Linn LP12 I channel louder?

After swapping about between my Jolida JD9a phono amp and the MC part of my Yamaha as-100 have isolated the source of one channel weakness to my LP12.Its a 1980’s vintage with a 2/3 year old Dynavector DV-20X2L.My question is where do I start to track down the left channel bias? Its slight but a twiddle of balance control makes a huge difference.Its not amps or speakers as CD, Radio, PC all good and balanced.