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Vintage Radio STC model 510 portable superhet from 1939 By Assoc. Prof. Graham Parslow This radio is not an outstanding design icon, nor is it among the most collectable Australian radios. However, it is rugged and an excellent performer. Although described as portable, it is really more like “luggable” at 10.2kg. The circuitry and chassis work are first-class, and the vinyl fabric covering was innovative and modern at the time. W orking on this radio took me back to my youth in country South Australia, but more on that later. First, let’s look at the electronic side of it. Electronic design The radio is a conventional superhet with the bonus of RF amplification. One significant problem for all portables in the 1930s was the antenna. Some radios like the Astor Porta had a telescopic antenna, similar to contemporary FM radios. For the model 510, a loop antenna is built into the back panel, with the ends terminating on the two hinges. This arrangement can be seen in the picture of the bare case from the rear with stubs of the connecting wires soldered to the hinge mounts. The upper yellow wire leads to the aerial coil, while the lower black wire goes to ground (the chassis). In many valve portables, the loop antenna is part of a tuned circuit, but not in this case. This means that the radio still functions with the back panel removed. The loop antenna is directional in receiving radio waves, and it can be rotated on the hinges to optimise the reception of a particular station. However, this is not a user-friendly solution because the rear panel is wide (~370mm) and bumps into any close items as it swings. It also looks untidy with the rear open. To overcome this, and get better reception, I connected an additional aerial wire during restoration. The designers of this radio took care to produce an aesthetically pleasing chassis by lining up the three tuned-circuit coils in identical canisters placed next to the capacitor gang that tuned each coil. Few portable radio chassis are as neat as this one. Circuit details The original circuit diagram is reproduced in Fig.1. The aerial coil has a tuned secondary connected to one gang of the three-gang tuning The STC model 510 has a hinged front and back cover with a small pocket, in the front cover, that is used to house aerial equipment. The set measures 375 x 295 x 300mm and comes in a “hogskin” finish cabinet. siliconchip.com.au An advert for the STC model 510 from Australasian Radio World, November 1939, page 42. siliconchip.com.au Australia's electronics magazine October 2022  101 Fig.1: the circuit diagram for the STC model 510 portable superhet radio. The set has a standard intermediate frequency of 455kHz. capacitor. Three gangs are the first clue that the radio has an RF amplification stage to optimise the reception of weak stations. RF amplification is essential for farm use (ie, in distant rural areas) while also compensating for a minimal aerial. However, I have encountered non-RF amplified radios with a threegang capacitor when the manufacturer decided not to modify the mountings or change inventory to use a two-gang capacitor. Three gangs can also be found when both sides of the aerial coil are tuned. Confirmation of an RF stage comes from counting the valves, in this case, five in total. That is equivalent to a sixvalve mains radio as they require an additional rectifier valve in the power supply. As for the coils, the third coil is for the local oscillator, while the two larger canisters are the IF transformers. All valves except the output pentode are shielded in two-section metal cylinders. In keeping with a high-end radio, all of the metalwork is plated with a copper-hued finish that is characteristic of STC chassis of the time. The RF preamplifier is a 1P5 tube. Specifically, for this radio, the valve is a 1P5GT where G indicates glass (not metal) construction and T indicates that the shape is tubular rather than bulb-like. The prefix 1 indicates that the filament voltage is notionally 1V (in practice, it is 1.4V). Following the mixer-oscillator stage, using a 1A7G The top side of the restored STC 510 chassis. 102 Silicon Chip Australia's electronics magazine siliconchip.com.au The chassis was supplied in a fairly battered condition, with cobwebs abound and the cabinet frayed. The loop antenna is wound into the back panel. valve, a second 1P5 valve is used as an IF amplifier. The valves are all of short stature and have octal bases. In this case, the reduced size is of little advantage because the valves are shielded by conventionally-sized aluminium cans. A 1H5 valve provides audio signal rectification and preamplification. In this application, there is only one diode; there is no second diode to generate an AGC signal. Instead, a 1MW resistor from the detected audio provides AGC to the grids of both 1P5 valves. The volume control potentiometer (500kW) feeds the signal to the grid of the 1H5 audio preamplifier. A simple top-cut tone control is connected to the anode of the 1C5 amplifier valve. The 1C5 data sheet claims its maximum output as 500mW with 150V on the anode. In this radio, the anode is at 90V, so it can only produce 200mW before clipping. It is surprising how The underside of the restored STC 510 chassis. siliconchip.com.au Australia's electronics magazine 200mW can even be excessively noisy in a quiet environment. The speaker in this radio is a 6-inch STC unit with high efficiency to make the most of the limited audio power available. A sculpted space at the front of the chassis allows the speaker to recess into the chassis. Two sides of the metal frame are cut back to allow the speaker to clear the large dial assembly. The large dial size is due to reusing the escutcheon and tuning The STC model 510 is described as having an “extralarge” dial, and station names are radially grouped per state. arrangement of the STC table-top model 528. Restoration It was a welcome surprise that the speaker cone was in pristine condition. In general, battery-powered portable radios survive in better condition than their mains-powered cousins. This is because there are no voltages over 90V, and little heat is generated to stress components. The only electrolytic capacitor in this radio is a low-voltage cathode bypass. Hooking up bench supplies of 90V and 1.5V instantly produced a working radio. Dropping the HT to 80V produced little degradation in the performance, but dropping the filament voltage to 1.3V noticeably cut its output. Through the 1920s, filament voltage control by a rheostat was often used as the volume control, with the advantage of conserving battery capacity at lower output levels. The STC valve filaments took 260mA <at> 1.5V (0.39W) and the HT required 14mA <at> 90V (1.26W) for a total power consumption of 1.65W. Even with batteries lasting months, it was expensive to buy two new 45V batteries plus a heavy-duty 1.5V battery. When this radio was new, the 45V batteries used were likely to be the Eveready type 762 that packaged thirty The set uses a 6-inch permanent magnet speaker branded by the same company. The chassis has a cut-out to make room for the speaker to mount next to the dial. Compared to the state of the rest of the set, the speaker was in pristine condition initially. individual 1.5V cells. The filament battery was likely to be an Eveready type 741. The STC model 510 has four battery wires ending with one centimetre of bare wire. The wires are clearly labelled and would be joined to brass Fahnstock clips on the top of the batteries. Dedicated plugs and sockets made battery connection more foolproof at a later time. To operate the STC 510, there are three current options for power: 1) 60 AA cells to produce 90V (or 10 x 9V batteries) plus D cells for 1.5V. 2) A DC-to-DC converter to generate the HT from a lower-voltage battery, using an oscillator and transformer. 3) A mains-powered battery eliminator. I chose option 3. Looking through my bits boxes, I found a salvaged transformer from which I made a voltage doubler based HT supply (see Fig.2) plus a separate 1.5V source from a different transformer. The 1.5V supply came from a full-wave rectified source of 9.5V DC reduced to 1.5V by a prebuilt step-down regulator module. With this, the radio performed flawlessly. I built the eliminator onto a piece of Masonite and placed it in the radio’s battery compartment, leaving space to pack the mains cord and aerial wire. Condition as received The pictures hardly convey the Australia's electronics magazine siliconchip.com.au ► The battery eliminator (partial circuit shown in Fig.2) was designed from a salvaged transformer and other components to power the set. The set came with a little bonus in the ► form of a Broadcast Listener’s licence. degraded appearance of the radio when I saw it in a large emporium of pre-loved objects at Minlaton, South Australia. The proprietor had a great knowledge of his stock and showed me several other radios that I was able to resist for various reasons. But this orphan radio struck a chord with me, and we decided that an exchange of $50 would make us both happy. A bonus attraction was a moth-eaten bundle of papers in the radio’s front panel pocket. The papers were the seven paid-up Broadcast Listener’s licences from 1949 to 1956. The most intact licence covered 1949-1950 and cost one pound (written as 20/- if you can read the handwriting). The fee rose to two pounds in 1952. That fee was subsequently increased when a combined radio and TV licence was sold from 1956 onward. Every individual radio needed a licence. The licence fees were substantial enough for evaders to ingenuously hide radios, TVs and aerials from inspectors. The saga of licences ended in 1975 when Gough Whitlam said “enough”. Johann Launer of Anderson St, Yorketown, SA was the licensee. The S preceding the license number indicates SA and other states had their own identifier. I was born in 1948, and for the period covered by the listener’s licences, I lived in Edithburgh, ten miles (16km) from where this radio was being used. Anderson Street is on the fringe of Yorketown, next to an open wheat field with a salt lake in the middle. So the location is ultra-quiet, and 200mW of audio would suffice for comfortable listening. I passed Anderson Street each school day from 1961-1964 when I rode a bus to Yorketown Area School. Discovering the contents of those licences brought back happy memories of the period. Restoring the vinyl The vinyl covering at the base was almost completely destroyed (dissolved) by the radio lying in a pool of oil. I scrubbed all of the intact vinyl surfaces with detergent, and they cleaned up well, while the oil-affected vinyl washed away. I used PVA glue to reattach the loose vinyl, but this left several bare timber patches. I used Montmartre-brand artist’s oil paint to paint over these spots in a colour that matched the original vinyl. I then coated the whole radio with clear polyurethane to get an even surface lustre. And so it was that this neglected radio came to have a semblance of its SC former glory. Fig.2: the circuit for a mains-powered battery eliminator that can be used to produce the HT supply for this set. The valves used in the set from left-to-right: 1C5, 1H5, 1P5, 1A7 and another 1P5. All these valves have 1.4V filaments. siliconchip.com.au Australia's electronics magazine October 2022  105