Probably just a broken mechanical connection to the volume tube shaft.
How does it work?
I have my father’s Onkyo TX-SR500 receiver. It has a stepless volume control. It uses a 2 digit volume indicator. My problem is the control does not work. I can turn it 720 degrees in either direction with no affect. Luckily, the volume can be controlled with the remote control.
So, how does it work and is it fixable?