After looking up because (1) microphone socket pins or bad solder joint loose. (2) the main circuit board of a microphone amplifier with digital delay circuit between signal lines shielded off. (3) bone (pull) the KFE switch S6 is poor. (4) microphone volume and tone potentiometer VR5 VR8 internal connection is bad. (5) digital delay chip-powered three-terminal regulator is damaged. After repair or replacement, returned to normal.wqnservice manual download site