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Vintage Radio Airzone 6552A Concert Star from 1947 This Australian post-war set is based on a design from 1941. The circuit is a conventional superhet design; its appeal lies mainly in the flamboyant cabinet styling, reminiscent of a concert hall. By Assoc. Prof. Graham Parslow T he almost-identical Airzone 6651A was first offered for sale in 1941 but the government stopped all domestic radio production when components were reserved for war manufacture. A contemporary advert read: Airzone have presented another star to join their constellation. It is a new brighter and better star in the mantel world – the sparkling Concert Star. Buy the best mantel in Australia for £17/10/-. The model 6552A was almost the same radio when it was released in 1946, although the retail price had increased to £20/12/6. The styling was comparable to other late-1930s Bakelite radios and it used heritage circuitry. The speaker in this radio is stamped September 1947; it is unclear whether this electromagnet field coil speaker would have been made in 1947 or pulled from shelves that had been storing components during the war. In 1947, Rola were making excellent permanent-magnet speakers that were a better choice unless old stock needed to be used. A friend acquired this radio and passed it onto me for checking and restoration. A quick glance was enough to 104 Silicon Chip confirm that it was not a quick plugin-and-return task. It never ceases to amaze what debris ends up inside old radios. In this case it was chicken bones! The dirtier the chassis at the start of a restoration, the greater the satisfaction with seeing a resplendent end product. This one was certainly dirty. My standard procedure is to remove all the valves first. For very dirty radios like this one, the next step is a blow-off with compressed air. Turpentine and a brush can then remove or loosen a lot of the surface accumulations. Then comes a final compressed air blow-off. The Airzone factory that used to be on Paramatta road. Australia's electronics magazine Extensive light surface rust on this chassis was covered with chrome paint. The transformer and valve shields were repainted. This was adequate to satisfy me that the result would be gratefully appreciated when it was returned. Ian Batty (another Vintage Radio author in Silicon Chip) informed me that perfection is the enemy of the good. In other words, a lot of time can be wasted in seeking perfection when something is adequate. A bit of Airzone history At age 30 in 1925, engineer Claude Plowman established a business fabricating components for crystal sets. This was two years after the introduction of public radio transmissions. He had judged the market well and learnt that delivering quality products was best done by manufacturing in-house. Plowman registered the trademark Airzone in 1926. The coils wound by Airzone stood out for excellence. There was continuous growth in the output from Airzone through the 1930s. Airzone produced some of the most collectable Australian radios from the 1930s, due to their beautiful designs and the quality of their timber and siliconchip.com.au Bakelite cabinets. The growth of Airzone was helped by wartime manufacture of instruments for radar testing, various communications items and making ASDIC (sonar) echo-location equipment. This led to opening a large factory at Paramatta Road in Sydney in 1944 (see the drawing opposite). Post-war, Airzone returned to domestic radio production and badge-engineered radios to be sold as Malvern Star, Mullard and Peal. Their success led to a company take-over by the large EMAIL (Electricity Meter & Allied Industries Ltd) group, although Claude Plowman remained as manager. The changes brought about from the takeover resulted in the termination of Airzone radio production in 1955. The Airzone legacy to collectable Australian radios is substantial. Circuit details The circuit has been scanned from page 25 of the Official Australian Radio Service Manual (OARSM) Volume 5, 1946. The valve lineup (6A8G, 6U7G, 6B6G, 6F6G and 5Y3) is a classic prewar superheterodyne arrangement, widely used in Australian mantel radios of the late 1930s and early 1940s. By the time this set reached the market post-war, the design was already conservative but well proven, offering reliable performance rather than innovation. R1 (10kW) is connected in parallel with the primary antenna coil. Per Roger Johnson (Electronics Australia, November 1998, p62), this broadens the tuning of the antenna circuit by lowering the secondary Q by damping the primary winding. Without the detuning lower frequency stations are emphasised over higher frequency stations. After this, the switching between MW and SW is relatively simple. The tuning is accomplished by conventional ganged variable capacitors. There is a switch linked to the band change mechanism, between two sets of dial lamps, that could be used to switch the lamps. However, on this radio, the four dial lamps remain on regardless of band selection. The representation of the 6A8 mixer valve uses an old convention of drawing crinkled electrodes much like a resistor. All of the electrodes are drawn stacked between the cathode siliconchip.com.au The top two photos show the rear of the cabinet and the chassis in their unrestored and very dusty condition. The bottom photo of the chassis is what it looked like after a good clean. Australia's electronics magazine May 2026  105 This is what the underside of the chassis looked like before any work was done. and anode, rather than set apart as a pentode and triode. The 6A8 was released in 1935 and is relatively common in pre-war radios. Two separate local oscillator coils pass a mixing frequency to the 6A8 via capacitor C2. The result is an intermediate frequency (IF) output at 456kHz. The 6U7 IF amplifier is a pentode, although it is drawn as a tetrode. The omitted electrode is connected internally to the cathode. AGC is applied to both the 6A8 and 6U7 via 1MW resistor R7, which connects to the detector diodes in the 6B6. A second IF transformer passes the signal to the detector diodes. Audio from the diodes passes to the volume control potentiometer, R12; the slider is connected directly to the top-cap grid of the 6B6. The negative voltage from the diodes is prevented from reaching the 6B6 grid by 10nF capacitor C11. The 6F6 output pentode is another venerable valve from 1935. It is drawn here as if it were only a tetrode. This valve needs a relatively high grid bias voltage to minimise distortion. A grid bias of -13V is specified in the circuit diagram. The 460W wirewound cathode resistor, R16, delivers that bias (measured as -10.7V in this radio). The 6F6 grid is held at Earth potential by resistor R15 (5MW), a resistor that is more commonly encountered as 500kW. C13, the 10nF audio coupling capacitor, needs to be leakage-free to prevent voltage from the 6B6 anode from driving the 6F6G grid positive. The use of a 5Y3 full-wave rectifier is another link to heritage parts. The 5Y3 is the old type 80 valve with four pins, repackaged into an octal base. It is an excellent rectifier, but has two drawbacks. First, the cathode is directly heated, so almost immediately from switch-on, the 5Y3 is generating a The recabled speaker and restored chassis are shown here. Note the replaced mains lead. 106 Silicon Chip higher voltage than the eventual voltage under load when the other valves have warmed up. This can stress components in the HT line, causing failure. The other downside is the need for a separate 5V AC transformer winding, increasing the cost of the transformer. At the time, a range of indirectly heated rectifier valves such as the 6V4 were available. 1947 was close to the end of the period when octal-base valves were used in radios, because new miniature 7-pin and 9-pin valves were becoming available. Due to large inventories and war surplus, octal valves continued to appear in radios well into the mid-1950s. Radio service people often had substantial numbers of spare valves. Such new-old-stock and salvaged valves have generated a valve bank of approximately 70,000 held for the use of members of the Historical Radio Society of Australia (see hrsa.org.au). Electrical restoration The two-core mains lead was replaced with a newly-manufactured cloth-covered three-core cable. That original old two-core cable was commonly also used for light fittings. The expense of using specially-made new cable is readily justified by keeping the external appearance of the radio true to period. More importantly, adding an Earth wire can both improve performance and enhance safety. Additionally, the dial cord was replaced and new dial lamps installed. The speaker cable was unserviceable and was rewired with coloured wire reclaimed from old switch-mode computer power supplies. The old computer wire is handy for medium-­ current applications and colour tracking separate lines. Under the chassis The first power-up was with without valves and the four globes illuminated with a stable power draw of 15W. 8W of this was just the globes. Sometimes all capacitors remain serviceable, and I had a good feeling about this radio. Accordingly, without the mains connected, I connected a bench HT supply ramped it up to 250V DC, allowing the electrolytics to re-form. In the end, it only drew 2mA at 250V. This tested all the paper capacitors subjected to high voltage with one important exception: C13, the audio coupling capacitor, which I replaced before a full power-up. With the valves installed and power applied, only two strong stations were received very faintly. A signal tracer showed good signal input from multiple stations was delivered to the volume pot. The wiper on the pot was broken and made no connection. After replacing the pot, the radio worked properly again, drawing 52W from the mains. At raised volume, the speaker was poling and distorting. Luckily, this was easily fixed by re-gluing the cone to the rim where it had separated. Restoring the cabinet The chief detraction from the cabinet was the soiled grille cloth. The cloth was folded into the theatre-­ curtain pattern that was in vogue in the late 1940s (recall that this radio is called the Concert Star). I took a gamble that worked in this case. I sprayed automotive degreaser onto the cloth and brushed it in. Copious water cleansing followed, to remove the degreaser, and the result was an impressively clean grille cloth unharmed by the harsh treatment. The case suffered from the kitchen ceiling disease, which is contracted when a ceiling is roller-painted without adequate protection to what is below. It has a light covering on the face, but a much heavier spatter was over the top. The globs of white paint were removed by careful rubbing with grade zero steel wool soaked with carnauba wax (car polish). This had the twin benefit of cleaning and polishing the SC cabinet. Fig.1: the circuit for the Airzone 6552A radio with the components labelled for convenience. siliconchip.com.au Australia's electronics magazine May 2026  107