It's not the coil because the small Fisher coil I got reacts the same way as the stock. My tones are yippy, crushed and not repeating on known good targets in clean ground. My Sun Ray pro Gold headphones wouldn't work on my F75, worked fine on my SE. However my Rat phones and Koss work on the F75. I called Ralph at Sun Ray a while back and he couldn't understand why the Sun Ray's didn't work on my f75 because apparently they work on other's F75s. Well, I just bought another pair of Sun Ray Gold headphones for my wife's Se and they don't work in the f75 either. Now I know it isn't my Sun Ray headphones, there is nothing wrong with them. An interesting thing and the reason I hadn't sent my f75 in for repair was because it bench tests fine with longer, normal response tones, But I never had headphones plugged into it when I bench tested it, so I think it's some kind of short in the headphone jack, has anyone else had something similar happen on theirs???? Also, my female adapter where the 5 pin plugs into the control box is too tight and it's a five minute wiggling chore to pull out the five pin to change coils, to the point that the black plastic on the five pin insert on the stock coil is starting to crack. Who's the @#@@$%% schmuck who engineered this stupid battery compartment???? Must have designed the coil shape and cover, good luck separating those for cleaning 100 times this year, my money's on it cracking by the third attempt. Nothing for nothing guys but if I ever get this thing to work right, I still will be disappointed in the lack of quality of this unit. I DON'T SEE THIS MACHINE BEING WORTH 950 BUCKS. Cause even after they fix it or replace it, as cheaply made as it is, it's going to break again and again. F75 plus 300 bucks an Explorer it does not make. I know allot of you guys love your machines, and that's great, I've had mine since May or June and I use it to attract dust in my garage. I'm sending it in and hopefully I won't have to go through two and three like some of you have posted.