The pot travel of 3/4ths of a turn is not unusual. Three of my amps do this, including a Cayin. The bias curcuit not functioning at all is a different matter. I don't own a A50t so I don't know but is it possible the bias circuit is seperately fused and a fuse has blown? Is it possible that the previous owner blew out a resistor(s) in the bias circuit that might affect all of the pots? You might take off the lid and see what is there. After its fully discharged of course.
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