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Vintage Radio

AWA R7077 Beat Frequency Oscillator

By Maurie Findlay, MIE Aust, VK2PW

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This war-time photo shows the AWA Works Cafeteria at Ashfield. Meal breaks were staggered to cater for the large staff numbers involved and for shift workers.

THE R7077 Beat Frequency Oscillator (BFO) described here was a very small part of the company’s output during the early 1940s. This well-made piece of test gear provides a good example of the techniques that were available at the time. Vacuum tubes (valves) were by then well-developed and reasonably reliable but the transist

Indeed, it would be another 20 years or so before equipment using semiconductors became available.

As a radio amateur after the war, I acquired a number of pieces of AWA equipment through the disposals outlets and adapted these for use on the amateur radio bands. I also remember having a famous American communications receiver (HRO) and an AWA receiver made in the same general format (AMR100). The workmanship and general quality of the Australian-made set was better than the original.

AWA’s associated company AWV manufactured valves locally and many, particularly those designed for battery operation, were more reliable than overseas types intended for the same job.

The R7077 BFO

A handbook describing the R7077 BFO at the time lists the features, applications and design of the instrument and these are summarised in the accompanying panel. It is interesting to note the technical style used in the 1940s, long before terms such as “Hertz” were introduced, both in the panel and in the following text which is also derived from the leaflet:

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Fig.1: the original circuit from the AWA handbook. The signals produced by the variable and fixed oscillators (V1 & V2) are mixed in V3 and fed to a 6V6G audio output stage (V4). V5 is the rectifier, while V6 is a “magic eye” beat indicator.

The audio output is produced by mixing the outputs of two high-frequency oscillators, selecting the lowest frequency component of the output and amplifying it to a suitable level.

One oscillator remains fixed at 100kc, and the other is variable from 100kc to 86.5kc by means of a variable condenser. The plates of this are shaped to give an approximately logarithmic law to the beat frequency scale calibration.

Frequency drift, due to variations of temperature and supply voltage, is reduced to a minimum by using silvered mica condensers and robust coil formers in the tuned circuits. In addition, a symmetrical layout is used for the two oscillators.

Both oscillators are of the resistance stabilised tuned grid type, using a triode oscillator electron coupled to a heptode buffer amplifier. A type 6J8G valve is used in each oscillator circuit, which allows the oscillator and buffer amplifier to be accommodated in the one valve. The output of each oscillator is fed to the mixer valve, the variable oscillator output being taken directly and the fixed oscillator output reaching the mixer through a filter.

A low-pass filter placed between the mixer and the output stage effectively prevents high-frequency components in the mixer output from reaching the output valve.

Inverse feedback is applied to the output stage, which is a beam tetrode (6V6G). This results in a reduction of the output harmonic content, improvement in frequency response characteristic and stabilisation of output impedance.

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