Service BulletinAmateur Radio Division

Subject: TS-120S - Hum (RF Feedback) in SSB TxDate: August 9, 1979

See also, SB-844 - TX Oscillation

Some TS-120S users may complain of 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. Poor regulation and insufficient filtering in the power supply (other than PS-30).
  5. Linear amplifier creating high RF field near the operating position.
  6. RF induced back into power supply causing false 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 0.01uF bypass cap across Mic input jack between pins 1 and 4.

Some AFSK operators may have similar RF feedback problems with the TS-520S or TS-820S. Bypass the Mic ground pin to the chassis, pins 3 to 4 and in extreme cases, install a ferrite bead on the Mic coax center conductor at the microphone connector.

©1979 Kenwood