Just set up my banjo with a pair of Schaller D-Tuners. Fairly straight forward operation but I'm finding that in turning the knobs through regular use, the centre screw can goes loose, causing the string to drop out of tune. The fix, and Schaller's own instructions, seem to be to just keep the centre screw tightened. However, it's irritating to have it loosen unexpectedly. I'm reluctant to go beyond hand tightening it, such as with a screwdriver. Have you found a way to tighten enough to keep it secure as long as required? I'm assuming the centre screen is for taking it apart, although I can't fathom that would be required.
I snug mine up with a fingerpick as needed.
I use a dime to tighten. If you put a drop of blue Lok-Tite on the threads of that screw, it will stop loosening though.
A drop of fingernail polish on the threads will work if you don't want loctite.
I had the same problem on one set.
In my experience this is caused by sloppiness in the knob fit (it rocks back and forth no matter how finger tight you adjust it and the screw loosens after using it a few times. If you tighten it too much it will be too tight to turn). The above advice for "blue" loctite on the threads only worked great for me. If you get it on the inside of the knob you may have trouble getting it off to position it after changing strings.
My auto parts store has small amounts of blue loctite at the checkout counter. You will probably have to use a coin or screwdriver to adjust after it sets up when changing strings.
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