Help! Setting "tuner" on an old White TR.

[arkansasboy]

My detector is an older Whites TR Discriminator(until I can afford something better). I have used it in the past but I get aggravated at it and I put it back down, then pick it back up some years later and do it all over again. I am a little confused about the "tuner" setting. I know you want a little bit of "hum" but at times it seems almost impossible to get it set right where it will remain consistent. What drives me nuts is if you have it set with a little hum then sweep over lets say a small clump of grass, or anything that changes the distance of the coil to the ground, then the hum jumps way up. The sensitivity needle will peg out if it is raised up the slightest bit but if I try to set the tuner with the coil a little higher from the ground then I can completely lose the hum at times. It seems like I constantly have to tweak the tuner knob to get what I need. Any ideas??????