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Vintage Radio 1928 1928 RCA RCA Radiola Radiola 60 60 superhet superhet radio radio By Dennis Jackson Made during the end of the Roaring 20s, just before the Great Depression, this set by RCA was definitely a luxury item, as showcased by the detailed escutcheon and likely mahogany cabinet. It sold for US$147 and weighed over 20kg – at least they decided not to make it into a portable. Of late, my most prized radio items have come from the collections of friends, who have reason to downsize and have donated parts and sets they constructed as radio amateurs. I firmly believe that such treasures should be shared and brought out into the open, and that is why I wrote this article. Radio technology, electricity and electronics in general mushroomed from small beginnings due to the genius of a dedicated few. As a result, these fields became a significant influence on our lives. fication were added on the input side of the detector. Once that had been done, why not add a couple of stages of transformercoupled audio amplification to boost TRF development in the 1920s Generally speaking, in Australia, the 1920s was the era of the tuned radio frequency (TRF) wireless receiver, at least as far as the general public was concerned. Simple single-valve regenerative receivers, although efficient, had their drawbacks. So typically two stages of separately tuned RF ampli- 100 Silicon Chip the output? That would allow a moving iron, rocking-armature or diaphragmtype horn or cone speaker to be driven, for all to hear. The resulting audio usually left a bit to be desired, sounding rather like a person speaking through a long tube, but who cared? This was wonderful wireless. These early, uncomplicated TRFs were very well suited to athome construction. I recently finished restoring an elegant Planet Waldheim TRF console wireless manufactured during the early 1930s. It came in an elaborate timber cabinet with a lift-up lid to house an optional electric turntable and heavy magnetic pickup, to play the latest 78s. An inverted-tray type iron chassis replaced the usual breadboard. It has three pentode RF valves, a triode detector, two 2A3 triodes work► The RCA Radiola 60 with a Radiola 103 tapestry speaker mounted on top. Australia’s electronics magazine siliconchip.com.au ing in push-pull to drive the electromagnetic moving coil speaker and all three capacitor tuning gangs mounted axially on a single shaft. There’s also a separate AC power supply using an 80 rectifier, with its main antiquity being the use of large paper filter capacitors in place of the yet-to-be-developed electrolytics. I mention this to demonstrate the very rapid advancement in wireless technology which occurred during the twenties, a fascinating period. Wireless sets such as these were the final burst of glory for TRF receivers. Enter the Radiola 60 While I had an example of the first commercial superhet, the RCA AR-812 (described in the August 2019 issue; siliconchip.com.au/Article/11782) and an L2 Ultradyne Superhet from 1924-25, I was missing a superhet radio which represented the late 20s/ early 30s, a time of significant radio technology advancement. The Radiola 60 superheterodyne, sold from September 6th 1928 to September 1930 by the Radio Corporation of America (RCA), fired my imagination. This was the first Superhet to connect to the AC mains via the lighting circuits, making use of the then-new UY227 triodes with indirectly heated 2.5V cathodes. These were designed to avoid the AC hum problems caused by the slight heating and cooling between AC cycles when using directlyheated cathodes. My chances of acquiring such a set were very slim, but miracles do happen; an RCA Radiola 60 eventually became available on eBay for “local pickup only”, which posed a problem as it was in Sydney and I am down in Tassie. Luckily, my two teenage nieces were flatting in Sydney while studying, and they even had a car. That old thing was very heavy to carry up all of those stairs, but they did it for their Uncle Den, who will be forever thankful. The set eventually arrived by courier at our home in Hobart, after the nerve-racking process of last-minute bidding was successful. rounded feet of pressed wood fibre, typical of the 1920s. The separate power supply chassis originally housed three large rectangular cans, one containing four 0.5µF rice paper dielectric capacitors (then referred to as “condensers”), and three similar 2µF caps, plus the final HT filter choke/audio-blocking inductor. One adjacent can contains two series-connected filter input chokes for the HT+ supply. Three filter chokes in series may seem to be overkill, but the paper filter capacitors had pretty low values, so a high inductance was needed to achieve sufficient filtering. The third can originally contained the power transformer with its type 80 rectifier valve alongside. There was a problem with the power transformer overheating after ten minutes or so of use. Its enclosing can was missing, with the unsecured transformer being held in place only by its connecting wires. This may have been the reason the seller had insisted on local pick-up only. The seller had informed me of this problem, and he had assured me the transformer had been designed for our 240V (now officially 230V) mains. A small brass plate on the chassis verified this. RCA had produced mains-powered TRF radio receivers before the Radiola 60 Superhet; namely, the Radiola 17 and 18 in 1917, using a similar chassis and layout. Maybe the composition of the iron laminations had yet to be improved. Some power transformers used silicon steel laminations to reduce hysteresis losses, which otherwise could result in overheating and a loss of efficiency. There do appear to be common problems with the Radiola 60’s power transformer, according to various web forums. I also considered that its problems might have been due to shorted turns, aged and brittle inter-layer insulation and/or leaky paper-dielectric filter capacitors. Before switching on, I changed the twin-conductor power cord to a regulation three-wire type fed through a cord clamp and grommet, and also fitted a 0.75A fuse in the Active line. I bypassed the wafer on/off switch, as I considered that it could be unsafe by modern standards, especially considering that it was nearly a century old. I replaced most of the wire-wound resistors in the power supply; their phenolic cores were charred. I had a look, but I could find no obvious problems within the radio chassis itself. After plugging in the nicely-restored RCA Radiola 103 rocking-armature tapestry speaker (described below), which had been part of the deal, it took a full 90 seconds for the indirectly heated valve cathodes to reach The power supply chassis The internal electronics are contained in an attractive table-top lidded case, which had been refurbished by the previous owner. It boasts prominent pot metal escutcheons, and short siliconchip.com.au The power supply chassis is shown with the new power transformer (right), the defective one is shown below. The small transformer on top of that is a multi-tap from Jaycar with secondary removed and rewound to supply 2.5V. Australia’s electronics magazine December 2020  101 The RCA Radiola 60 was a simply designed and operated radio when compared to others of the time. It had single dial tuning (right), a single control for volume, a power supply integrated into its case and good reception. The only problem might lie in its non-linear tuning range (the frequency division over the MW band is non-linear over the 1-100 scale range). emission temperature. The set then performed well for some minutes, until there was a definite smell of hot tar, so I immediately switched it off. The type 80 rectifier valve draws a filament current of 2A at 5V. To lighten the load on the overheating power transformer, I removed the type 80 and substituted two 800V power diodes. This is a much more efficient arrangement, but has the drawback of causing the full 300V+ open-circuit voltage to be applied to the HT filter components for the minute or two it takes for the valves to conduct and draw the voltage down. This helped, but it did not entirely cure the overheating problem. My next move was to disconnect some leads and measure the resist- ance across the seven 0.5µF/2µF paper dielectric filter capacitors in the power supply cans. All were very leaky, having resistance values of just 100-500kW. So I left them in place but disconnected them, to preserve originality, and substituted 250V polyester types with the same values. I would have preferred to use caps with a higher voltage rating, but didn’t have any on hand. After fifteen minutes of use, the power transformer windings were still becoming reasonably warm to the touch, but as the set would be used only for demonstration purposes, it was probably good enough. Still, I was not happy. I once had wax dripping from a power transformer catch alight during a soak test. Above: the valve layout diagram for the Radiola 60. Left: the internal connections of the filtering, bypass & output condensors and choke of the power supply. 102 Silicon Chip Australia’s electronics magazine There was little hope of me finding a direct replacement, but an idea came to mind. Fixing the overheating transformer I decided that the simplest solution was to replace the original transformer with a modern version with 240V centre-tapped HT windings and 6.3V filament supplies, with the latter voltages reduced by dropping resistors to get 5V AC for the type 71A output valve filaments. I did some experimentation and found that 4.5W series resistors gave 4.5V AC at the filaments, which worked pretty well. The HT winding centre tap also proved suitable to derive the bias for the control grid network. To supply the 2V heaters of the remaining six valves, I decided to use one of those handy multi-tapped dualbobbin transformers available cheaply from electronics stores with a rewound secondary. I unwound the original secondaries to figure out the number of turns per volt, then wound three separate parallel coils of 1.25mm diameter enamelled copper, ten turns each, in the same direction and paralleled. The resulting output was 2.1V, matching the original transformer. I fitted this filament transformer into a small timber box together with a fuseholder in the mains Active lead. siliconchip.com.au Volume Control Tuning Dial R14 450W 450W C10 / C18 0.5µµF 0.5 R10 2kW 2kW 3rd IF C16 1µ 1µF 0kW 2nd IF R11 3kW 3kW R15 4 1st IF R12 40kW 40kW C15 2400pF C17 580-640pF Osc. Tracking C28 (1400kHz) / C27 (600kHz) L2 Mixer L1 RF L3 Local Osc. An oddity with this set was the requirement to remove the tuning capacitor before performing alignment, as the IF and neutralisation adjustment trimmers are under the tuner, along with needing a “dummy” UY-227 (one heater prong removed). This was probably done in standard RCA fashion to prevent meddling. I screwed it to the inside back of the case, opposite the power transformer. A 1.2kW 4W wirewound resistor immediately after the diode rectifiers and before the first input filter choke reduced the HT to around 185V DC, just above the recommended value. This should also give a measure of protection if a major fault occurs. This arrangement is now working reliably and giving surprising results. The B+ plate voltage is somewhat critical because this has some influence over the grid bias to the amplifying valves via the resistive network, of which the volume control is a part. To operate correctly, the anode bend Here is the type 60 IF transformer removed from its can. siliconchip.com.au detector must be biased at cutoff or distortion will occur. Testing the radio The Radiola power supply connects to the receiver via eleven screw terminals. Connecting it up and switching it on resulted in nary a sound from the speaker. I have become accustomed to these antics over the years, so anything which works on the first go frightens me! The set had worked previously; only the power supply had been tinkered with. So I resolved to check all supply voltages, while keeping in mind most of the circuit is floating above chassis, resulting in unusual readings. I have learned to take voltage readings from terminal seven, which goes to the HT centre tap on the power transformer, as this also provides the bias for the valve grids. The voltages seemed to be correct, with all HT readings being around the 170-185V DC mark. I spent a pleasant hour or two checking all valves for emission with a University valve tester. The Radiola 60 originally used all type 27 or UY227 valves in the RF sections; mine collected three type 56 substitute valves at some point. These are drop-in replacements; most tested in the 7080% range. One type 27 proved to have low emission, so I replaced it. Another later proved to be the cause of the Australia’s electronics magazine sound fading away after some minutes of operation. A check along the signal path with my Radio & Hobbies senior signal tracer (June 1954) soon isolated the fault to the area around the first IF, although all seemed in order after a further resistance and voltage check. It was now time to draw upon experience, remembering similar timewasting problems in other early sets due to socket contacts making unreliable contact with valve pins. A gentle wriggle sometimes reveals this problem, but not this time. After removing all the valves and carefully bending the brass contacts closer together, the set finally came slowly back to life, again taking 90 seconds to warm up and another 30 to reach full volume. My next job was to make a record of all working voltages, taken between terminal 7 (the HT centre tap) and the plates, cathodes and grids of all valves on the receiver chassis for future trouble-shooting reference. To better understand the circuit, I redrew the schematic larger using coloured pencils to mark out the various circuit operations and then matched this to the physical layout of the radio. Circuit description This circuit deviates from later Superhet designs by having two RF amplification stages in front of the mixer, or first detector as it was then known. December 2020  103 The Radiola 103 speaker is a rockingarmature type, which at the time were starting to fall behind electrodynamic models (below). This Magnavox R-3 14-inch horn speaker (1922) works well with the Radiola 60, but needs a 6V accumulator to energise its field coil. 104 Silicon Chip This is an autodyne type, suiting the single-purpose triode valves of the twenties and early thirties. This set also appears to use an anode bend detector (or plate detector, as it was known in the USA). The first RF valve is untuned, the aerial being connected directly to its grid. A 2kW resistor to chassis Earth provides a measure of grid bias. All signals from the aerial are amplified. RF transformer T1 is in the form of a tubular coil. It and a variable ganged capacitor tunes the secondary, coupling the selected station RF carrier from the plate of V1 into the grid of V2. A third smaller winding shown below T1 serves as part of a neutralising circuit, preventing parasitic oscillations due to the internal capacitances of V2. The amplified RF signal is coupled into V3 via a similar RF coil RF2, without neutralisation. V3 and T3 form the second detector (mixer), where the incoming RF is mixed with the local oscillator to result in the fixed intermediate frequency (IF) signal. In this set, the IF is 180kHz. The local oscillator is driven by a separate triode (V6), unlike newer sets which use a single purpose-designed pentode to drive the resonant oscillator ‘tank’ and also mix the signals. In this case, the oscillator is a modified Armstrong design. Feedback to keep the tuned circuit running is accomplished inductively via a tickler coil coupled to the valve plate. The first two sets of variable capacitors making up the tuning gang simultaneously tune the transformers feeding V2 and V3 to the station frequency, while the third set controls the oscillator frequency. An adjustable ‘padder’ capacitor, or capacitors, are in series with the oscillator tuning gang to reduce its effective capacitance, so that the local oscillator tracks 180kHz above the tuned station frequency. Following the mixer, there are three single-tuned intermediate stages of fixed RF selectivity and amplification, with the last also serving as an anode-bend detector, biased at or near cutoff by the voltage divider network between the power supply connecting terminals 6 and 7. Probably because indirectly heated output valves were then not readily available, a type 71A directly-heated Australia’s electronics magazine triode supplied from a separate 5V AC heater winding is used as an output valve. This could be a source of mains hum due to slight heating and cooling of the cathode over the mains AC cycles. To reduce this effect, the 5V AC heater circuit is centralized by two 8W resistors, which also form part of the C-negative grid bias circuit. Being at the end of the amplification chain, any induced hum would probably be well below that of the audio signal, minimising its effect. User controls Contrary to earlier wireless sets, controls are sparse on the front panel, mainly because single-point tuning is used and there is no need for filament rheostats. A wirewound volume control pot varies the bias to the grids of the amplifying valves via a resistive network. A simple wafer on/off switch completes the lineup of only three controls. High-value carbon track potentiometers which could be simply inserted into the audio line after the detector seem not to have been available then. Also lacking were electrolytic capacitors for the power supply, and purpose-designed valves for each section were not yet common. It is interesting to note that the grandfather of all valves, the type 80 dual-anode rectifier, was present in one of its forms; it is still used in guitar amplifiers to this day. Overall, I think set designers did an excellent job with what they had to work with. siliconchip.com.au This article would not be complete without a brief description of the Radiola 60’s matching 103 rocking-armature/balanced-armature loudspeaker. Similar speaker cone drivers were used in the model 100 drum type loudspeaker which came out with the first superhet, the AR-812 and the model 100A Mantel, which had a pot metal housing. The mechanism consists of a large horseshoe magnet, the poles of which are continued across to the centre to provide a small magnetic gap bridging both ends of a soft iron armature about 40mm long and 3mm square. This armature, which lays parallel to the arms of the magnet, is firmly pivoted in the middle, having only a minute amount of springy play. It is surrounded by two bobbins of fine wire, each with a 1000W impedance, firmly fixed to the frame at either side of the pivot point and with a small air gap in the centre to allow armature movement. The inner end of the armature is several millimetres longer and connects to a thin rod, transferring vibrations down to the apex of the speaker cone. My interpretation of the action is like two pairs of men sawing in unison at each end of a log pivoted in the middle, on a sawhorse. Both saws always move in opposite directions, but with varying velocity. Both bobbins need to be connected in the correct phase. The allowed movement is so small one must wonder how this mechanism can work at all, but it does, and it gives reasonably good reproduction. The RCA Radiola 60 also works very well with a movingcoil speaker and matching output transformer, plugged in directly. The 103 speaker frame is constructed from pressed and moulded wood fibre with a fabric bonnet covering the works at the back. Conclusion This is a wonderful piece of history. It still looks great, taking pride of place in our living room. When this became available, the public finally had a radio which had singlepoint tuning, making it very simple to operate. And could be heard all over the house. Unlike the RCA AR-812 of 1923 (the first superheterodyne set), there is no double spotting or heterodyne whistles when tuning. The set has a reasonable range and good volume and fidelity. It works well with the thennew moving coil speakers as well as the earlier rockingarmature types. Its price in its first year of sale was $147 US plus speaker; a bit less than half the cost of the AR-812. Last, but certainly not least, this was amongst the first radios to relieve its owners of the tedious and expensive routine of replacing batteries by being mains-powered. Previously, this feature was restricted mainly to AC-powered TRF types like the RCA Radiolas 17 and 18. The Radiola 60 has stood the test of time, and a few are still working in original condition after ninety years or so. I would say that the Radiola 60 was the most electronically advanced of all domestic radios when it first hit the market in September 1928. Although the superhet still had a way to go developmentwise, the Radiola 60 certainly set a precedent proving the supremacy of Edwin Armstrong’s concept, and lead the charge in replacing TRF sets as the standard. A good write-up on the Radiola 60 can be found at: siliconchip.com.au/link/ab4b SC siliconchip.com.au The circuit diagram for the Radiola 60; the power supply section is on the right-hand side of the terminal strip. Many of the capacitors, and some resistors, are unlabelled. This may have been because of the values varying during construction depending on testing by the workers. Every valve in this set except the rectifier is a UY-277 type with an indirectly heated cathode, reducing hum. No reflexing is used in this set. The RCA balanced armature loudspeaker Australia’s electronics magazine December 2020  105