All American Five - Potential Hazards of The Design

Potential Hazards of The Design

Many early examples of the 'All-American Five' posed a shock hazard to users. Lacking a mains transformer, the chassis of the AA5 radio was directly connected to one side of the mains electric supply. The hazard was made worse because the on/off switch was often in the wire of the mains supply which was connected to the chassis, meaning that the chassis could be "hot" when the set was either 'on' or 'off', depending on which way the plug was inserted in the power outlet. Many power plugs had two identical pins, and could be plugged in either way round. The metal chassis securing screws were sometimes accessible from the outside of the Bakelite or wood case, and there were many examples of owners receiving a shock by making contact with these screws while handling a set. Ventilation holes could be large enough to allow children to poke their fingers, or metal objects, through. The same type of hazard was present in European AC/DC sets, at twice the voltage.

The hazard was eliminated from later sets by the use of an internal ground bus connected to the chassis by an isolation network. Underwriters Laboratories required the adoption of the floating chassis, as isolation from the mains (the exact circuit and component values were not specified although the leakage current allowed was specified) to limit the shock to a "safe" current level. The chassis was maintained at RF ground (for shielding) by a bypass capacitor (typically 0.05 µF to 0.2 µF) usually with a resistor connected across it (typically 220 kΩ to 470 kΩ, although values as small as 22 kΩ were sometimes used or the resistor was simply omitted). See here for a typical schematic of a 1948 model AC/DC radio with a 220K isolation resistor.

Over the years, these paper capacitors often become leaky, and could allow sufficient current flow to give the user a shock.