I am on page 46 ( audio amp. & gen test ) SN # 4239 Testing for the first time with head set plugged in I go to ST-L hold edit and hear the 600 cycles with a display of 40. As I vary the VFO up the display numbers go up but the volume does not change. Turning the VFO in the op. direction the numbers go down but the volume still stays the same. any sug?