In both Australia and New Zealand during the valve radio era, it
was standard practice to use a mains transformer to convert the 240V AC mains to
other AC voltages, as required. By using a transformer, the lethal mains voltage
was isolated from the receiver’s metal chassis and this made it safer for both
users and servicemen.
However, there were some exceptions to this convention as there
were areas with DC mains and areas that relied on 32V DC house lighting plants.
Many AC/DC sets were made for use in country areas and these often had one side
of the mains directly connected to the chassis!
This meant that, depending on which way around the mains was
wired, the chassis could operate at 240V AC with respect to earth. For the
unwary, they could be a real death trap and were always dangerous to work
on.
Because of this, AC/DC receivers were always totally fully
enclosed in a cabinet (ie, with closed backs) and the controls were often fully
insulated from the chassis. That wasn’t always the case though. Many AC/DC sets
had metal-shafted controls which were attached to the chassis and if a knob came
off, users could get a nasty if not fatal shock from the bare control shaft!
To overcome this problem, some sets did not earth one side of
the mains so that the chassis itself could be earthed. In these sets, all items
that would normally have been earthed to the chassis were instead connected to a
bus bar (often a thick solid-core tinned copper wire). This bus was then earthed
as far as RF was concerned using a large high-voltage paper or mica capacitor
wired between it and the chassis.
In addition, the antenna coil primary winding was often
completely isolated from the mains, with one end going directly to the antenna
and the other going directly to an outside earth.
