01-05-2022, 04:59 PM
Yes, you probably were not feeding in enough signal to activate the AVC. You will need a somewhat strong signal as the 690 has a delayed avc circuit.
The 6J5 AVC rectifier cathode is biased a few volts positive, and the grid (used as the plate) is biased negative, so the IF signal amplitude must be high enough to overcome the initial bias before AVC voltage is generated.
The initial 2.4V negative bias applied to the grid of the 6J5 is what you are seeing on the IF 6K7 grids. It should increase with a strong enough signal.
The 6J5 AVC rectifier cathode is biased a few volts positive, and the grid (used as the plate) is biased negative, so the IF signal amplitude must be high enough to overcome the initial bias before AVC voltage is generated.
The initial 2.4V negative bias applied to the grid of the 6J5 is what you are seeing on the IF 6K7 grids. It should increase with a strong enough signal.
