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Vintage Radio National R-72 “Toot-a-Loop” radios by Ian Batty Is it a musical instrument? Is it a telephone? No, it’s a radio! This quirky transistor set comes in a unique, colourful plastic case that’s sure to attract attention. Inside, the circuitry of this six-transistor set hid some surprises. W ho doesn’t remember the 1960s? The Beatles, the Vietnam War, moon landings, Mao’s Great Leap Forward, Woodstock, and cars with massive tailfins. Fashion designers shook free the drab aesthetic of the 1950s, releasing more and more flamboyant, colourful, exciting designs in a frenzy to capture the new postwar economic boom. By the late 1960s, transistor radio engineering had pretty much settled on the standard set: three radio frequency (RF)/intermediate frequency (IF) stages, a volume control, an audio driver and a push-pull Class-B output stage, likely powered by the then-­ ubiquitous 9V PP6 battery that’s still in use today. Matsushita’s National brand, unable to use that name in the USA due to an existing company of the same name, had rebranded as Panasonic. They put forward several remarkable offerings; the R-72 Toot-a-Loop (appearing at the end of the 1960s) is one of the most distinctive. It’s another example of National’s successful marketing strategy: 94 Silicon Chip visually attractive radios using sound electronic design. Like the R-70 Panapet (March 2025; siliconchip.au/ Article/17800), the Toot-a-Loop is unique. Even if you have no idea of its provenance, you’ll be impressed by its styling. As an early ‘wearable’ radio, it’s a standout. The quirky design was complemented by bright colours to create a radio rivalled in its overall effect only by an identical offering from RCA subsidiary Japan Victor Corporation (JVC). The Toot-a-Loop came in white, red, blue, yellow, orange and lime, with the last two options being specific to the Australia/New Zealand markets. Our models were badged National JIS transistor coding Prefix Type 2SA High-frequency PNP BJT 2SB Audio-frequency PNP BJT 2SC High-frequency NPN BJT 2SD Audio-frequency NPN BJT 2SJ P-channel FETs (JFETs & Mosfets) 2SK N-channel FETs (JFETs & Mosfets) Australia's electronics magazine Panasonic and were advertised as a “Sing-O-Ring”. Between my yellow and red sets and what’s online, I’m aware of three different circuits for this radio. Let’s look at the R-72S yellow set (serial number 88009) first. Ernst Erb’s Radiomuseum has the circuit, showing a classic six-­transistor set; my redrawn version is shown in Fig.1. It runs off a battery of two AA cells, giving a nominal 3V supply. The six transistors comprise one converter, two intermediate-­ frequency gain stages, one audio driver, and the transformer-­coupled Class-B output pair. My example matched this with a few exceptions. The audio driver transistor, rather than a metal-can germanium PNP 2SB475/AC125 type shown in the original circuit, is an epoxy silicon NPN type, the 2SC828. The RF/IF section varied even more; the converter uses an epoxy silicon NPN 2SC829 transistor, while the IF strip has only two IF transformers, one coupling the converter to the first IF amplification stage, with the second siliconchip.com.au coupling the second IF stage to the demodulator. It has two IF transistors, both ceramic-cased silicon NPN 2SC920s, with resistance-capacitance coupling. The red R-72 set (serial number 11878) runs from a PP6 9V battery. The RF/IF section is similar to that in the original R-72S circuit, with three metal-can PNPs (2SA102, 2SA101 & 2SA101). These are drift types, with typical ft values in the 25MHz range, an improvement over the preceding alloy-junction OC44 with its typical 15MHz ft. It’s a very similar circuit to that of the previously reviewed R-70 Panapet. Does that mean my red R-72, apart from the supply voltage, is similar to the R-72S? Well, no. It shows a notation for a 2SC829 silicon NPN converter with the correct symbol, but it’s wired into the circuit with the correct polarities for a PNP device. It’s odd that it says 2SC829 (OC1044), since the OC1044 (2SA101) is definitely a PNP germanium type, which is what I found installed. Also, the audio section’s transformerless design uses two epoxy-cased NPN transistors and one PNP type. Online searching revealed the Philips 20RL012 long-wave-only set using PNPs for the converter and IF amplifiers, with a complementary output stage using NPN and PNP transistors. It’s an unusual design, as long-wave broadcasting had begun declining by the time the Toot-a-Loop arrived. So I ended up with two chimeras – not quite the classic lion’s head, goat’s body and snake’s tail, but close enough. Finding no authoritative circuit for the yellow set, I resorted to tracing it out as-built. This was complicated by the extreme compactness of the design and by almost all the resistors being printed onto the circuit board. Where I would usually lift one end of a resistor to measure it, I had to apply my ohmmeter with both polarities and take the higher reading (to prevent transistor junctions giving a false reading) or, bravely, short-­circuit bias resistors to ground, measure the short-circuit current, then apply Ohm’s Law. You may not want to try this at home! The resulting circuit for the red set is shown in Fig.2. Both sets are built on double-sided PCBs with most of the resistors siliconchip.com.au printed directly onto the phenolic substrate. This would reduce the amount of mechanical assembly, as resistors don’t need to be placed prior to wave-soldering the PCB. It’s also a great way of reducing the overall size of the set, and would be highly reliable. It does make circuit tracing harder, especially when the as-built set differs as much from available circuits, as these two do. So we have three different circuits for just two sets. Let’s look at the RF/ IF sections first. Circuit details The 3V (Fig.1) front-end uses Q1, an NPN 2SC829 converter transistor. With a typical ft of 230MHz and being recommended for “RF amplification, oscillation, mixing, and IF stages of FM/AM radios”, it’s similar to the more familiar BF115. The base bias for Q1 is supplied from the decoupled supply via resistive divider R1/R2, with emitter resistor R3 stabilising the circuit, bypassed by capacitor C4. I’ve never seen a tuning gang returned to the emitter (or cathode) of a converter before, but this does conform to the single-point grounding technique. Be aware that the tuning gang’s ‘cold’ RF connection is above ground and must not be used as an earthing point during testing. Q1 is configured as a self-­oscillating converter, with the usual ‘Japanese-­ style’ oscillator feedback from LO transformer L2 via 10nF capacitor C3 to the base. Since connecting a signal generator to the base stops the oscillator, I used a low-value series capacitor to inject to the top of the tuned antenna circuit. This confirmed the high sensitivity borne out in testing (more on that later). IF injection is also reliable. The converter feeds the tuned, tapped primary of the first IF transformer, IFT1. Its secondary feeds first IF amplifier Q2, a 2SC920 transistor with a typical ft of 250MHz. This stage’s emitter goes directly to ground, eliminating the usual emitter resistor and its bypass capacitor. It gets weak Fig.1: both of my sets had a different configuration from the ‘standard’ published circuit. This is how my yellow set was built. It uses many different transistor types from the standard circuit and has one fewer IF transformer, with RC coupling taking its place. Australia's electronics magazine July 2026  95 forward bias via R4, and gain-control voltage from the demodulator via R10. Load resistor R5 measured as 1.3kW. The signal is then resistance-capacitance coupled to Q3, another 2SC920, via 22nF capacitor C8, working with fixed bias. Voltage divider R6/R7 measured as 30kW and 16kW, while emitter resistor R8 measured at 210W, bypassed by 22nF capacitor C9. IFT1 was confirmed as the first IF transformer (converter circuit) by its yellow adjusting cap, and IFT2 as the final IF transformer (demodulator circuit) by its black adjusting cap. The usual second IF (white cap) was missing, confirming (i) only two IF transformers and (ii) R-C coupling. My familiarity with two-stage audio preamplifiers initially suggested a direct-coupled design, but the application of AGC would presumably have disturbed DC operating conditions excessively. Q3 feeds final IF transformer IFT2’s tapped, tuned primary, shunted by 100kW resistor R9, presumably to provide some damping and broaden the IF bandwidth. IFT2’s secondary feeds demodulator diode D1, a miniature allglass silicon diode. Jim Greig’s January 2025 article in Radio Waves magazine on the Sanyo RP-1250 shows a circuit that’s very similar to the R-72. The IF circuit, on pages 50-51 of that issue, is very similar to that of my Toot-a-Loop. Given the relationship between Panasonic and Sanyo, a shared design makes sense. That circuit notes the silicon rectifier diode as a BAY41. A silicon type is needed, rather than the usual germanium type, as its forward conduction voltage must match that of silicon transistor Q2 for proper AGC action. As usual, the diode is weakly forward-biased by the controlled IF amplifier’s bias circuit, in this case, 22kW resistor R4. The rectified IF signal is filtered by RC network A1. The demodulated audio is sent to volume control potentiometer VR1, and the rectified DC component is sent back to the first IF stage via 7.3kW resistor R10. Audio stages The as-built circuit of the yellow set’s audio stages was close to the published R-72S circuit, with a few oddities. The audio driver, rather than a metal-cased PNP 2SB475, was an epoxy-cased silicon NPN 2SC828 type. The R-72S showed an adjustable resistor of some kind in the lower end of the 2SB475’s base bias divider, but no emitter stabilising resistor. This would be consistent with laser-­ trimming, where the set would be put on test and the resistor element carefully vaporised to give the correct circuit voltages/currents. I was unable to discover any evidence of laser-trimming, though. As the 2SC828 is an NPN type, its emitter returns to supply ground, and it gets base bias via resistor divider R11/R12. Its collector feeds the primary of driver transformer T1, with its ‘cold’ end going to the battery’s positive terminal. Top cut is applied to the audio signal via collector-base feedback capacitor C12 (10nF). T1 phase-splits the audio signal and applies it to the bases of transistors Q5 and Q6 in anti-phase. Both are metal-can germanium PNP 2SB475s. They get around 200mV of forward bias via the R15/R16 divider and temperature compensation via 240W thermistor R14. Output transformer T2 feeds the 8W speaker via the earphone socket, with the usual disconnection of the speaker when the earphone jack is plugged in. Red set front-end My circuit (Fig.2) may seem unusual but, as it uses a positive supply, the audio section is easily understood. This does ‘invert’ the all-PNP RF/IF section, with the IF transformer primaries going to ground and transistor emitters returning to the decoupled positive supply. The RF/IF stage is a completely ordinary all-germanium circuit, which should be similar to that of the R-72S. My set differs from the R-72S in that the latter shows a 2SC829 silicon NPN converter wired in-circuit as PNP! It does correctly show its equivalent as a germanium PNP OC1044, however, which is equivalent to the 2SA101. The equivalence is confusing; the OC1044 is described as a ‘junction’ type (with a typical ft of 15MHz), while the 2SA101 is a drift-field type, with a typical fαβ of 25MHz (not an identical specification to ft but usually close). As built, converter Q1 gets bias via divider R1/R2 (5.6kW/33kW), with emitter stabilisation via 1.2kW resistor R3, bypassed by 22nF capacitor C4. R2 is one of four discrete resistors, probably used because printed-circuit types (marked on the 20RL012 circuit as “imp”) could not give sufficiently high (R2/R12) or sufficiently low (R14/ R15) values. Note, though, that the 20RL012 circuit shows R2 as a printed type. LO transformer T1’s tuned, tapped secondary feeds back to Q1’s emitter via 4.7nF capacitor C3. The tuning gang uses a cut-plate LO section, so R14 R9 R15 C13 Both variants (R-72S, left; R-72, right) are built on a double-sided PCB, with only two components on the bottom side. 96 Silicon Chip Australia's electronics magazine siliconchip.com.au there is no padder. The bias voltage on the converter transistor is only some 100mV, confirming the Class-B operation needed for the conversion process. Converter Q1 feeds the tapped, tuned primary of the first IF transformer, IFT1. Its untuned, untapped secondary feeds the base circuit of the first IF amplifier transistor, Q2, noted as a 2SA101/OC1045. Although the drift construction method improved high-frequency performance over that of alloyed-junction devices, the resulting collector-base capacitance was still significant, so Q2 is neutralised by 2pF capacitor C8. Q2 gets weak forward bias via 100kW resistor R4, bypassed for audio by 10μF capacitor C7. Q2 feeds second IF transformer IFT2’s tapped, tuned primary, with its untuned secondary feeding second IF amplifier transistor Q3. Unusually, transistor Q3 is also gain-controlled. This did not give much better output constancy with changes in signal strength, but it did give a very early onset of gain reduction, demanding an abnormally high input signal to give the standard 50mW output. More on this below. Q3 feeds the tapped, tuned primary of third IF transformer IFT3. Q3 operates without neutralisation, possibly due to demodulator diode D1’s loading of IFT3 giving a lower gain in this second IF stage. Demodulator diode D1, an OA70, feeds the IF filter block C11/C12/R7. Audio is fed, via 1kW resistor R9 and 1μF coupling capacitor C14, to the volume control. D1’s DC output is fed, via 10kW series resistor R8 and 10μF audio filter capacitor C7, to the bases of Q2 and Q3 for AGC, as noted above. Red set audio stages The red set’s audio section appears complicated, but it became the design of choice and remains so to this day. You’ll find its principles everywhere, from the LM386 audio chip to highpower amplifiers in the kilowatt range. Its circuit stability surpasses that of previous designs, while the removal of transformers and most capacitors allows manufacturers to deliver any level of performance, from low-power AM radio quality up to hifi systems of previously unmatched fidelity. Transistors Q5 (NPN) and Q6 (PNP) form a complementary pair. Feeding an AC signal to their bases will see Q5 siliconchip.com.au biased on for the positive half-cycle, with Q6 coming on for the negative half-cycle. They’re both configured as emitter-­ followers, so they provide roughly unity voltage gain. They do, though, provide considerable current gain, with high input impedances. A highgain driver stage can take the millivolt-­ level signal from a radio’s demodulator and amplify it up to speaker levels, with the output pair matching the low-impedance speaker load. In detail, audio from the volume control enters the circuit via coupling capacitor C16, arriving at the base of preamplifier/driver transistor Q4. Q4’s collector current, flowing via D2 and R13, becomes the driving voltage for Q5/Q6. Their emitters (via R14 and R15) connect together to drive the speaker via C20. The circuit is able to deliver almost the entire supply voltage (as a peak-topeak AC signal) to the speaker, around 8V peak-to-peak in this circuit. A quick calculation gives a potential output power of around 200mW into this set’s 40W speaker. It’s important that the circuit is biased correctly. This demands a quiescent (idling) current of a few milliamperes in Q5/Q6, temperature compensation to ensure that Q5/Q6 do not enter destructive thermal runaway at high temperatures, and that the Q5/ Q6 emitters set close to half the supply voltage, to allow maximum undistorted output, ie, equal magnitude positive and negative half-cycles. The quiescent current is set by the base-to-base voltage of Q5/Q6. Diode D2 is designed for a breakdown voltage of just about 1.2V, which is roughly twice the normal Vbe for silicon transistors. D2 also has a negative temperature coefficient (NTC). As the ambient temperature rises, D2’s forward voltage will fall, compensating for the 2 × -2.5mV/°C fall in Q5/Q6’s total Vbe. For the emitter voltages of Q5/Q6, we need to look at the DC feedback path via 560kW resistor R12. For example, a rise in the emitter voltages will supply more bias current (via R12) to Q4. This will raise Q4’s collector current, drawing its collector voltage down and lowering Q5/Q6’s emitter voltages. Fig.2: the circuit of my red set is different again, with most of the transistors having different polarities than in the yellow set! Australia's electronics magazine July 2026  97 A fall in the emitter voltages of Q5/ Q6 would result in less bias for Q4, allowing the circuit to send the Q5/Q6 emitters higher. It’s a simple feedback loop that stabilises the entire amplifier’s DC conditions. The final problem is to get enough voltage swing at the bases of Q5 & Q6. They need several milliamperes of base current to deliver full current into the speaker at signal peaks (the speaker current divided by their hfe figures). Pulling the bases down to switch Q6 on hard is easy; Q4 can readily pull Q6’s base to ground and supply many milliamps of base current. Pulling up seems harder. Let’s send Q4 to cutoff. Now, Q5’s base is fed via 820W resistor R13. Assuming we need about 2mA base current to bias Q5 fully on, we’ll get a drop of around 2V across R13. If only we could supply R13 from a higher voltage than the battery. That’s the job of R13’s connection to the speaker. With no signal, this point will be at around 9V. As the Q5/ Q6 emitters start to swing positive, so will the speaker voltage. But the speaker is already at 9V, so the positive half-cycle will see the speaker connection increase above 9V on the signal’s positive peaks. In theory, this point can get to The R-72 and R-72S share the same dial and case. Apart from the colour, the only other external difference is on the nameplate. around 13V. So that means that the voltage drop across R13 is fairly constant at around 4V. It’s known as a ‘bootstrap’ circuit, based on the principle of pulling oneself up by one’s bootstraps! This particular design offers circuit protection; if the speaker is open-­ circuit, there’s no DC supply to the top of R13 and the circuit simply fails to operate. Restoration Confusingly, a very recent search on Radiomuseum turned up an R-72S circuit that shows the RF/IF correctly, but retains the PNP audio driver! My advice is to always check any circuit against the as-built equipment. Both sets came in acceptable condition, although the red set’s coin slot had seen excessive force and was a bit mangled. They both cleaned up nicely with a spray wash and some automotive polish. I’ve had the yellow set for some years now. When I first tried it back at Harcourt, it failed to impress. I was Fig.3: the original circuit; it seems like it may be representative of only a minority of the R-72 sets that were manufactured. 98 Silicon Chip Australia's electronics magazine siliconchip.com.au able to pick up Melbourne stations, but at less than full gain. I put the lack of sensitivity down to the small ferrite rod antenna and the oddball resistance-capacitance coupled IF channel. But when I started to test the set, I found that the solder tab connecting the top of the ferrite rod’s tuned winding had broken, open-circuiting the connection to the tuning gang and leaving the antenna circuit completely untuned. The low sensitivity was one clue, but I also couldn’t get a peak at the 600kHz alignment point. If the antenna circuit was not being tuned, it would not resonate at 600kHz, so adjusting the LO to maximise the 600kHz sensitivity would be fruitless. As the antenna circuit would just be acting as a simple untuned pickup coil, the set would work about as poorly no matter the LO frequency. With that fixed, and with a quick tweak, 3WV Warrnambool rocked in at full volume; not bad for a station over 200km from my previous place at Rosebud. The red set was dead, though. No output, nothing. Connecting my monitor amp to the earphone socket got it going, and the lack of output was found to be an open-circuit speaker. Not expecting to get it rewound (does anyone repair/rewind three-inch speakers?), I got a replacement online. Its diameter was a little smaller than the original, but I turned a collar using a circle-cutter on my bench press drill from an old ice-cream container. That done, it was onto the test bench for alignment and performance analysis. We’ve just moved to Malvern, where the local levels of EMI even intrude on 774 ABC Melbourne’s powerful signal. Taking both Toot-a-Loops for a walk in the park, though, I was easily able to pull in my favourite 3WV at full speaker volume, albeit with a bit of noise. How good is it? I was able to test the yellow set at the standard audio output of 50mW. The audio response from volume control to speaker is around 220Hz to 4.6kHz; from antenna to speaker, it’s some 160Hz to 1.1kHz. Driving the output stage to clipping gave 150mW at 10% total harmonic distortion (THD), while the 50mW output (4.5% THD) and 10mW output (2.8% THD) are creditable for a Class-B push-pull output circuit. siliconchip.com.au Volume control Demod Output transformer 2nd IFT Driver transformer 2nd IF Output 1st audio 1st IF 1st IFT Oscillator coil Converter 3rd IFT 2nd IF Volume control Demodulator 1st audio D2 Output 1st IF 2nd IFT Converter 1st IFT Oscillator coil R2 The insides of both the R-72 and R-72S with some of the components labelled. Australia's electronics magazine July 2026  99 Some Panasonic history National Panasonic’s founder, Konosuke Matsushita, was born in 1894 to a family that fell on hard times. Young Konosuke was forced to leave school at age nine to find various jobs until he made his first invention, a light socket. While his product surpassed many others in quality and cost, he found marketing this product very difficult. The experience showed him the need to find marketing outlets for his products. He formed a strategy that reduced the energy put into manufacturing in favour of the establishment of a sales force that led to a retail store network. His next outing seems equally humble today: a bicycle lamp. Still, it was battery-powered, making it far superior and much more convenient than his competitors’ candle- and oil-based offerings. Matsushita was in danger of being removed as president of National after the end of WWII. One of General Douglas MacArthur’s strategies to rebuild the Japanese economy involved breaking up the Zaibatsu (large national corporations). A 15,000-strong petition from National employees changed MacArthur’s mind and, in 1947, Matsushita gave brother-in-law Toshio an unutilised manufacturing plant to manufacture bicycle lamps. This company eventually became Sanyo Electric. It’s tempting to cast National as the also-ran to Sony, especially given Sony’s remarkable rise from the ashes of WWII. Against this, we can consider Sony’s vulnerability as industry leader and the VCR wars of the 1970s and 1980s, which saw Sony’s Betamax outsold and finally obsoleted despite continuing improvements that ultimately delivered CD-quality audio. The competitor to Betamax, Video Home System (VHS), had been developed by Japan Victor Corporation (JVC) and was strongly supported by National, with Mitsubishi, Hitachi and Sharp adding their marketing power to the VHS push. It’s fair to say that Sony’s innovative energy and design flair would always be challenged by products that, while perhaps not the cutting edge of technology, were sound, reliable and well-marketed. And the leader of that group was Matsushita’s company, National. The Panasonic “Toot-a-Loop” radio could rotated into a ring, which was designed to be wrapped around your wrist. And all it needed was a single 9V battery or two AAs. 100 Silicon Chip The set needed 140μV/m at 600kHz and 110μV/m at 1400kHz for signal+noise:noise (S+N:N) ratios of 18dB and 13dB, respectively. For 20dB S+N:N, the levels were both 190mV/m. AGC control was as expected for a single stage: a signal rise of around +30dB gave an output rise of +6dB. Selectivity was ±2.3kHz for -3dB down and ±44kHz for -60dB. The skirt selectivity, especially, is very wide for a transistor set of this era, confirming the reduced selectivity expected from the use of only two IF transformers. Selectivity reduction is also increased by R9 shunting of ITF2’s primary. As for the red set, its unusual two-stage AGC showed very early onset, needing an artificially high signal to get to the Australia's electronics magazine standard 50mW output. Accordingly, I tested at 10mW output, so all measurements on the diagram are for a 10mW output. The audio response from volume control to speaker is around 125Hz to 2.7kHz; from antenna to speaker, it’s about 65Hz to 1.7kHz, although the unusually low bass response is wasted with the tiny speaker. The top end of just 2.7kHz seems low for an output-transformer-less (OTL) design, but 22nF capacitor C18, connected from Q4’s collector to ground, gives a significant amount of top cut. Driving the output stage to clipping gave 150mW at 10% total harmonic distortion (THD), while the 50mW output (2% THD) and 10mW output (3% THD) show the value of a well-­ designed OTL circuit. RF performance was also creditable; noting that I tested at 10mW audio output, the set needed 75μV/m at 600kHz and 120μV/m at 1400kHz for S+N:N ratios of 8dB and 7dB, respectively. Without the early-onset AGC, these should have equated to about 170μV/m and 270μV/m, respectively, for 50mW out. In reality, the red set needed 450μV/m and 550μV/m to achieve 50mW. Despite the two-stage AGC, the input change for a +6dB rise was only about +30dB, the same as for the single-AGC stage design. Selectivity was ±1.6kHz for -3dB and ±12kHz for -60dB. Would I buy another? I could keep going and get the complete set. It’s a striking example of packaging; take a popular commodity that’s gotten a bit ho-hum and wrap it inside an exciting, attractive case that makes it stand out from the pack. My research turned up the R-72, R-72S and Wadley RF-72 FM version from South Africa. There’s also the identical AM R-720 from Citizen Electronics. For a vividly ‘interesting’ design, hop onto Radiomuseum and look for the JVC Balance (8008). Special handling When replacing batteries, take your time and be kind to the coin slot. For more information on this series of radios, visit the following Radiomuseum links: R-72(S): siliconchip.au/link/acb1 RF-72: siliconchip.au/link/acb2 8008: siliconchip.au/link/acb3 SC siliconchip.com.au