minelab x-terra 505 questions

as I understand it your not dropping it down or raising it but just runs on a different channel where the least EMI is where your hunting ... but hopefully Longhair will chime in

That is my problem/question I don't seem to be having any EMI problems but rather better performance running it -1 with better tid

Actually you're not having any AUDIBLE problem with EMI. It can have effects that aren't apparent. That's part of the issue with EMI, and exactly why I tend to trust the auto NC on the 705 to pick the least affected channel to run on.

So chock it up to experience. You now know that at least at that site it runs better in that NC channel. At a different site you may find that the machine runs better in a different channel.
Proper GB actually causes what you're describing more often than NC, but GB is also influenced by EMI as is ID.

Out of curiosity, did you happen to re-GB after changing NC channels?

Yes I am trying to make a habit of constantly gb'ing every 10-15 min. I managed to pull up 3 nickels all about 6-8" deep.

Well, the amount of frequency shift between the highest and lowest NC channel isn't enough to have a dramatic effect on target response. You're talking less than a ½kHz total. That amount of shift is really only significant for it's intended purpose of avoiding resonant frequencies or harmonics.

Here's something that Randy wrote on the subject.

"VFLEX does reduce the effects of interference due to the digital signal being processed between the coil and the housing, opposed to an analog signal on most detectors. However, don't overlook how changing the Noise Cancel channel or changing the actual coil (and thus the frequency) can also assist you when you get into an area with a high level of interference. It is the resonant frequencies that induce interference. Not the actual frequency of the interference being generated. As discussed on page 84 in the eBook, in each model of the X-TERRA, the resonant frequency is Noise Cancel channel 0. This is set up in the coil's microcontroller when it is manufactured and calibrated at the factory. The amount that the operating frequency is "offset" is determined by the frequency of the particular coil. Lower frequency coils have smaller increments than higher frequency coils. The 3 kHz coils "step" up and down in increments of .035 kHz. The 7.5 kHz coils "step" up or down in increments of .040 kHz. And the 18.75 kHz coils step up or down in increments of .080 kHz."

And here's a chart that shows it all.