I need a little help here in understanding the primary discrimination knob. Here is my observation. 8X9 coil, Using a lead pellet as a test target, buried 3", when placed in All Metal the target sounds off loud and clear. When switched to the discriminate mode, knob far left (zero discrim,) I don't receive any signal. I would think that at the lowest discriminate setting I should be getting a signal from iron on up. Is it possible the internal discriminate circuit out of adjustment?