The reason is: (1) the signal input board and motherboard signal line screen layer lead off or weld. (2) the first stage amplifier circuit VR1 - VR4 ground. (3) the signal selector switch S1, the straight switch S3 contact. (4) pre amplification circuit VR1-VR4 damage and + 15V three terminal voltage stabilizing block damage, filter capacitor failure, Op Amp integrated block performance is bad. Repair welding or replacement, return to normal.jCxservice manual download site