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VINTAGE RADIO By RODNEY CHAMPNESS, VK3UG Beginner’s radios – as they were All people interested in electronics or Vintage Radio have to start at the bottom, as an absolute novice. Gradually, you come to recognise the jargon and understand the technology. This is the story of one teenager who become interested in radio many years ago. Back in the 1950s, a teenage lad who lived on a farm near a small country town picked up a copy of “Radio & Hobbies”. He read it cover to cover, understanding perhaps one word in 10 and he was immediately hooked on radio. A few of the radio suppliers had adverts displaying a G Marconi one-valve radio kit for the sum of five pounds; a lot of money for a young school boy with no money in the piggy bank. However, pocket money was scrupulously saved and the kit was duly ordered by post. He anxiously waited for it to arrive. While waiting he installed a long wire aerial around 100 feet long (31m) and strung between the 40-foot (12m) high 32V windlight tower and the 25-foot (7.6m) windmill tower. The earth was a rusty old pipe driven into the ground around a metre deep. The set arrived and he then feverishly set about assembling the kit and trying it out. It didn’t work. Oh dear; doom and gloom prevailed. The radio was checked and re­ checked to make sure assembly had been done correctly. The conclusion was that the valve must be faulty and it was sent back to the supplier. A replacement valve was received and then the set operated as it should. The lad was now getting really enthused and he tuned in regularly each night to see what he could hear. This is the rebuilt G. Marconi 1-valve set in its cabinet. It mightn’t look much but it was my first working receiver. 82  Silicon Chip One night the family had been out and returned after midnight. Our young enthu­ siast turned on his radio and heard a weak radio station. What was so exciting about that when they are all rather weak on a one-valve set in the bush? In the 1950s, radio stations did not transmit 24 hours a day and most of the Australian stations had then closed for the night. He had a book which listed most radio stations, their powers and frequencies. He came to the conclusion that he was hearing an extremely powerful broadcast station in the Philip­ pines. A suitable jig around the bedroom was called for. I doubt his parents shared his enthusiasm. A radio station was a radio station and as long as Dad (a farmer) could hear the local news and weather forecasts on 3WV and Mum and younger sister could hear all their serials on 3LK, who cared! G Marconi’s 1-valve radio The set was easy to assemble. It had a “breadboard” layout, using a 9mm A view of the works of the 1-valve receiver. It was built using a simple “breadboard” layout www.siliconchip.com.au Fig.1: the “G. Marconi” was a basic regenerative radio receiver employing a single 3V4 valve. Its output was coupled to lowimpedance headphones via an audio step-down transformer. thick board with all the components and wiring layout stencilled onto the board in black paint. The valve socket had a lug soldered onto each pin. Each pin, along with a Fahnstock clip (made out of brass), was secured to the board with small wood screws. The components and leads were then attached to the vari­ous Fahnstock clips as per the stencilled overlay. No soldering was necessary. Larger components such as the audio output transformer, tuning and reaction capacitors and the coil were mounted sepa­rately. The front panel of the set was printed cardboard. The instruction booklet consisted of six pages not including the front sheet and enough detail was included in the instructions to help constructors get it right first time. All in all, it was a very basic receiver and spartan methods of construction were used. But it was the lad’s first radio and it worked. The circuit (Fig.1) is typical of most one-valve sets of the era, with a few departures from the norm. A capacitor was wired di­rectly between the This is the instruction booklet that came with G. Marconi kit. aerial and the top of the tuned circuit with no aerial coupling winding. A 3V4 valve was used instead of a 1T4. The 1T4 would have used only half as much filament current. Expensive headphones were not included. Fig.2: the instruction book included detailed diagrams on the coil winding. www.siliconchip.com.au An audio step-down transformer was used to couple between the valve and the low-impedance headphone, a cheap single ear­piece unit. It had a metal band that went over the head and dug into the side. It wasn’t comfortable but when you are so enthusiastic it doesn’t matter. Naturally, being so enthusiastic, all sorts of things were tried (much like our early experimenters), with about as much direction as a rudderless ship and with similar results. Ul­timately the set was rebuilt and now resides in a roughly built cabinet that housed the set and the batteries. An on/off switch was added, an aluminium front panel and a phone jack so that better quality headphones could be used. By that time, slightly more comfortable headphones had been obtained. Instead of the Fahnstock clips used for interconnecting the various parts of the circuit, our young enthusiast took to sol­dering the wires. I can tell you he didn’t know much about sol­ dering – dry joints were rather common. He didn’t know much about how to make the metal free of oxides, nor much about how to tin wires. He was October 2001  83 A front view of miniature 2-valve receiver. The design appeared in “Radio & Hobbies” in late 1950s and was capable of driving a loudspeaker. more aware of how plumbers’ soldering irons were used and things like Spirits of Salts (hydrochloric acid). Fortu­ nately, he’d read that the use of such things caused radio wires to be eaten away so that mistake wasn’t made. Receiving faraway stations was of prime interest but the transmitting side of radio was intriguing too. It was known that his radio interfered with reception on his parent’s set when the reaction control was turned up and the set squealed. He’d read somewhere about how transmitters operated and how voice and music were impressed on radio signals. So being a bright young lad, he tried to make a transmitter out of his-one valve set. With the set oscillating and a speaker transformer and speaker connected This is the above-chassis view of the miniature 2-valve receiver. A 20kΩ potentiometer was used to control the amount of reaction. in place of the headphones, he yelled into the speaker. His cousin wandered around the back yard with the family portable radio. He could hear what was being said around 15 metres away over the radio – as well as direct! It was time to swap tasks and our keen enthusiast then heard his “transmitter” too. Satisfaction! There were no other radio enthusiasts for miles, so the idea of transmitting again was put on the back burner until his late teens when he got involved with the Emergency Fire Services (EFS) and was issued with a real transmitter. But that is another story. In recent times I did an overhaul of that set and it still operates quite well. Its tuning range is from 590kHz to 2100kHz and its sensitivity is around 3mV for a reasonable level of audio on received stations. This set does form a special part in my collection – it was my first successful set! Silver fish have eaten part of the booklet that came with the kit but some idea of the detail that was included in it can be seen in the excerpts (Fig.2). The circuit diagram shows that the receiver was very simple; ideal even today for those keen on building replicas. I progressed to making all sorts of things, some that worked and some that didn’t. During this period I purchased a 1000 ohms per volt multi­ meter, and this really did help me sort out any problems that I had. A 2-valve miniature “Radio & Hobbies” had a design for a miniature two-valve radio in the Fig.3: the “Radio & Hobbies” miniature 2-valve receiver. A 1T4 valve was used as a regenerative detector followed by a 3V4 audio output stage. 84  Silicon Chip www.siliconchip.com.au mid to late 50s. I’d about done all the experiments that I could think of with a one-valve set, so a “big” high performance two-valve set was the ideal next step. By this time, I had more experience and had a semblance of an idea of how to lay out a set. The axiom of “keep inputs away from outputs” was gradually seeping into my brain. As can be seen in the circuit diagram, it uses a 1T4 as a regenerative detector followed by a 3V4 audio output stage. It was claimed to be able to drive a loudspeaker on nearer stations. The Reinartz coil was a commercial miniature unit. The regeneration was controlled by a 20kΩ potentiometer across the reaction winding. When the potentiometer wiper is at the end nearest the tuned winding on the circuit, maximum regeneration and oscillation is achieved. Conversely, when the wiper is at the far end, the radio frequency (RF) energy in the plate circuit of the 1T4 is shunted to earth through the 500pF capacitor, hence no regen­eration. In place of a bulky RF choke in the plate lead, a cheap alternative was used; a 20kΩ resistor. In many circuits, a resistor is more practical compared to an RF choke and it is cheaper. An RF choke is essential where very little DC voltage drop across the component can be tolerat­ed, whereas this can be substantial across a resistor. Back bias for the 3V4 was obtained through the 1kΩ resistor and 10µF capacitor network in the negative HT line. The total current drain from the 67.5V battery was 4mA and 150mA from the 1.5V torch cell. The performance of the set is superior to the one-valve set, as it should be. The tuning range is 510kHz to An under-chassis view of the miniature 2-valve receiver. Point-to-point wiring was used between the valve sockets, the coil and the other hardware items. 2,000kHz. Its sensitivity is such that a 300uV signal is heard reasonably well with the detector not oscillating. It is capable of detecting a signal that is one tenth the level required by the onevalve set for the same performance. Extra valves do help. If the detector is oscillating, signals as weak as 3µV can be heard. This goes to show why very simple receivers were quite adequate to hear Morse code signals worldwide years ago. I was very pleased with my miniature set which measured 125mm wide, 110mm high and 85mm deep, including the knobs. It was the smallest set I’d seen and it was complete with the batteries inside the case. I built many other simple receivers, and in the Australian Radio College instructional kit of the 50s, there were many projects to build to help aspiring radio enthusiasts improve their ability. I remember building a one, a two and a three-valve receiver. The three valver was a good performer and even had shortwave on it, which widened my horizon of interest in radio. Like many other projects that I built over the years, they were stripped down to make way for the next one, with the exception of the two above items which are all that remain of my early days in radio. I now regret that I “improved” my little G Marconi set but at that time very few people were interested in Vintage Radio – which is SC a part of our heritage. UM66 SERIES TO-92 SOUND GENERATOR. THESE LOW COST IC’S ARE USED IN MANY TOYS, DOORBELLS AND NOVELTY APPLICATIONS 1-9 $1.10 10-24 $0.99 25+ $0.88 EACH INC GST www.siliconchip.com.au October 2001  85