If you’re mechanical or electrical inclined, you might remove the cover to take a look. Might be something simple like a loose connection on or around the attenuator. You can find the service manual at hifi engine, https://www.hifiengine.com/hfe_downloads/index.php?onkyo/onkyo_tx-sr500_500e_service_en.pdf
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?