The General Coverage 550kHz to 30MHz SWL
was a Low End or Beginners Receiver that came on the market in 1971 and
produced until 1982. The SW-717
was a Solid State design and it replaced the Tube design GR-64
that came on
the market in 1964, which in turn replaced the GR-91
that came on the market in
My receiver suffered heavily from hum.
The main reason was that the aluminum chassis was used as the zero
voltage reference and also as a conductor between components. I have
connected copper wires between the components and also replaced the
original mains transformer with a toroid transformer. The toroid
transformer is better in general but the difference it made in this
case is negligible. Instead it was the use of the aluminum chassis as a
conductor that caused the hum.
The band-spread mechanism was of poor design and did not work. I have
redesigned it and now it works.