On the same general topic, I once had a problem with my XLT and after trying everything I knew to correct the problem (I don't even remember now what the problem was) I boxed it up and sent it in to Whites for repair. They charged me a minimal repair fee and replaced something on the circuit board, and said my coil was defective, and replaced it - that was the most expensive item on the repair bill.
Interestingly, the coil I sent in with the detector had NEVER BEEN USED. It was the original 9 1/2 inch coil that came with the detector. I used the 5.3 bullseye coil that also came with the detector until I purchased a bigfoot coil which I used exclusively thereafter. So was there anything wrong with the 9.5 inch coil? I guess if White's said so, there must have been, but if so, it must have came from the factory that way or "deteriorated" while sitting, unused, in the box. Now, I don't know diddly-squat about what's on the circuit board, but I have a hunch there is a trimmer capacitor or coil (or both) used to null the coil when it is manufactured, and I suspect that deterioration in one of these items (especially loss of capacitance by the capacitor) is much more likely to cause the "coil to go out of calibration" than a coil sitting, unused, in a temperature and humidity controlled room. But then, what do I know....??
