I purchased a used explorer II and received it this last weekend and it seems to work great. I put it together and went to a park, where there is not anything old, but I knew there would be some clad for testing and learning. I used the pre-set settings and found 2 dimes 2 zinc pennies and a copper penny. My question is about the threshold hum. It seemed when I got real close to power lines I would loose the background hum. Is this like nulling? Do I need to change some settings or is this normal? I am going to love this machine. Thanks Randy