On Kenwood TS-904's that do not have the optional Voice Synthesizer
module VS-1 installed, you may notice a hum in the RX audio. The
already present molex connectored cable for this module should be
dressed to the top of the shield case where the voice synthesizer
mounts. However, on some units they have been left hanging free
in the vicinity of the PLL and digital A units and are quite good
at picking up noise from these circuits.
The Fix, is simply to secure this cable to the top of the shield
case where the VS-1 would mount. Plastic tie straps, or even
simple electrical tape works very nicely.