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Vintage Radio UDISCO L6 TRF Radio from 1926 or 27 By Dennis Jackson There was a least one Australian company that developed the batterypowered triode valve TRF wireless receiver of the early 1920s to its limit: UDISCO, the United Distributing Company. When advertising, UDISCO would often draw attention to the single dial control they used. According to the 1993 Electronics Australia publication “The Dawn of Australia’s Radio Broadcasting”, wireless telephony in Australia kicked off after the end of The Great War (also called World War 1) in 1918. The first direct wireless telegraphic messages between England and Australia were received on the 22nd of September 1918 by Ernest Fisk (later Sir), one of the founders of AWA. On the 13th of August 1919, Ernest Fisk demonstrated audible sound reception across Sydney without wires. Various professional experimenters and amateurs were to add their talents to the development of wireless transmission, until licensed broadcasting became available to the general public in early 1923, after the government enacted the necessary regulations. 90 Silicon Chip As one who witnessed the advent of television around 1960, I can imagine the excitement of the times. Peoples’ daily lives were taken up by manual effort, and there was little opportunity to understand the broader world. The cost of owning a wireless was far higher then compared to today. I have memories of conversations with those who lived during that era. One uncle told of his excitement when receiving his first feeble signals after months of experimenting with a simple single-valve regenerative receiving while living on the family farm. But not all were wholly in favour. A common belief was that wireless sets could distract women from their domestic duties during the day or affect peoples’ social lives in the evenings. The simple crystal wireless might Australia's electronics magazine suit a boy lying in bed listening to his favourite cowboy show, but was not of much use in a family situation. Once tuned radio frequency (TRF) sets became available, they were soon the instrument of choice. Two stages of tuned radio frequency amplification selected and amplified the station of choice, followed by a detector to separate the audio signal from its carrier frequency. Two stages of transformer-coupled audio frequency amplification were used to power a loudspeaker, giving a somewhat distorted output of less than 1W, still sufficient to amaze all listeners in an average room. UDISCO’s history UDISCO was founded in Australia in 1911, selling household goods and importing electrical components. siliconchip.com.au Fig.1: this circuit was initially traced by hand with sparse few known values filled in. As we couldn’t find a circuit diagram online for the UDISCO L6, this should be the next best thing. The company produced a wide range of sets between 1925 and 1929, ranging from kit sets sold under the brand name UMAKIT to advanced receivers like the UDISCO Super Six, a TRF set selectively tuning over six wavebands from 2000m to 20m with no gaps. Early sound technology and valveera radio, particularly from the 1920s and 1930s, have fired up my imagination as far back as I can remember. Around 30 years ago, I came by one of my more valued wireless finds at auction. The name engraved in the bottom corner of the front panel reads “UDISCO Model L6 Made in Australia by United Distributors Ltd Patent No. 20643 US No. 1610918 No.176”. The only reference I can find to my UDISCO receiver on the internet is one photo. My first task was to gain some understanding of how it worked. The circuit is more elaborate than the usual simple five-valve TRF receiver. I admit to at first being puzzled, so I began to sketch a rough circuit. Three attempts later, it began to make sense. It’s an upmarket TRF set with single-­ point cable-ganged tuning, housed in a heavy 300mm-high 819 × 380mm solid oak stained cabinet. The cabinet is meant to last for a generation or two and is typical of its time. The circuit has four RF stages with unusual choke-capacitive coupling (including the detector), plus the usual two stages of transformer-coupled audio. siliconchip.com.au The four RF coils are of the binocular type, resembling a single coil cut in half and bent over on itself, each half having an equal number of windings that are effectively wound in opposite directions. Both confined RF fields are intended to oppose each other, preventing interaction between adjacent coils and unwanted incoming signals making neutralising, shielding and angular placement unnecessary. The four sets of binocular tuning coils are mounted out of sight under one of the four sub-panels fixed to the baseboard. As can be seen from the circuit diagram (Fig.1), the first half of the first binocular tuning coil (L1) is tapped and switched to facilitate aerial matching. The binocular tuning coils seem to work, making this set very stable in operation with clear reproduction. It tunes stations without the howling or whistling common in TRF sets using simple triode valves. The choke-capacitive coupling between the four RF valves is via four large circular honeycomb-wound RF chokes mounted between the sub-­ panels and the front panel. They block RF from the B+ 90V supply and divert the RF signal through a 4nF mica capacitor to the tuned grid of the next stage. Valve lineup Most Australian-built radios from the 1920s that I have seen used Philips Bakelite-based triode valves with dumpy glass envelopes. This set initially used Philips A609 6V triode valves, designed for use with a 6V accumulator for the A filament supply. The A609 was first manufactured An example photograph of the ‘binocular’ type RF coils. Australia's electronics magazine January 2023  91 in 1926, the same year as this set. Its oxide-coated filament drew only 60mA and used the same four-pin base as the popular USA-manufactured UX201A. The UX201A had a thoriated filament drawing 250mA at 5V, making them interchangeable with some adjustment of the filament rheostats, while Philips had a sales advantage due to reduced battery drain. Mounted along the top edges of sub-panels two, three and four are six metallic tubular adjustable capacitors of a few picofarads each. My interpretation of their purpose is that C14, C16 & C18 provide a small measure of feedback to their respective valves giving some regeneration. C15, C17 & C19 also appear to be part of this network. This is just an educated guess, they could actually be for balancing out inter-electrode capacitance within the RF valves. Possibly confirming my determination, removing the adjustable sliding rods inside the insulated tubes gives a modest reduction in sound volume. The grid bias to V1, V2 & V3 is from the negative filament line via taps on tuning coils L1, L6 & L9. Controls The front panel looks uninteresting, with only two controls. There is a reduction dial for tuning and directly under it, a smaller knob for adjusting the rheostat (R1) controlling the plate-anode current to the four RF valves for volume control. R1 is bridged by capacitors marked C21 and C22, which are in series and centre tapped, going to the positive filament line. Choke L13 in the anode circuit of V4 has the primary of L14 (the first audio transformer) taken from its more positive side instead of directly from the anode, as one might imagine, but it works better that way. A long, narrow sub-panel just under the hinged lid holds additional knobs. The first on the right controls a vaned trimmer capacitor (C1) across the first of the four tuning capacitors, to adjust for any misalignment as stations are tuned across the bands. The second knob controls a rheostat (R2) in series with the A+ battery supply to adjust the valve heater current according to the battery voltage. It also affects the volume (along with the external knob mentioned earlier). Knobs three (C2), four (C3) and five (C4) perform similar functions as knob one, tuning capacitor trimmers. Knob six (C9), marked “control”, adjusts the positive feedback from the plate of V5 to the grid of V4 (the detector) to provide regeneration. Restoration Opening up the lid reveals a series of aditional knobs connected to the chassis. The knob at the top of this photo is connected to trimmer capacitor C1. 92 Silicon Chip Australia's electronics magazine At least one of the audio coupling transformers was replaced sometime during the history of this set. I also noticed that, at some point, radio-­ frequency choke L3 had been added across choke L4. It seemed unnecessary, so I removed it. I inspected the set and couldn’t spot any more apparent problems, so I decided to switch it on and see if it worked. I connected to an aerial and Earth plus my most trusted horn speaker before wiring in my special battery eliminator power supply and making voltage adjustments. As is typical, there was not even a buzz, and no amount of knob twiddling could coax this set into the faintest whisper. I should have performed a closer inspection by checking the voltages on the valve pin sockets. Using a signal tracer, I found a signal at the grid of V1 but none at the plate. Also, detector V4 lacked HT on the plate, indicating there were open-circuit anode chokes. The very fine-gauge cotton-­covered wire used in these large honeycomb-wound coils was adrift from the respective terminals. Worse, both siliconchip.com.au C1 R2 C2 C5-C8 C4 Reaction Feedback Bias C22 C21 Adjustable Capacitors L14 L15 Anode choke coils I removed the chassis from the cabinet to effect some repairs (the 9V battery was used for testing and is not part of the set). Four of the six knobs adjust trimmer capacitors across the tuning caps. wires on L4 had broken close to the coil. The outer was easy enough to pick up, but the inner close to the coil former had only a couple of millimetres of stub left. With no second chances, several careful scrapes with a razor blade exposed a streak of clean copper and I gently added a dab of solder to join another thin wire. I then added a small blob of Blu Tack to keep it rigid. But there was still no continuity. With fading hopes, I decided on a closer inspection under a large magnifying glass with good light. A tiny green spot of verdigris was visible. I poked it with a needle to expose two short, stubby wire ends, which I then bridged and set in place with another blob of Blu Tack. It then had continuity which was a considerable relief. A tiny drop of acid solder flux, probably splattering during manufacture, had corroded the wire through in subsequent years. That explained why RF choke L3 had been soldered across it. Now that I’d fixed L4, it was no longer necessary. With both anode choke coils now repaired, there should have been some siliconchip.com.au response from the horn speaker, but it is never that easy. I re-checked everything twice more; all seemed good, but there was still no response. One or more of the six A609 valves must be low on emission, so I would have to set up my Paton valve tester, which has a four-pin UX socket to suit these early triodes. All valves displayed less than 50% emission, with one being a total dud, probably resulting in this set’s retirement. These old Philips 6V triodes are very seldom for sale now. I keep a few known-good UX201A 5V triodes for replacements, so I fitted them after re-adjusting the filament ‘A’ supply. This brought forth a hint of croaky reception from the ancient horn speaker. Some careful adjustment of the single tuning control on the front panel, together with the anode voltage rheostat and then all six knobs along the under-lid sub-panel, resulted in surprisingly ample sound. A rocking armature speaker gave a less strident output; no doubt, a further improvement could be obtained by fitting a moving-coil unit through its output transformer. Australia's electronics magazine The similar AWA Radiola C54 At this stage, I remembered that I had previously purchased an AWA Radiola Battery Six model C54 from around 1928. Electronically, it is a similar set but probably as basic as a six-valve TRF wireless could be. The point of interest was that the model C54 also used four sets of binocular RF tuning coils. In that set, the more typical inductive coupling was used between all four RF stages instead of choke-­ capacitive coupling. I decided to try to get both TRF sets working so I could compare their performance. Unfortunately, both coils in each AWA audio coupling transformer had gone open circuit. Someone had worked around that by inserting the high impedance speaker in the HT circuit of the first audio valve and feeding its grid through a 100nF capacitor from the detector anode. The final audio stage had simply been disconnected, making this a five-valve set instead of the original six. I have successfully rewound open circuit windings on audio coupling transformers using very fine enamelled January 2023  93 An advert from Wireless Weekly, June 1927 showing a UDISCO Neutrola which uses a case that is very similar to the L6. copper wire (0.1mm/4-thou diameter). Patience and a gentle touch are required. Fortunately, I was able to scrounge two working replacements from the junk boxes of friends, with the originals perhaps to be rewound sometime in the future. Two of the six valves were missing. I tested the remaining four British Marconi Osram valves for emission, and three came up good. The Philips B406 appeared to be similar, and once added and everything connected and tuned in correctly, this set now gives good reception for our two main local stations. Both sets are inaudible when the aerial is removed and are free of any sign of oscillation in everyday usage, possibly due to the use of binocular tuning coils. Sensitivity is limited in TRF sets due to the low RF gain of the front-end when compared to my two superheterodynes from the same period. Still, the output volume is good considering the meagre gain of these early triode valves, particularly in the output stages. Substituting a 71A or a UX112A power output valve (both have 5V 0.25A filaments) gives a noticeable increase in audio volume. These valves are compatible with UX201A types and became available in early 1927. In conclusion, the UDISCO model L6 is a good user-friendly receiver, making up for its plainness in ornamentation by its sheer bulk, complexity and exceptional performance. SC A photograph of the AWA Radiola model C54. Like the UDISCO L6, it is also a six-valve TRF set and uses four sets of binocular tuning coils. 94 Silicon Chip Australia's electronics magazine siliconchip.com.au