The tuning knob has no function other than setting the level of the threshold audio. When the knob is turned counter clockwise into the "Metal" zone it increases the volume of the threshold hum, and the further you turn it in a counter clockwise direction the louder it gets. The "Null" zone is just that, turning the knob clockwise nulls the threshold, makes it go silent, and also reduces depth. The optimum setting is to set the threshold level so you barely hear it. With it set that way targets that are discriminated out will make the threshold go silent and lets you know there's something under the coil that's below your discrimination setting.
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