Radio receivers were still very much at a developmental stage in the early 1930s, the very first sets having been built just 30 years earlier, around 1900. During that time, they had developed from modest “breadboard” pieces of equipment through to the “coffin-style” cabinets of the 1920s and then to steel chassis sets from the late 1920s onwards.
A steel chassis made life so much easier when it came to design and manufacture. It meant that each receiver made would consistently perform according to specification, provided of course that the correct components were used and the wiring had been correctly carried out.
Of course, some designs were “dogs” due to poor design and construction but many manufacturers did have good engineers who designed excellent equipment. Some of that early equipment is still around today and can still turn in a good performance. One such receiver is the Airzone 503, described here.
Fig.1: the circuit is an early supherhet design, with the first valve (6F7) acting as a mixer/local oscillator stage. The second stage (78) functions as an IF amplifier, while the 77 functions as an anode bend detector. A type 41 valve is used as the audio output stage, while an 80 is used for the rectifier.
The Airzone 503
This particular Airzone 503 belongs to a friend and it had been restored several years ago. As a result, there wasn’t much I had to do to get it running at peak performance.
As shown in the photos, the set is housed in an attractive medium-sized wooden cabinet. It has a matte finish applied to the timber, which makes it really look the part. However, being built around 1933/4, it only has a rudimentary tuning dial, a feature it shares in common with many other sets of that era.
Basically, the dial consists of a smallish knob with a moulded pointer on it to show what part of the band the set is tuned to. This knob is connected directly to the twin tuning gang and so the tuning is quite direct. However, this isn’t really a problem as the IF (intermediate frequency) bandwidth is quite wide.
The volume control is basically a rheostat. It rotates through 330° and uses a knob that’s identical to that used for the tuning. However, the controls are reversed to what we normally expect, with the tuning control on the left and the volume at right.
The above-chassis components are well laid out and everything is quite accessible. One unusual feature is the use of a curved metal sheet to form a shield between two of the stages in the receiver. I’ve never seen anything like that before.
A look at the circuit
Initially, I had problems finding a circuit diagram of this receiver. That was until I got onto the Internet and found a reference to the Airzone 503 that steered me to a publication that I had. A quick check in that publication then turned up the circuit diagram for the old Airzone.