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Vintage Radio The National R-70 Panapet AM Radio By Ian Batty That’s no moon... The National (Panasonic) R-70 Panapet is a sixtransistor superhet shaped into a unique spherical case (pictured at the centre). We even have a “blue moon” immediately to its right. I reckon I know how not to sell a radio: “This offering is a boring old six-transistor superhet with an autodyne converter, two intermediate frequency amplifiers which are necessitated by the limited stage gain of around 30dB per stage, blah, blah, blah…” By the time the Panapet was released, anything apart from the ‘standard six’ was unusual and would need extra investment to make it work. So Panasonic used the combination of a highly unusual design and a special occasion to sell the Panapet. It looks remarkable – maybe siliconchip.com.au nobody trusted the chain well enough to snap the key ring over a belt loop and let the radio swing about on the end of the chain, but it must have been tempting! The recessed tuning dial, added to the two silvered control knobs, really do make it look like some kind of weird ‘pet’ just begging to be given a home. It was released in the early 1970s; if they had only waited a few years, they could have called it the Death Star and the shelves would have been emptied pronto! It was released in bold colours: red, blue, green, yellow and white. There Australia's electronics magazine was also an elusive purple version, which is pretty rare. So why not maximise its impact by showing it off at an international occasion? How about the 1970 World Expo (https://w.wiki/Ay$K) in Osaka, Japan? With visitors from across the world coming to a six-month-long festival promising “Progress and Harmony for Mankind”, what better time and place to present this cheeky offering, and showcase Japanese design? A review from Future Forms states, “First exhibited at the World Expo in Osaka, the Panapet perfectly captured the playful pop spirit of the early March 2025  101 1970s. With its boldly futuristic spherical design and space age styling, it was an instant hit with the young and youthful-at-heart when it burst onto the scene” (siliconchip.au/link/ac1t). Circuit description This radio follows the design that had stabilised by the mid-1960s. As shown in Fig.1, it’s the familiar six transistor superhet. Although the R-70 uses PNP transistors throughout, ground connects to battery negative. While this does not affect the set’s operation, all emitters go to the supply and all collectors go to ground. Where we’d usually find emitter voltages of up to 2V and collector voltages close to supply, the R-70 upends that idea. Converter TR1, a 2SA102, is a drift type developed from the successful alloyed-junction design (as detailed in my article on transistors in the April 2022 issue – siliconchip.au/ Article/15272). Drift transistors used graded doping across the base area, giving improved high-frequency performance. The 2SA102 offers a minimum transition frequency of 20MHz, compared to the OC44’s 7.5MHz. This circuit uses collector-to-base feedback. It’s pretty much a signature non-European design. I’m making that distinction as most Australian, European and US designs continue the plan used in the first transistor radio, the Regency TR-1, which used feedback to the emitter. 102 Silicon Chip Operating the local oscillator (LO) in grounded-base ensured that the grown-junction converter, with its limited high-frequency specification, would operate reliably over the broadcast band. Base-injected circuits have stopped working in the past when I’ve dropped my signal injector onto the converter base, so I’ve developed a workaround. This set’s LO tuning capacitor section uses the cut-plate design. As this naturally forces the LO to track 455kHz above the incoming signal frequency, no padder capacitor is needed. Transistor TR1 appears to work with almost zero bias, but that implies that it’s working close to Class B, as we’d expect with an autodyne (self-­ oscillating) converter stage. The component side of the R-70; note the two output transistors sandwiched between the two transformers at the bottom of the PCB. Australia's electronics magazine siliconchip.com.au Fig.1: the R-70 Panapet circuit diagram with suggested test points and expected voltages. It’s ‘upside-down’ with ground at the top and the positive supply at the bottom, because that’s how the original was drawn. Slug Colour Function Red Local oscillator Yellow First IF White Second IF Black TR1 feeds the tuned, tapped primary of first intermediate frequency (IF) transformer T1, in the familiar ‘silver can’. It is permeability tuned by an adjustable ferrite slug. T1’s secondary feeds the base of first IF amplifier transistor TR2. As this has automatic gain control (AGC) applied, its base resistor (R4) has a relatively high value of 100kW. This allows the AGC control voltage to significantly reduce TR2’s bias on strong signals, thus reducing the stage gain and helping to keep the audio output constant across a range of station strengths, from weak to strong. TR2 feeds the tuned, tapped primary of second IF transformer T2. Like T1, it’s the familiar silver can type. T2’s untuned, untapped secondary feeds The R-70 uses a simple design for the dial. siliconchip.com.au Australia's electronics magazine Third (final) IF the base of second IF amplifier transistor TR3. TR3 gets its bias from the same source as TR2. This is unusual, as most designs only apply automatic gain control to the first IF amplifier. We’ll soon find out whether this improves the AGC performance over other, more conventional designs. TR3 feeds the primary of the tuned, tapped third IF transformer, T3. Its secondary feeds demodulator diode D1, and the demodulated audio goes to IF filter M1. This is an integrated device, comprising two capacitors and a series resistor. It’s a simplified version of the Couplate used in the Emerson 838 hybrid radio (described in the October 2018 issue – siliconchip.au/ Article/11276). The audio signal from M1 goes to the volume control potentiometer, R8. This also develops the positive-going AGC voltage that is fed back to TR2/ TR3 after being low-pass filtered by 10kW resistor R6 and 33μF capacitor C7. Audio from the volume control goes to the base circuit of audio driver transistor TR4, which uses combination bias. TR4 feeds the primary of phase-­splitter transformer T4. The output transistor pair, TR5/TR6, operates in the usual Class-B mode. Bias is derived from resistive divider R13/R14, with temperature compensation by thermistor RRT. Its notation of “251” is probably a type number rather than its resistance at 25°C. Top-cut is applied by 1.5nF feedback capacitors March 2025  103 testament to this set, it can just pick up 774 ABC Melbourne inside my screened room – no easy feat. The converter’s 455kHz sensitivity of 9μV for 50mW output backs up the air interface figures. As this converter uses base injection, it wasn’t possible to inject test signals to the base, so I used my standard workaround of coupling to the ferrite rod’s tuned primary via a 10pF capacitor. This has the advantage of minimal detuning of the circuit and giving a repeatable indication for testing. The injected signal levels were 2.5mV at 600kHz and 550μV at 1400kHz. The IF bandwidth is ±1.7kHz for -3dB and ±26kHz for -60dB. The AGC allows some 6dB rise for a 40dB signal increase. The audio response from antenna to speaker is 600Hz to 2700Hz for -3dB. From the volume control to the speaker, it’s around 700Hz to a bit over 5kHz. At 50mW, total harmonic distortion (THD) was around 5.5% with clipping at 120mW for a total harmonic distortion (THD) of 10%. At 10mW output, THD was 7%. The low battery performance was good; with a 4.7V supply, it managed a useful 35mW at clipping, albeit with visible crossover distortion due to the voltage-divider bias circuit. Audio response The tuning gang trimmer and volume control pot are mounted on the plastic chassis. The earphone jack can also be seen in the lower half of the case. C14/C15, while some local feedback is provided by common 12W emitter resistor R15. TR5/TR6 drive the output transformer, T5, and its secondary drives the internal speaker, or an earphone plugged in to the earphone socket. The circuit and service notes are available online. As the Panapet uses PNP transistors with a positive supply, their circuit voltages are shown as negative with respect to the positive supply. I have used the conventional method and taken all voltage measurements with reference to ground. Restoration The review set was in good cosmetic condition, so a light clean had it looking just fine. Initially, it seemed deaf, only giving a signal in the low 104 Silicon Chip milliwatts with a strong input signal. The problem was light oxidation on the earphone socket. With that fixed, it responded well to my radiating ferrite rod test setup. How good is it? It’s better than its specifications state. National quote 150μV/m for 5mW output, but I was able to get 50mW output from a signal of 120μV/m at 600kHz, some four times the specification. At the upper end of the broadcast band, 1400kHz, it needed 190μV/m for 50mW output. The signal+noise to noise (S+N/N) figures were 13dB at 600kHz and 15dB at 1400kHz. For the more standard 20dB S+N/N, 300μV/m is required at both 600kHz and 1400kHz for 50mW output. In Australia's electronics magazine So, the audio frequency response is not very good, as shown in Fig.2, but why? Could my test set have a driedout electrolytic capacitor? Usually, a dried-out cap affects gain across the audio spectrum, but it was worth checking. On the basis that ‘if it’s worth doing, it’s worth over-doing’, I replaced emitter bypass C12 (10μF) with a 100μF type, and coupling capacitor C11 (330nF) with a 4.7μF type. However, there was virtually no improvement. Then I performed a frequency sweep and recorded the signal voltage at the collector of TR4. If there was some weird low-frequency deficiency, it should have been evident at the primary of driver transformer T4. Despite the constant input signal of 8mV, the voltage developed at the primary of T4 ranged from only 280mV at 200Hz (where the audio output was only 1.1mW, 17dB down) to a substantially constant 1.3V (giving 50-60mW) from 1kHz to 5kHz. T4’s primary inductance is clearly siliconchip.com.au inadequate, as shown by the falloff in developed voltage below 1kHz. The problem is worsened by TR4 having a high output impedance of around 30kW. I connected the low-impedance output of my audio generator to T4’s primary and drove it directly with a 1.3V signal across the audio band. This improved the 200Hz output to 22mW, just a little worse than 3dB down. My audio oscillator’s low impedance (as a voltage source) partly overcame T4’s low impedance at low frequencies, giving a much better bass response. It may seem counterintuitive that the driver transformer should need a higher primary inductance than the output transformer. However, this is needed to give a sufficiently high impedance to get a useful signal current through the transformer at lower audio frequencies. While the driver and output transformers are roughly the same size, it’s mainly the driver transformer that causes the poor low-frequency response observed here. Yes, it’s a charming, must-have gadget, but considering that the human voice’s fundamental frequencies lie between 95Hz and 230Hz, don’t expect the dulcet tones of your favourite actor to come through at all well. And the bass fiddles in the Choral Symphony? Pardon? Transistor coding The Japanese Industrial Standard (JIS) semiconductor coding is a little more helpful than the chaotic RETMA system. We can at least distinguish polarities, technologies and Fig.2: the measured audio response peaks at 2kHz and is down by over 20dB in the critical voice range of 80-250Hz. As a result, voices tend to sound rather tinny. applications based on part codes, although chemistry (germanium/silicon) and power ratings are not coded for. The prefixes are: 2SA: high-frequency PNP bipolar junction transistors (BJTs) 2SB: audio-frequency PNP BJTs 2SC: high-frequency NPN BJTs 2SD: audio-frequency NPN BJTs 2SJ: P-channel FETs (both JFETs and Mosfets) 2SK: N-channel FETs (both JFETs and Mosfets) Special handling The Panapet is easily dismantled for servicing. Be aware that, depending on the serial number, the circuit board may be secured by one or two screws. My white one (serial #40322) used two, while the blue (serial #50593) used just one. It uses a dial cord mechanism. Both of mine were still OK, but you would need the service notes for re-stringing. Would I buy another? Having two now, it’s tempting to collect the entire set. That would mean finding the very rare purple version, as well as the French Radiola RA010, which tunes the long-wave band of 150~250kHz. It’s an oddity, given that long-wave would have been in its final years of broadcast usage by the mid-1960s. Further Reading • R-70 service manual: siliconchip. au/link/ac21 • Radiomuseum Panasonic R-70: siliconchip.au/link/ac22 • Radiomuseum Radiola RA010: SC siliconchip.au/link/ac23 The R-70 has a striking appearance and was available in a variety of colours (red, blue, green, yellow, white and purple). Readers should also look at the service manual (siliconchip. au/link/ac21), as it has a very good quality drawing of the circuit and PCB wiring diagrams. March 2025  105