Why is the sound erratic when using volume control on my receiver?

Why is the sound erratic when using volume control on my receiver?

I have a Realistic STA-111 Digital Receiver. It works but when I turn the volume control, static runs through the speakers and the volume in one speaker will turn very low. I have to have the volume at certain levels for one speaker to work. Also, I also receive static when changing the balance alone.

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