I don't know your radio so I don't know if the volume control would be a rotary encoder type (usually identified by just a number stamped on it) or a potentiometer, usually identified by a number with either a k or k Ohms after the number denoting x thousands of Ohms resistance stamped on it).
If it is a rotary encoder try cleaning it and see if that resolves the problem.
Here is a link to a video that shows in principle, what to do.
Here is a link to the ifixit Grace Digital Mondo Repair guide which may be of some help to gain access to the encoder.
If it is a faulty potentiometer (or even an encoder that has failed mechanically) hopefully there will be information stamped on the case of the component which will help if you have to order a replacement part, as I cannot find a service manual for the radio online from which to get the part information.
Hopefully this is of some help.