[Mondial]

Probably the reason you hear the buzzing with the 18, but not the 20 or 80 junior is that the 18 has AVC while the 20 and 80 do not. When not tuned to a station, the 18 AVC line drops to 0 volts and the gain controlled stages are running flat out at maximum amplification. As soon as you tune to a station, the AVC voltage rises, reducing the gain and along with it the residual received RF noise level. The 20 and 80 with manual gain control do not automatically increase the amplification as you tune off a station, so the background noise level does not rise noticeably.

The 18 is also a much better receiver and has a lot more reserve gain than either the 20 or 80, so the off station background noise is more apparent than with the lower sensitivity sets.