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Another suggestion if you happen to have a bias box (a low voltage variable DC supply): Put the positive lead to ground, the negative lead to: (schematic's too blurry to read) the Test Point on the AGC bus, below the 2nd IF tube. Vary the voltage to see if you can get optimal contrast with no clipping or overloading.
Some early low end sets had no AGC circuitry at all, and actually used this method in lieu of a contrast control, sometimes calling it the 'Picture' control.
Or you could rig a 'bias box' very easily with a 9V battery, a resistor and potentiometer. (Of course it will disable the set's regular AGC action.)
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