Some TS-120S users may complain of a hum during SSB operation, which may be caused by RF feedback. There may be various reasons for this complaint.
Prevention is the best cure. For those cases where all normal measures
have been tried, try installing a .01uF bypass cap across the mic input
jack between pins 1 and 4.
Bad feedline and antenna system ground.
Poor Mic hookup or shielding.
Vertical antenna with poor or no radial system.
For base operation with other than a PS-30 Power supply:
Poor regulation and insufficient filtering in the supply.
Linear amplifier creating hi RF field near the operating position.
RF induced back into the power supply causing falsie protection (other
RF pickup by any cables to or from ancillary equipment.
Some AFSK operators may have similar RF feedback problems with TS-820S or
TS-520S. Bypass the mic ground pin to the chassis, pin 3 to 4 and in
extreme cases, install a ferrite bead on the Mic coax center conductor at
the microphone connector.
Please annotate your service manual.