From the early 1990's until 2002 Fishers were my main detectors for relics and deep coin hunting. I had quite a few of them, including a 1265, two 1266's that have almost identical circuitry as the 1265 and five CZ's, and never had any problems with any of them. The 1265's and 1266's had preset ground balance, so if the guy was trying to ground balance a 1265 he had to pull the board and do it with a trimmer pot, R205 if I remember right. The trimmer pot only requires a very tiny movement to make a big difference in how the ground balance is set and it's right beside an amp that's easily damaged if a slip is made, so it's very easy to screw up.