My first thought is; if this potentiometer is to regulate the volume, then it may have cracked or burnt open-circuit due to its age. Get a multimeter set to resistance on the pins on the back. There are usually three pins, the outer pins are across the whole resistance and the middle one is the variable one. Check you are getting a variable resistance. (Often two pins are connected together, so you will only get a variable result from one pair of pins.) I will have a hunt to see if I have a circuit diagram for this board.