Determine the recommended bias setting for your recorder for the tape formulation and tape speed you'll be using. This information is usually available in the operation or maintenance manual for your recorder.
Locate the bias adjustment potentiometer on your recorder. Adjust the recorder's position if necessary so that you can access this potentiometer during operation. The adjustment must be performed for each channel if there is a specific bias potentiometer for individual channels.
Load the blank tape onto your recorder and connect the signal generator to the recorder's input.
Input an appropriate reference tone (10 kHz for 7.5 and 15 inches per second, 20 kHz for 30 inches per second) using the signal generator and adjust its output so the level reads 0 VU on your tape recorder.
Record the reference tone to tape, and set the VU meter to monitor from tape, not from input. While it is recording, turn the bias potentiometer counterclockwise with your screwdriver or alignment tool until the level on the VU meter falls by several dB.
Adjust the bias potentiometer clockwise. The VU meter will climb again, but only to a point: continuing to turn the potentiometer clockwise will eventually cause the VU meter to fall again. Note the peak level on the VU meter before it begins to fall.
Adjust the bias potentiometer clockwise until the VU meter falls past the peak level by the amount specified as the bias setting for your machine. For example, if the peak level on the VU is +1 and your recommended setting is 3.5, turn the potentiometer clockwise until the VU reads approximately -2.5.