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Vintage Radio STC Radiotym model 5160 from 1956 By Assoc. Prof. Graham Parslow Clock radios from other manufacturers all followed a basic pattern of a rectangular case with a clock set where a speaker might otherwise be. However, the STC Radiotym 5160 (a modified Pixie radio, original shown above) stands out stylistically from its competitors. I n the 1950s, every major Australian radio manufacturer offered a clock radio, usually as a variant of a radioonly model. They were good in the kitchen, lounge, or bedroom to tell the time and provide entertainment. They can also be set to mechanically switch on or off at set times (eg, to be used like an alarm clock). The clocks all have synchronous motors locked to the 50Hz mains frequency and are impressively accurate. Neither the STC Pixie nor the Radiotym was a great marketing success if judged by the small number that survived in collections. I have seen several Pixies, but only the one Radiotym. I was lucky enough to purchase the one featured here from a Historical Radio Society of Australia auction (HRSA). I previously described the STC Pixie in the March 2023 issue (“Three ‘kindred’ radios from STC”; siliconchip. au/Article/15705). 104 Silicon Chip The colour choices for the Radiotym were black, cream, Chinese-red or grey. These STC radios were not styled in Australia, but in the USA by ITT, the parent company of STC. Relative to Australia, radios from the USA through the 1940s and 1950s were generally more flamboyant in style, with multiple vivid colours. For a US-designed radio, the Pixie and Radiotym are on the conservative side. Australian Pixie and Radiotym radios were made from local components in Sydney because high tariffs made it uneconomical to import assembled radios from the USA. Circuit details The Radiotym circuit and dial string arrangement details are shown in Fig.1. Many contemporary radios in 1956 featured a ferrite rod antenna. However, the front end of the Radiotym Australia's electronics magazine has a conventional aerial coil with a trimmer capacitor linking the primary to the secondary. This trimmer optimises the sensitivity to higher frequencies which would otherwise be partly shunted to earth by the 100pF capacitor in parallel with the primary winding. The secondary of the aerial coil provides tuning through the MW band by resonance with tuning capacitor C3. The 6BE6 valve (V1) designed by RCA is a commonly-encountered frequency converter in radios built from the late 1940s to the mid-1950s. The 6BE6 in this radio is branded Mullard. The local oscillator that produces the superheterodyne frequency is a Hartley circuit tuned by parallel capacitors C4 and C5. The tuned signal, converted to 455kHz (the intermediate frequency [IF]), passes through the first IF transformer for amplification by a 6BA6 valve (V2). The 6BA6 is another siliconchip.com.au Fig.1: the circuit diagram for the STC Radiotym 5160, which is a clock-controlled radio. Note that C3 & C5 are ganged capacitors, while C8, C10, C12 & C15 are valued at 75pF. RCA-designed valve released at the same time as the 6BE6. The high performance of the 6BE6/6BA6 combination contributes to the STC claim of a sensitivity of 10μV for reception across the MW (AM broadcast) band. The amplified IF signal is detected by the 6AT6 valve (V3; at pin 6) and produces an audio output at the secondary of the second IF transformer. The second diode at pin 5 in the 6AT6 generates a negative voltage proportional to signal strength. This negative voltage is fed via R3 (1MW) to the control grids of the first two valves for AGC (automatic gain control). Back-bias resistor R13 (330W) in the high tension line makes this delayed AGC, so that weak signals are not affected by AGC action. Only once signal strength crosses a certain threshold does the AGC circuit start to reduce the set’s gain. Resistor R13 is also the source of grid bias for the 6CH6 output pentode. The 6AT6 valve provides a triode section to preamplify the audio signal fed to the grid from the volume control, P1. The 6AT6 grid also receives negative feedback from the speaker via 1kW resistor R15, reducing distortion and keeping the amplifier section siliconchip.com.au stable. When reconnecting a speaker, preserving the original polarity (phase) to maintain stability is essential. Sometimes a guess has to be made during restoration – the wrong choice is given away by greatly increased distortion or howling oscillation. The 6CH6 (V4) output pentode is Photo 1: from the rear view of the chassis and case, you can just see the speaker sitting at the bottom. It is there because the normal location for a speaker is instead occupied by the alarm clock that is covered by a sheet of plastic. Australia's electronics magazine February 2024  105 Photos 2 & 3: the empty STC Pixie case, which uses the same moulding as the Radiotym model 5160. The populated case can be seen below. The two radios use identical components except for the Pixie having a mains switch in the volume control. the most unusual of the valves found in this radio. The only other radio I have encountered this valve in is a Tecnico model 1050 (the ‘fortress’). The valve was released by Brimar in 1951 as a video output pentode and can easily produce 3W of audio power due to a high anode current and heat dissipation. Brimar is a UK subsidiary of the STC-ITT group of companies, so that explains the choice. This valve has a mutual conductance (gm) of some 11mA/V, almost three times that of the more common 6AQ5 output valve (4.1mA/V). Since voltage gain depends on gm, the 6CH6 boosts the radio’s sensitivity by almost three times compared to an identical set using a 6AQ5. This gain contributes to the high sensitivity quoted for this set in the service notes. Restoration The primary of the speaker transformer in my set was open circuit, so I installed a replacement transformer. The coupling capacitor C17 was leaky, but all other components were serviceable. Many valve radios from the late 1950s have likewise mostly functional components. Adding a clock to the Pixie Photo 4: you can see the speaker grille from the underside of the model 5160 case. A downward-facing speaker is rare for vintage radios. 106 Silicon Chip Australia's electronics magazine The Pixie has a front-mounted speaker that is attached to the main chassis. The same four-inch (~100mm) speaker is used in the Radiotym. The empty Pixie case (see Photo 2) shows that the front plastic moulding inserts into the main case, leaving a vent for the speaker and a rectangle for the dial. The Pixie dial features the call signs of all Australian states, the same as for almost all Australian contemporary radios. The grille moulding for a downward-facing speaker is also included in the Pixie case (Photo 4). The Radiotym uses the same case moulding. A custom aluminium panel in the front accommodates the clock and provides a window to the dial cursor. That frequency-calibrated dial is the same as for the USA, where local station markings were impractical due to the large number of stations. The calibration numbers run from 5.5 to 16, representing multiples of 100kHz (kc/s at the time). Most Australians in 1956 would not have been familiar with locating stations by frequency. Downward-facing is the worst of siliconchip.com.au Photo 5: there is a label on the power transformer showing the valve layout. Photo 7: the Japanese Copal flip-clock radio, which was a later development in the sphere of clock-radios. all the alternative locations for a speaker. The reproduction quality then depends on the acoustic properties of the surface below. However, there is also a backwave that contributes significantly to the sound after emerging from the rear moulding of the radio. As a result, the listening quality is not great, but passable when there is nothing to compare it with. MSP speakers were made in Sydney by AWA. They used the Manufacturers Special Products label to provide original equipment manufacturers (OEMs) with items not branded with a competitor’s logo. Rola speakers were considered acceptable to use because there were no Rola radios. The chassis and component placements are identical for both the Pixie and Radiotym. In the photo showing the Radiotym components mounted below the chassis (Photo 6), the plastic shroud covering the clock against dust can be seen through the rectangular punch-hole. In the same view for a Pixie chassis, the front-mounted speaker can be seen through the rectangular hole. A simple but effective mounting to lock the downward-facing speaker to the chassis is provided by two screws tapped into the alnico (aluminium/ nickel/cobalt) speaker magnet. There is one component difference between the Pixie and the Radiotym. The Pixie has a mains switch incorporated with the volume control, while the Radiotym does not. Clock Radios use the clock to control on/off switching as well as timer functions. This can be a trap for today’s unwary collector of a clock radio. For example, a collector told me his radio was not working. I replied, “Try turning it on with the clock control knobs”. A miracle followed – it worked! Radiotym variant of the Pixie was not a huge success in Australia. However, STC also offered a locally-designed Bantam radio that was better attuned to Australian tastes in the 1950s. I also covered the Bantam radio in the article on three STC radios (March 2023; siliconchip.au/Article/15705). Before the second world war (pre1939), STC aimed for a prestige market, but after the war (post-1945), they introduced a series of Bantam radios aimed at the middle market. The Chinese-­red model Bantam is one of my favourites. For the Tymatic clock radio, STC performed a radical internal reorganisation of the Bantam to create a linear tracking dial with the speaker behind it and a clock in the usual speaker grille. I have seen several examples of the Conclusion Photo 6: the underside of the STC Radiotym model 5160 chassis, and MSP (Manufacters Special Products) speaker made by AWA. As mentioned previously, the siliconchip.com.au Australia's electronics magazine Tymatic, so this met the market more successfully than the Radiotym. I suspect that one reason is the large, clear station information on the Tymatic dial. As transistor radios displaced valve radios in the 1960s, the conventional clock radio vanished. This niche was inherited by the low-profile bedside clock radio with a digital front panel. Initially, these bedside units had a mechanical flip-over set of numerals giving a digital read-out. By this time, Australian radio manufacturing had succumbed to cheaper, wellmade imports. A photo of a Japanese Copal flipclock radio (see Photo 7) is presented above to conclude this sketch of clock radios. It shows the next step in the SC evolution of this genre. February 2024  107