I have an old Triplett 610 that I use at times. On the AC scale it is way off suddenly. 115 AC was pinning the meter in the 300 volt AC position so I tried the 1200 scale and it went almost to the end , about 1150 volts for 115 volts AC. Am I missing an obvious cause ? Any idea on a cause and or/cure? Tks, Gerry