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How to check old transformers?

Discussion in '"Boat Anchor" & Classic Equipment' started by N8FF, Jun 2, 2021.

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  1. N8FF

    N8FF Ham Member QRZ Page

    I have a National SW-3 that I'm getting ready to start diving into, so first thing-of course I'm going to replace the PS capacitors, but there's still that worry about the old irons. How does one check them out short of plugging them in and hoping for the best?
    Those contraptions are 70-80 years old by now and made with 1930/1940's technology and materials.
    A friend and I once tested the PS in a Swan markII amp to failure and the amount of smoke was frightening.
  2. N2EY

    N2EY XML Subscriber QRZ Page

    Do you mean power transformers or audio transformers?

    Here's how I'd check a power transformer in a vintage piece of gear:

    1) In the case of a built-in power supply, remove all the tubes from the set. In the case of a separate PS. remove the rectifier and disconnect the set the PS powers.

    2) Using an ohmmeter set on the highest scale, check that the various windings are not grounded unless they are supposed to be grounded. Also check that they are insulated from each other.

    3A) Apply power to the primary of the power transformer gradually, using a Variac.


    3B) Apply power to the primary of the transformer through a "dim bulb" tester. Such a tester is nothing more than an ordinary incandescent lightbulb of appropriate wattage connected in series with the 120 volt line, so that if the power transformer being tested draws a lot of current, the bulb will limit the current.

    If all of the above works, apply full line voltage.
    WA1GXC, N8FF, KX4OM and 2 others like this.
  3. KP4SX

    KP4SX Premium Subscriber QRZ Page

    What transformer and PS caps in a SW-3?
  4. N8FF

    N8FF Ham Member QRZ Page

    Power supply transformer, and thanks for the tips. I don't have a variac but I do have an incandescent bulb or two.
    N2EY likes this.
  5. W9WQA

    W9WQA Ham Member QRZ Page

    been messing with old/new junk for 65 yrs. never had a bad trans. changed 4 caps.
    you guys are making the cap makers rich,,,!!!
    i just plug them in. had a 75 yr old wire recorder. worked fine.

    waaayyy tooo much woorrring goin on heah...
    K4KYV likes this.
  6. WA1GXC

    WA1GXC Ham Member QRZ Page

    Your luck's about to run out.

    Halon is best fire-fighting agent for Class-C conflagration; no residue.
    Now illegal to manufacture . Find older extinguisher at a marine consignment store.

    N2EY likes this.
  7. KI4ZNV

    KI4ZNV Ham Member QRZ Page

    1. Replace capacitors.

    2. Plug the thing in with a 100W lamp in series. The lamp will illuminate bright as the capacitors charge, and then dim down when the radio stops pulling so much current. If it doesn't dim down... then you have a transformer issue or a rectifier issue or a capacitor issue. If it doesn't light up... then you have a transformer issue. Run it for a while like that, then apply line power.
  8. K5MIL

    K5MIL Ham Member QRZ Page

    Pull the rectifier tube. Bring it up on a Variac while monitoring the current draw. A shorted power transformer will draw excessive current as soon as you start to bring up the voltage.

    Bill - K5MIL
  9. W9GB

    W9GB Ham Member QRZ Page

    An Old Radio/TV bench Technician Trick.
    Disconnect Transformer being tested (Primary and Secondary Leads)

    Use a Filament Transformer with a 12 VAC Secondary (3 to 5 amps usually sufficient).
    Use that 12 VAC output to drive Unknown transformer Primary.
    using your DVM or VOM, then read Secondary Voltages (should be expected divided by 10).

    N3RYB, N5XH, N8FF and 1 other person like this.
  10. N8FF

    N8FF Ham Member QRZ Page

    That's a great idea there.

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