SB-048 Hum in SSB TX, TS-120S and others 8-9-79 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. 1. Bad feedline and antenna system ground. 2. Poor Mic hookup or shielding. 3. Vertical antenna with poor or no radial system. 4. For base operation with other than a PS-30 Power supply: Poor regulation and insufficient filtering in the supply. 5. Linear amplifier creating hi RF field near the operating position. 6. RF induced back into the power supply causing falsie protection (other than PS-30). 7. RF pickup by any cables to or from ancillary equipment. 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. 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. JEB/yn (c) 8679TKC