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@ChasnotRobert cheers. Is that an easy fix?
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you could try plugging the turntable into another input on the amp and see if sound comes out of both speakers. it'll be very quiet, but should be enough to tell if both channels are working.
if that is the case, then a separate phono stage would be a solution. or open up the amp and look for dry joints, if you are a bit handy with wires and solder.
seems most likely that there's a fault on the phono input stage, or else on the output of the turntable.