformer

Discussion in '"Boat Anchor" & Classic Equipment' started by N6UH, Sep 8, 2019.

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  1. N6UH

    N6UH Ham Member QRZ Page

    My Favourite Rx at the moment (SX-117) went quiet last week. The audio output transformer primary went OC for no apparent reason.

    Here's what I found when I removed the crimped on clamp and sidecovers:


    All the 'I' lamination parts together on one end, not alternate as you would normally see. A quick squeeze with the grips and it looked like this:


    The only things holding the transformer together were the clamp and the varnish.As far as I can tell it was original as it was still riveted to the chassis. Is this normal for transformers from that era?

    The bobbin is now sitting on my coilwinder unwound and awaiting some 37 awg wire. Ugh!

    In the meantime, I have removed the output transformer from my long term project SR-150 and the construction is the same apart from the orientation of the mounting clamp. There's a bit more iron in it but the 'I' laminations are stacked on one end.

    When I come to rebuild the original is there any reason why I cannot put the laminations back in the conventional way and not totally rely on the varnish and the crimps on the clamp for a second time?

    Ian N6UH aka G4LPL
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Transformers can be fabricated in a number of different way. The engineers who designed the radio did not randomly throw parts into the box. :)

    Rebuild it as original. Audio transformers do not carry great current. As long as there is no significant humming or vibration, there is no reason to have high clamping forces. Good on ya' for attempting a rewind!!!! b.
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Last edited: Sep 8, 2019
  4. N6UH

    N6UH Ham Member QRZ Page

    Thanks for the response.
    I emailed Manfred this very morning, he's helped me a lot in the past.
    I wasn't questioning the original design after all it has lasted almost 60 years. I'm not sure the crimping tangs on the bottom of the mounting bracket will stand a second use. Building it conventionally will allow me to through bolt the laminations to the end covers.
    KA0HCP likes this.
  5. W1BR

    W1BR Premium Subscriber QRZ Page

    That is very common. The small gap between the E and I cores helps to prevent saturation due to the DC current in the primary winding
    WQ4G, N2EY and N6UH like this.
  6. KP4SX

    KP4SX Premium Subscriber QRZ Page

    I gather Radiodaze, etc don't have a suitable replacement or do you just want to rebuild it for fun?
  7. W1BR

    W1BR Premium Subscriber QRZ Page

    Even a six volt filament transformer wired backwards would do in a pinch. Single ended audio transformers at that power level are not rare.

  8. W1BR

    W1BR Premium Subscriber QRZ Page


    Being Hammond, besides radiodaze they are probably carried by Mouser, DigiKey, etc. Or, steal one from an AA5 junker kitchen radio.
  9. N6UH

    N6UH Ham Member QRZ Page

    Thanks for that, Manfred XQ6FOD also responded by email with the same information. He also spotted in the picture a piece of what was a thin paper insulator keeping the core sections apart

    Not sure now! :eek: Winding it shouldn't be an issue but clamping it back together might be as it seems the laminations are held against each other and the 'E' and 'I' sections are squashed together by the mounting bracket before varnishing, Re-using the original bracket would mean opening it out a bit on 3 sides to fit the laminations and end covers in and then pressing the 3 sides flat again.

    Radiodaze and others have generic replacements that will do but if I want to keep it as original as possible rewinding it is most likely the only way. The original also has a 500 ohm winding underneath the primary for some kind of headphone output.

    It's currently working ok with the transformer " borrowed" from my SR-150 project so I have time to decide

    Ian N6UH
  10. W1BR

    W1BR Premium Subscriber QRZ Page

    Go over to the IO Hallicrafters group and ask if anyone has a junker SX-117. Hallicrafters may have used the same transformer in other models as well.

    BTW--the SX-117 was a real sleeper. It has a very good reputation performance wise but was largely ignored by the collectors.
    KD2ACO likes this.

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