It could be a phono input problem . Try reveresing the tonearm output cables to see if the dead channel moves or stays in the same one .
Also , you could ever so briefly test for continuity between the cartridge output pins [ red / green , white / blue ] . It will let you know if it is an internal wire problem .
Also , you could ever so briefly test for continuity between the cartridge output pins [ red / green , white / blue ] . It will let you know if it is an internal wire problem .