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How to Tune Rodgers 321?

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  • How to Tune Rodgers 321?

    Hello all,
    So, now that the humidity is back down, my Trio 321 is working rather well again. So, I'm thinking it would be good to finally tune it. I know how to tune my Allen Carousel, but this Rodgers is eluding me. On the Allen, there are little metal pots you turn for each note. I can't seem to find anything like that on the Rodgers.

    I see what looks like some magnets, but I can't figure out what to turn. I'm also a bit confused by the layout. Everything was so well labelled and accessible on the Allen. This Rodgers definitely has a very different layout.


  • #2
    Rodgers used cup-core inductors made of ferrite. It's like a small jar with a flat lid piece on tip. Here's a link to a picture of one oscillator with the inductor on the left end of the board: https://tinyurl.com/y2od8llf

    The flat top is about 1/8 inch thick, and it rotates against the cup underneath it which must remain stationary. You rotate the top with your finger tips to raise and lower the pitch. If it is stubborn and won't rotate, use a hair dryer to loosen it up--they used wax as a lubricant which can stiffen over time. If you do that, you need to rotate the cap piece to free it up, but come back later to get the tuning right, as the heat will change the tuning a little.

    Allen was one of the early users of pots for tuning, as it's a poor choice to control tuning of an oscillator. Resistors vary value with temperature, so stability isn't great. Allen used them originally because they used air core inductors which can't be adjusted.

    If you are tempted to unscrew the nut on top of the oscillator, be extremely careful when tightening; ferrite is very brittle and can easily be cracked (think of it as glass). Rodgers recommended never loosening these nuts. I'm sure the factory installed then with a torque wrench, but I don't know what the torque level was, and it would require a precision torque wrench, not the type you use on your car--more likely a special clutch version of an electric driver.


    • #3
      Thanks for the helpful advice. Hopefully sometime next week I'll be able to mess around with it. Got a few notes down low that are pretty far out.


      • #4
        I checked some old service information on tuning the ferrite core oscillators. The cautions note: "Do Not loosen the nut on the cap holding spring", and "The coil leads (very fine wires) are drawn out just under each core cap. Use care when turning the cap that your fingers do not break the wires". They are very fine gauge enamel coated.
        Hold the base of the core with one hand and turn the top with your other hand. To raise the pitch---turn the cap counter clockwise. It takes very little turning to change the note pitch.
        Also, check the tremulant for the out of tune notes. If there is no or very little tremulant, that can change the tuning. The tremulant problem would have to be repaired first.
        Happy tuning,