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Vintage Radio Philips Minstrel radios By Assoc. Prof. Graham Parslow The Minstrel series of radios from Philips in the early 1950s was intended to be affordable and cheerful. Comparable kitchen radios are the Astor Mickey, HMV Little Nipper, AWA model 467MA and Healing model 404. The Philips Minstrel cost £21, similar to its four-valve competitors. 84 Silicon Chip Australia's electronics magazine I managed to get a copy of the original advertisement showing the nine beautiful colours that the cabinet came in via Glen Oriss, a member of the Facebook group The Real Bakelite and Antique Radio Page. As well as the models noted in the introduction, Philips also competed against themselves with the Jubilee model 122. But this comparable fourvalve radio was much more conservatively styled in dark Bakelite. The performance of these radios is excellent on local stations due to the progressive refinement of the three valves in the radio circuit (the fourth valve is the high-tension rectifier). More expensive five-valve sets added audio preamplification and are usually indistinguishable in performance for city locations. The Minstrel radios were moulded from solid-colour plastics. These new polymers were used in many items in the 1950s when plastic was fantastic. Before these plastics, light colours were often produced as factory-­ painted Bakelite. The standard Minstrel is the model 138. When they added a clock, it became the Chronoradio model 145. The circuit and construction of the two models are otherwise almost identical; the clock radio has an additional socket on the chassis that allows the synchronous-motor clock to connect to the 240V 50Hz mains and switch the radio on at set times. The clock radio has a low profile 4-inch speaker (100mm) mounted at the top and delivers sound through a grille moulded into the top of the case. This produces reasonable sound, but not as good as the 5-inch (125mm) speaker mounted at the front of model 138. The station markings on the dial depended on the target states for sale. The blue radio pictured opposite features WA and SA stations. The clock radio included at the end of the article has all states on the one dial. Circuit details The original circuit for the model 138 clock radio is shown in Fig.1. The aerial coils of the 1950s were well-evolved to make the best of whatever aerial was connected. The Radiotron Designer’s Handbook 4th Edition from 1957 says, in the summary of design for aerial coils: “It can be seen that the common siliconchip.com.au Fig.1: the circuit for the Philips Minstrel Four 138 radio is nearly identical to the model 145. The 145 has one less tap on the secondary of the power transformer (L14 is removed) and some of the resistors have been changed by ~10% in value. An excerpt from the model 145 circuit showing the clock portion is shown on the left-side with a grey fill. loose coupled primary and secondary for MW radios is most satisfactory because it readily lends itself, with minor modifications, to applications using balanced or unbalanced aerial systems.” Random lengths of wire connected to a domestic radio certainly fit into the unbalanced category. In looking at the aerial coil circuit for this radio, I was motivated to dig a bit deeper to work out what the hook shape at the top of pin 2 indicates. Whatever it is, it was logical that it would be equivalent to many other front-ends that connect a 15pF capacitor between pins 2 and 3. C1 (100pF) makes a resonant circuit with L1, and ideally, that resonant frequency will be below 550kHz at the bottom of the MW band. This avoids an impedance peak in the MW band that would give uneven matching between primary and secondary over the span of the band. Even so, the signal coupling will deteriorate as tuning goes from 550kHz to 1600kHz. Adding a small capacitor between pins 2 and 3 boosts the signal at higher frequencies to even out the sensitivity over the MW band. I went to my salvage shelf and found another Philips radio with the same siliconchip.com.au aerial coil and removed it. I melted off the protective wax covering using a heat gun to reveal the wire connections shown in Fig.2. The primary of this transformer measured 25W and the secondary 2W. At first, this seems a paradox until looking at the wire gauge in the different coils. The primary has many more turns of fine-gauge wire, giving a higher inductance than the secondary, so its resonance with a 100pF capacitor (C1) is below 550kHz. The separation between the primary and secondaries (loose coupling) makes the tuning characteristics more robust to whatever aerial is connected to the primary. This Minstrel is originally blue; the colour is solid through the case. Re-sprayed radios can usually be detected by having a different colour inside the case. Australia's electronics magazine November 2022  85 100pF 3 4 4 2 Aerial coil with wax coating Secondary To pin 1 Primary 3 Loop of gimmick wire between secondary coils connects to pin 2 To pin 2 Connection between loops of the secondary Fig.2: an aerial coil taken from another Philips radio. The protective wax coating was melted off to reveal the connections shown above. At last, the nature of that hooked line (a gimmick) on the circuit diagram from pin 2 to 3 was revealed. It is a loop of wire sandwiched between the secondary coils and provides capacitive coupling to augment the higher MW band frequencies. Valve lineup V1, the mixer valve, is a 6AN7, possibly the most common valve for this application through the 1950s. Philips released this 9-pin miniature triode-hexode valve in 1949, so it was new technology for the Minstrel. The local oscillator (L3 and L4) is an Armstrong type with feedback from the triode anode to sustain oscillation. The double-gang tuning capacitor is the compact brass-plate type introduced by Philips in the early 1950s and used right up to the early Philips transistor radios. The compact IF coils in the Minstrel were another new standard for Philips radios that would span the 1950s. These IF transformers are configured to tune both the primary and secondary with slugs adjusted at the top. They are a cause for some apprehension because the IF coils are set in resin, so the common occurrence of open circuits due to spot corrosion condemns them to the bin. V2, the pentode IF amplifier and twin diode detector, is a 6N8. This is also a Philips-designed valve, released in 1949. The 6N8 is not reflexed as an audio amplifier, so the detected signal is passed directly to the 6M5 output pentode via potentiometer R7 (0.5MW). Unsurprisingly, the 6M5 (V3) is another Philips design released in 1949. At the anode voltage of around 210V used in the Minstrel, the audio output from the 6M5 is comfortably 2W. This is a fair match to the 5-inch (125mm) round speaker made by Philips that fits snugly into the moulding in the case. R13 (160W) generates a negative bias for the 6M5 of -6.5V. R13 also serves to generate a negative grid bias voltage for the 6AN7 and 6N8. The first two valves additionally receive negative feedback (AGC) from the audio detector. Follow the circuit from the intersection of R4 and R5 to trace the AGC. Because of R13, the AGC does not work on weak signals and comes into The front of the model 138 chassis, with the power transformer visible on top. 86 Silicon Chip Australia's electronics magazine effect as signal strength increases (ie, this set has delayed AGC). The high-tension (HT) circuit is conventional using a 6V4 valve (V4). The Minstrels were assembled at Hendon in Adelaide, and the components were largely Australian-made (including valves) with occasional European imported stock. A European-made EZ82, equivalent to the 6V4, can also be found in these sets. Interestingly, the indirectly-­ heated 6V4 valve has the heater powered by its own 6.3V transformer winding. This allows the heater and cathode to be connected to avoid any high tension arc-over between these elements. Later Minstrels had an alternative transformer with only one 6.3V winding and no connection between the cathode and heater. There is no tone control and no feedback from the speaker to modify the tone and maintain stability. Even so, the sound is cheerful. The radios typically consume 28-30W; included in that figure is the power for a single dial lamp. The hardware Disassembling a Philips radio is invariably a challenge. A minor nuisance with the Minstrel is that the captive speaker obliges the connecting wires to be desoldered. A trap for the unwary is to overlook disconnecting the dial cursor from the dial string before pulling the chassis out. Forcing the chassis out breaks the dial string, and restringing these radios is one of life’s greater challenges, particularly without the stringing diagram. The underside of the model 138 chassis, showing the output transformer. siliconchip.com.au When the chassis is out, these units are relatively easy to work on because they are happy to stand up resting on the power transformer at the bottom. The filter electrolytics (2 x 24μF) are both mounted in a single can. With the original paper capacitors in place, the 6N8 and 6M5 bases are inaccessible. Fortunately, it is only a minor chore to replace the old units with small modern capacitors and continue with a complete re-cap. Two Minstrel radios I have worked on had C10 measuring negligible capacitance, preventing the frontend tuning circuit from functioning. C10 is an Earth return from pin 4 of the aerial coil. It is soldered into a cramped position with one lead tightly folded back, probably pulling an endcap away from the internal foil with time and heat. Case restoration One Minstrel that I acquired on eBay was apparently posted via the post office branch that assesses survivability after ‘robust’ handling. It was packed in a cardboard carton with only crumpled newspaper pages for padding. The outcome was instructive (or should that be ‘destructive’?). Fortunately, I was able to glue the shards of the case back in place reasonably well with thin-CA (cyanoacrylate) glue. After that, I applied a twopart epoxy body filler, then abraded it back to a smooth finish. I then sprayed it with an undercoat, sanded it back and repeated. I needed to make four applications before I was happy with the adhesion and quality of the surface. I then finished the radio in powder While the case arrived cracked, it glued back together quite well. blue, a slightly lighter shade than the original Philips blue. In the end, there was no external hint of the distress suffered by the case. The inside of the case was left cream so that it could not be passed off at a future time as an original blue radio. The clock radio All major manufacturers offered a clock variant of their low-end models so that they could serve as a kitchen or bedroom set. The clock radio shown below did not work when I received it, due to a faulty capacitor C10, which was not a surprise. However, another unexpected fault was a 27kW resistor installed as a replacement for two 50kW resistors in parallel (R2 and R3). This determines the screen voltage to the 6AN7 and 6N8, and the screens should be 55V. On this radio, it measured only 40V. Replacing it with an 18kW resistor restored the correct screen voltage. The clock is accommodated by moving the speaker to the top and using a low-profile clock. There are no markings to indicate where those clocks were sourced or made. Most Australian clock radios use a Smiths synchronous movement that is too bulky for the limited space at the front of this radio. The 4-inch Alnico speaker in the clock radio was carefully chosen to fit between the chassis and clock at the top of the case. To be fair, the installed speaker did an adequate job for kitchen or bedroom listening. I tried replacing the original speaker with others that had better specifications, but they fouled the clock. The conservative Minstrel case was utterly compliant with the norm at the time – a rectangular shape with rounded edges. The distinctive niche of the Minstrel radios was to introduce the world of coloured plastics to radios SC made by Philips in Australia. The Philips model 145 radio also includes a clock on the dial. siliconchip.com.au Australia's electronics magazine November 2022  87