I don't own a Spectrum, but I do own a White's of a different variety, so I'm speaking only for my White's, and making an assumption that the Spectrum is similar.
When I talk about "null", I'm talking about tuning. I'm still assuming here, but my Whites has several different setting, some are "basic" settings that you start out with and then you experiment with different discriminations (I find, with my White's, the lesser the discrimination, the better).
The rest is experimental, for your surroundings, amount of trash, etc., where you reach a point where it can let go of most of the ground matter and still pick up good signals. That's MY null position.
Now, I don't know specifically about yours, but my Whites is not a "turn on and go" machine. I have to re-do it every single time I turn it on.
(also if I hit a rock or something - it needs to be reset). Also, I have a little collection box, it has a penny, nickle, dime and quarter in it, along with a couple of different types of ring tabs - with my White's, it scans different kinds of tabs differently - and a couple of small pieces of different metals - I use that when I start out for the day, and if I knock it out of null, to reset it. If I can detect these (and I don't insist on the nickles), then it will pick up just about anything, in most types of ground.
B
PS - One thing I forgot - you really need to listen to the sounds your detector makes . My White's has a gauge to tell you what is in the ground, but it is not always right - the noises it makes, after you use it for a good period of time - I can usually tell if it is something good or not. (depending on what you call good, of course - I like digging up horseshoes.)
