Not sure if this will solve your coil issue or not but may be worth a try if you havnt solved the problem yet, its not in the user manual & if nothing else its definitely an interesting hidden feature of the xterra range.. came across it on another forum a while back..
"Hello All,
Early this year I was out detecting with my "Xterra 705". I was loosing alot of depth even with the sensitivity up as high as it goes.I waved my 9ct gold ring flush against the coil "Coiltek 6inch Digger" with no tone at all. So I sat there and started to press buttons just to see what happens. To my surprise I found if you press the + & - buttons together "hold" then power on the machine it constantly makes a tone and scrolls through all the settings and Icons until the coin and the pick icon show up and in bettween them is a chequered flag to say its complete. Turn the machine OFF then ON and its back to normal and synced my coil to the maching so it was running at optimal performance. What this did for me was a Diagnostics check to see if the machine was running correctly. It did fix my depth issue for the time being but once I swapped coils it happend again thus having to re do it again. I rang the Minelab service center in S.A and I was told the same thing that's how they do a Diagnostics Check to see how its performance through the machine was before they pulled them apart."Mine was missing a one portion/icon on the discrimination bar on the bottom of the screen thus showing an issue. Long story of chit chat short I had a part loose connection on the circut board and an issue with a connection on at the back of one of the battery terminals or something along the lines of that I was told by the technician. I was also told it sync's your coil to your machine and you should only do it once per coil so it runs as optimal as possible. I hope this help any of you out there, It sure helped me"
Husky
