Fullscreen HTML5Med Res (??MB)HTML4

Vintage Radio Silvertone Model 18 AM/FM mantel radio from 1952 The Silvertone model 18 is an excellent example of a 1952 US radio. The plastic case shows some heritage from the Bakelite era. From left-toright, the knob controls are for volume, AM-FM selection and tuning. By Associate Professor Graham Parslow M any Bakelite radios of similar appearance were made in the 1940s. However, thermo-mouldable plastics, which were new in the early 1950s, were significantly cheaper and faster to produce. The old celluloid dial covers that degraded to become brittle and opaque were replaced by clear polystyrene that is naturally transparent. One downside to polystyrene is that organic solvents, including acetone, can degrade the surface, leaving permanent splotches. The dial cover on this radio is in excellent condition after seven decades. Brass and gold-tone features were common on US radios at this time, as embodied by the dial cursor, the stylistic “S” and the knobs. US dials were calibrated by frequency, not call signs, due to the sheer number of stations that would clutter the dial. Transmissions at the same frequency should not be offered as the reason. There is no problem with putting multiple stations at the same frequency on the dial because they would be at considerable geographical separation. Silvertone is a house brand This label attached to the interior of the Silvertone 18 case has company information, the serial number and some basic servicing guidelines for the set. siliconchip.com.au Australia's electronics magazine The label glued to the bottom identifies the radio as Silvertone catalog number 18, October 1952. The list price was US$37.95. Sears, Roebuck and Co is an American chain of department stores founded in 1892 by Richard Sears and Alvah Roebuck. The company began as a mail-order catalogue company, progressing to retail locations from 1925, beginning in Chicago. The 110-storey Sears Tower in Chicago (now known as the Willis Tower) was the tallest in the world in 1974. Sears filed for Chapter 11 bankruptcy in October 2018, but a restructure allowed them to continue trading at a reduced scale. August 2025  91 Fig.1: the FM tuner is across the top of the circuit diagram, with the AM section below it and the power supply at lower middle. A 3PDT switch selects between FM and AM modes; it also switches valve V4 so that it operates as an FM IF amplifier in FM mode and an AM IF amplifier in AM mode. Capacitor C38 allows the mains cord to operate as an FM antenna for strong stations. Sears and Roebuck had used the Silvertone brand going back to the 1930s. Howard W. Sams & Co were the radio manufacturers in this case. Likewise, in Australia, Myer stores contracted manufacturers of convenience to produce the in-house Aristone branded radios. Commendably, this radio has the circuit pasted onto the side of the 92 Silicon Chip case. The service notes provided by the manufacturer can be downloaded from siliconchip.au/link/ac1x From the ten pages of impressively detailed service notes, one page is shown here that itemises components on the top of the chassis. The photo of the top of the chassis also shown overleaf shows the tuning capacitor shield in place. Australia's electronics magazine Another page of the documentation shows the itemisation of components beneath the chassis. I have not seen documentation from any Australian radio manufacturer of the period as comprehensive as this. The history of FM radio in the United States Edwin Armstrong (born December siliconchip.com.au 18, 1890) served with the US army in France during WW1. During this period, he developed the superheterodyne receiver system. The superheterodyne radio shifts the high-­ frequency radio signal of interest to a lower ‘intermediate’ frequency. The original aim was to get the frequency down to a range better suited to amplification by early triode valves. siliconchip.com.au It had the serendipitous effect of achieving precise tuning at the broadcast frequency, that otherwise needed a cascade of tuned circuits (called TRF circuits, TRF standing for tuned radio frequency). In this radio’s circuit (Fig.1), that mixing is performed by ‘converter’ valve V3 (a 6BE6 heptode). It has two control grids, one connected to pin 1 Australia's electronics magazine and one at pin 7. The incoming signal, tuned by variable capacitor A6 and the secondary of transformer L8, is applied to pin 7. At the same time, a transformer-­ coupled oscillator circuit, developed by Armstrong and named after him, acts as the local oscillator, which tracks at a higher frequency than the tuned signal (a fixed interval above). August 2025  93 The oscillator is formed by transformer L9 and tuning capacitor A5, and its output is applied to the control grid at pin 1. Feedback to maintain oscillation comes from the valve’s cathode, at pin 2. The alternative Hartley oscillator is cheaper, using a coil with only one tapped winding, but the Armstrong oscillator has proved highly reliable and is more commonly chosen. The output at the anode (pin 5) contains the amplified RF signal, oscillator signal, plus their sum and difference products. It is the difference product, at the intermediate frequency, that passes through the following tuned stages to ultimately be demodulated to produce audio. Introducing FM radio Armstrong began developing FM (frequency modulation) based radio in 1928, and argued strenuously for its adoption to replace AM. The time was not ripe, and early attempts to commercialise FM in 1941 in the USA faltered, in part due to allocating only 40 channels spanning 42–50MHz. In 1945, the FM band was reassigned to 88–106MHz. By 1952, limited FM stations were transmitting, so most buyers would pick a cheaper AM-only radio. Stereo FM broadcasting in the US took off in the 1960s, and by the 1970s became the dominant music source, relegating many AM stations to talk-only programs. FM has the virtue of rejecting the majority of the electromagnetic interference (EMI) that plays havoc with AM transmissions. As a result, the signal-­to-noise ratios are far superior to AM. Another virtue is high fidelity, transmitting the full range of human hearing, due in part to greater frequency separation between stations. Unfortunately, the 5-inch (127mm) speaker in this radio does little justice to the potential for high fidelity. Power supply The top view of the Silvertone 18 chassis and a matching diagram which has every component (on this side) labelled. Note the large tuning capacitor shield, which was removed in the diagram. 94 Silicon Chip Australia's electronics magazine US radios of the period were commonly transformerless, using serieswired valve heaters with the high-­ tension rectified directly from the mains. Mains-direct radios are hazardous to work with, particular if an auto-transformer (variac) is used. Happily, this radio uses a transformer to interface with the US mains that is nominally 117V. This radio was restored using a step-down transformer. Fortunately, the transformer on this radio was substantial enough to not heat up excessively with a 50Hz source rather than 60Hz. The mains supply in the USA is nominally 110120V at 60Hz. 220-240V is also available for larger appliances, due to there being two out-ofphase 110-120V conductors in the grid. Because of the lower frequency of our mains, even if the voltage is adapted using a step-down transformer, some US equipment will not be happy running at 50Hz. There is typically less iron in their transformer cores, as less is required given the higher operating frequency. siliconchip.com.au They can therefore saturate at lower than expected currents and overheat when running from 50Hz mains. Thankfully, the transformer in this set seems to have a generous core and that did not appear to be a problem during my testing, with the transformer remaining cool. The rear of the radio shows the AM loop antenna with a threescrew connection strip below. The connections are for an external AM antenna, external FM antenna and Earth. In my location, the loop antenna was all that was needed for AM. The AM receiver The tuning capacitor, designated A6, tunes from 540kHz to 1600kHz, while tuning capacitor A5 tunes the oscillator from 995kHz to 2055kHz to generate a 455KHz intermediate frequency (IF). As mentioned earlier, the IF signal emanates from the anode of the 6BE6 mixer valve (V3). The first IF transformer is A3/A4 (L12), delivering the IF signal to V4, a 6BA6 IF amplifier. The second IF transformer, A1/A2 (L14), passes its output to detector valve V6, a 6T8. When the switch is set to AM, detected audio passes via L14 to the volume control potentiometer, R1. The dial for this radio is printed onto a metal sheet, and is in excellent condition given its age. The FM receiver The FM section can work on high signal stations without an external aerial due to coupling the RF input to the mains lead via 100pF capacitor C38 (ie, the mains lead acts as an antenna if the station is strong enough). The untuned RF signal is amplified by V1, a 6BA6. The desired signal in the 88-108MHz band is tuned by variable capacitor A13 and heterodyned with the output from the oscillator, tuned by A12 and L6. The intermediate frequency is 10.7MHz, so the oscillator tracks 10.7MHz above the tuned RF signal. The first half of the 12AT7 (V2) is the converter, with the RF signal fed to the grid (pin 1). The same grid receives local oscillator input generated by the A12 cluster on the circuit diagram. The second half of the 12AT7 is an IF amplifier. V4 is an additional IF amplifier that does double service as an AM IF amplifier, depending on the band that is selected. The 6BA6 designated V5 is another IF amplifier. V6, the 6T8 ratio siliconchip.com.au The back view of the chassis shows the loop antenna and the external antenna terminal (the yellow wire). The second terminal is for an optional FM antenna and the third is Earth. detector, generates detected audio at R21 that passes via the FM selector switch to the volume control pot, R1. A ratio detector has two diodes conducting in opposite directions connected to a centre-tapped transformer secondary. In this case, both diodes are within a single envelope (V6). Ratio detectors have the significant advantage for FM demodulation that they do not respond to AM signals, making them more resistant to interference. Australia's electronics magazine If this was a full-wave rectifier, the output would be one polarity, but a ratio detector passes positive signals at one diode and negative at the other. The output is the sum of the diode voltages and the centre tap voltage. The output signal from across the diodes is filtered by a high-value capacitor, C3 (4µF) in this radio. It is bled to Earth by R20 (1.5kW). The combination of the capacitor’s opposition to voltage changes August 2025  95 The other two 6BA6s also measured as open circuits on their heaters. Previously swapping 6BA6s within the radio had done nothing; it now became clear why. It was shades of the movie True Lies – they were all bad! Inserting two replacement 6BA6 valves into the FM section instantly produced FM reception. Why would three identical valves all fail? My best guess is that some transient surge blew the most vulnerable heater filaments in the 6.3V AC line. Perhaps someone connected the radio to 230-240V. There was a crack in the case that was repaired using two-part epoxy car filler. This photo was taken before the epoxy was refined using an angle grinder. and the resistive loading produces a nearly constant amplitude for the output. This action gives FM its superior immunity to electrical interference. The set applies AGC for AM operation as usual, but it also applies AGC in FM operation. FM usually relies on the last IF stage being driven into overdrive and acting as a limiter to deliver a constant-amplitude signal to the ratio detector. The use of AGC implies that the last stage does not provide limiting for all FM signal levels, so it needs the AGC to provide the same volume for all stations. For FM, the audio output is converted to a DC level by 220kW resistor R23 and 5mF capacitor C17 and is used to bias the input signal and the signal applied to the first IF amplifier, V4. That path is disabled when V4 is used as an AM IF amp. (the final working power was 49W). So something was not drawing (enough) power. Valve V4 (6BA6) is common to both the AM and FM functions. With AM selected, injecting a 400Hz-modulated signal at the IF frequency of 455kHz to the grid produced nothing. However, a signal to the anode passed through as 400Hz audio. So it was a matter of working backwards to find the fault. Fortunately, it soon became evident what the problem was. V4’s 10kW screen resistor had 0V across it, as did its 68W cathode resistor. Everything indicated a non-­ conductive valve V4. This is classic for a non-functional cathode heater. Sure enough, its heater pins 3 and 4 were open-circuit. Replacing V4 from my stock got the AM function working, but not FM. Case restoration Examination of the crack at the bottom of the right-hand side revealed that a substantial chunk was missing. I fixed this gap using two-part epoxy car filler. To achieve this, I covered a section of aluminium sheet with a 90° flange in masking tape to make removal of the former easy after the epoxy had set. I laid the radio on its side and filled the gap coarsely with epoxy. The external surface of the radio had the epoxy set flat by conforming to the former. Once it had hardened, I used an angle grinder to profile the inside to size. I then painted over the pink epoxy with satin black paint, and it effectively disappeared. Conclusion The Silvertone 18 is a high-quality set with good performance. Its style is of its time, but offering FM was defiSC nitely ahead of its time. All three 6BA6s in the set had opencircuit heaters. Replacing them was all I had to do to get the set working. The audio section After the volume control, a 6T8 triode acts as a preamplifier for the 6V6 beam tetrode, V7. This 6T8 triode is actually part of V6; the 6T8 encapsulates one triode and three diodes. The 5-inch (127mm) speaker has an impedance of 3.5W. There is no negative feedback from the speaker transformer secondary, so the distortion due to the transformer is not reduced. Electrical restoration When I got it, the radio was dead. I ruled out the usual causes of complete failure in my preliminary assessment. The AM/FM switch checked out OK and the audio circuitry worked from the slider on the volume pot. The valves were all well-seated and HT was good at 201V/182V across the π filter. The initial power draw was 39W 96 Silicon Chip Australia's electronics magazine siliconchip.com.au