THIS 1948 BREVILLE 730 receiver was obtained by a friend of mine (Marc) in almost original condition. In fact, it’s quite rare to come across a set as original as this one, as most sets have had some routine servicing and parts replacement during their life.
Hopefully, any work that has been done on a set will have been carried out by a competent serviceman. An incompetent servicemen or hobbyist can leave a set with more faults than it started out with and can sometimes even destroy hard-to-get parts.
In the case of Marc’s Breville 730, the only evidence of any service work was on the band-selection indicator. In fact, the condition of this 65-year-old set is so good that it has obviously been used in a lounge room for most of its life. It had eventually failed when the ECH35 converter developed a short circuit (as confirmed by a valve tester), after which it had been carefully stored away.
As a result, virtually no damage has occurred to either the cabinet or chassis, other than the normal ravages of time.
Fig.1: the circuit is a fairly conventional 5-valve superhet design, although the 3-coil first IF transformer is somewhat unusual. The IF stage is tuned to 446kHz and has switch-selectable bandwidth.
The Breville 730 has a conventional superheterodyne circuit that’s similar to many other receivers of the era. However, it does have some features which, although not unique, are not seen in many other receivers.
Fig.1 shows the circuit details. The signal from the antenna is fed to an input tuned circuit and the position of the band-change switch determines whether shortwave (6-18MHz) or broadcast band tuning is selected.
As shown, the primary of the shortwave antenna tuned circuit (top) is in series with the primary of the broadcast-band antenna tuned circuit. Capacitor C2, a 100pF capacitor across the broadcast-band coil, performs two tasks: (1) it acts as a low impedance to earth for the bottom end of the shortwave antenna primary and (2) it tunes the primary winding of the broadcast-band coil to below the lowest frequency on this band.
This technique enhances the performance at the low-frequency end of the broadcast band.
Note that the primary of the shortwave antenna coil has little effect on the performance of the broadcast-band antenna tuned circuit and may even boost its performance slightly. This circuit works well and simplifies the band switching.