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Vintage Radio The Pye PHA 520 “Colombo Plan” Radio circa 1960s The Pye PHA 520 radio was developed to help improve education and cohesion in Southeast Asia, along with strengthening ‘soft power’ in the region. By Alby Thomas Circuit Description by Ian Batty F ollowing World War 2, there was a fear among Commonwealth countries that the scourge of communism would filter from China down through the Asia Pacific region. A meeting of major Commonwealth nations (including Australia) was held in Colombo (Ceylon, now Sri Lanka) in 1951, with the view of improving standards of administration and commerce in the developing Asian countries. At times, the assistance was misguided, with tractors sent to areas of labour excess and tiny farm holdings. While not a Commonwealth country, the USA funded educational programs, scholarships and medium-­ powered broadcast transmitters. The Colombo Plan still exists today, with 28 member countries, including Chile and Japan. As part of the Australian effort to improve education and cohesion in Southeast Asia, a network of radio broadcast transmitters was set up in Asian Commonwealth countries. Radio receivers were supplied, with contracts to manufacture these radios were granted by the Department of Supply in 1963 to Pye Australia. The HRSA’s Kevin Poulter advises that these receivers were made by Pye Communications, well-known for its A close-up view of the instruction sheet which is attached to the top of the case and the Pye PHA 520 dial which is... 92 Silicon Chip Australia's electronics magazine siliconchip.com.au The telescopic antenna for the set is attached along the rear edge of the plywood cabinet. This antenna and a separate Earth stake are connected to connectors also visible on the rear. taxi radios, not Pye Domestic, which would have typically manufactured radio receivers. AWA supplied medium-powered (5kW) HF transmitters as well as a transistorised receiver similar to the PHA 520 for use overseas. I have seen one of these with no ARTS&P label (Australian Radio Technical Services and Patents), no Australian stations shown on the dial and similar coverage to the Pye sets. It was similar in appearance to Radiola’s model 893P. Before that, in the early-to-mid 1950s, AWA provided valve radio sets for the Colombo Plan. They were the model 1548MA, a five-valve set operating from 110-240VAC, and the model 546PZ, a five-valve dry-cell battery set. Around 1000 of each model were produced. Pye was awarded contracts amounting to just over £A245,000 (about $8,000,000 today) for transistorised radio receivers and associated equipment. The sets are pretty large at 280mm (11 inches) high, 395mm (15½ inches) wide and 190mm (7½ inches) deep, with a large 15 × 23cm (6 × 9-inch) speaker. They weigh just under 7kg each without batteries. The cabinet is plywood with a grey/ green vinyl fabric covering. There is no internal loop or ferrite aerial, but the sets were supplied with an Earth stake and a long aerial wire that connected to terminals at the rear of the set. Power was from six D cells fixed in place in their carrier by a metal bar. There is no provision for an external power source. The sets tune from 525kHz in the AM broadcast band to 30MHz (shortwave) in four continuous bands. The set lid operates a switch that controls the power. The set I have (serial number 4244) was found at a dealer in Geelong under some boxes of other radio gear. I thought it was so ugly that I just had to have it! A two-metre-long telescopic whip aerial had been screwed to the side of the set as an afterthought. Another of these radios (serial number 0882) is owned by the HRSA’s Ray Gillett, purchased at a Ballarat flea market, while a third set (serial number unknown) was presented to a Pye manager, then passing through different hands until it reached the AVRS’s Warwick Woods. Other sets would have been brought back from Asia to Australia by migrating families and will be out there somewhere. As purchased, my set was dead. I had no circuit or other documentation, so I traced out the circuit, revealing a reasonably standard superhet with an RF stage, germanium transistors and a transformer-coupled output stage. ...using knobs sourced by Alby Thomas (rather than the ones shown in the lead photo). siliconchip.com.au Australia's electronics magazine September 2025  93 Unusual was the use of back-to-back 25μF electrolytic capacitors to get a non-polarised high capacitance. I had heard of this principle but had not seen it in practice. Editor’s Note: this approach is almost always cheaper and more compact than using a bipolar electrolytic capacitor Testing showed that all electrolytics (11 total) were either short-­circuit or open-circuit. Replacing them all did not bring it back to life until I replaced the OC171 RF amplifier transistor. The set’s construction is robust and neat. It performs very well, with good reception on all bands, although there are not many usable shortwave transmissions to tune into. Circuit details by Ian Batty Pye’s diagram follows the drawing conventions of the day. Transistors are prefixed with “TS”, while germanium diodes are prefixed with “MR” (“metal rectifier”). Band change switch SWA’s labelling was only partly legible on the best available manufacturer’s diagram, so I have renumbered its sections from 1 to 12. Pye’s original diagram is very dense (especially the tuner section), observing the need for compactness on the page. I have expanded the diagram for legibility and ease of description in Fig.1. This has displaced some components from Pye’s original locations. For compatibility, I have retained Pye’s component numbering. Legibility problems may have led to numbering at odds with Pye’s. I welcome any feedback on this, especially a clearer example of the original circuit! I have retained Pye’s coil numbering in the tuner coil set. I have put Band 4 (broadcast) coils at the top of my diagram for convenience. This is the opposite of Pye’s placements, but I have preserved their numbering. This has placed the coils in apparent reverse order from top to bottom. For example, broadcast aerial coil L4 appears at the top of my diagram. Because of this, the 3~30pF trimmers are also designated in reverse vertical order. The four bands In common with other shortwave radios, band numbering starts with the highest band: Band 1 (red): 14.8~30MHz Band 2 (green): 4.8~15MHz 94 Silicon Chip Band 3 (yellow): 1.6~5MHz Band 4 (blue): 525~1600kHz The tuning gang is cut for straightline frequency, so Band 2 to 4 scales show equally divided calibrations. Band spreading on Band 1 (a 1:2 ratio) causes the calibration to deviate, so Band 1’s scale is not equally divided. The tuner circuit The tuner section, in common with multi-band equipment, is complex at first glance. It’s also complex at second glance. The aerial circuit and local oscillator circuits, especially, switch different component configurations for each band in addition to the expected changes in coil sets. SWA/1 conveys the incoming RF signal to the selected aerial transformer, L1 (14~30MHz) to A4 (Broadcast). A series capacitor (C2, 220pF) is connected for all bands except Band 1. This series capacitor compensates for an aerial that is shorter than the ideal quarter-wavelength for the broadcast band. Bands 2, 3 and 4 give the usual 1:3 ratio for frequency coverage. Band 1’s coverage, in contrast, is only about 1:2. This demands a reduction in the tuning gang’s capacitance swing. Capacitor C7 (180pF) pads Band 1’s aerial circuit, reducing its span to 1:2. Band 1’s RF transformer is also padded by 180pF capacitor C20. SWA/2 connects the signal from the appropriate aerial transformer to the base of RF amplifier transistor TS1. This connection also conveys bias from the automatic gain control (AGC) line, via the selected transformer, to TS1’s base. SWA/3 connects the appropriate transformer to the tuning gang’s aerial section, C9. All inductors in the coil set are closely packed, creating the possibility of interaction between selected and unselected coils. SWA/4 shorts out the other three unselected aerial transformers to prevent interaction. The tappings on RF transformers L5 to L8 are driven from TS1’s collector, as selected by SWA/7. The tappings match the medium-to-high collector impedance of TS1 to the higher impedance of the selected RF transformer, ensuring maximum selectivity. Although TS1 operates as a common-­ emitter amplifier, it is not neutralised for two reasons. Firstly, RF amplifiers have low gains compared to fixed-­ frequency IF (intermediate frequency) Australia's electronics magazine amplifiers, so collector-base feedback is less likely to load the input circuit or cause oscillation. Secondly, Philips’ OC169/170/171 series of transistors use alloy-diffused construction, coming between the preceding alloyed-junction OC44/45 and follow-on ‘all-diffused’ Mesa transistors such as the AF139. The alloy-­diffused collector-base feedback capacitance of some 1.5pF apparently has no serious effect on this circuit, with its maximum frequency of only 30MHz. The RF amplifier gang connects to its selected coil via SWA/5, with unselected transformers being shorted out (as for the aerial circuit). The RF section uses SWA/6 for this. The converter Converter transistor TS2 receives both the incoming RF signal and the local oscillator (LO) signal on its base. The RF signal from the secondary of the selected RF transformer (L5~L8) comes via SWA/8. LO injection is more complicated. Each LO transformer is permanently connected, either via a tap (L11/L12) or a secondary winding (L9/L10) to the ‘bottom’ end of its companion RF transformer secondary. In concert with SWA/7’s selection of the active RF transformers primary, the selected RF transformer secondary’s combined RF and LO signals (selected by SWA/8) are conveyed, via 100nF coupling capacitor C21, to the base of converter TS2. TS2 works with fixed combination bias (R2/R3/R8/C23). The IF channel begins with a bandpass filter comprising IF transformers T1 and T2, with associated tuning capacitors (C34/C39), resistors R13/ R14 and coupling capacitor C37. While any IF amplifier, by virtue of its design frequency, is a bandpass filter, the term is usually reserved for circuits with several coupled resonant circuits and no amplifiers between them. T1 receives the converter’s four output signals: the input signal and LO signal, as well as the LO+ input and LO– input products. As this receiver uses ‘high-side’ LO injection, the IF strip selects LO– input, ie, the 455kHz signal. The local oscillator Local oscillator transistor TS3 (an OC171) operates in grounded-base mode. This configuration allows the siliconchip.com.au Fig.1: the Pye PHA 520 circuit is dominated by the tuning and band-changing circuitry (the top section). Band change switch SWA is a 12-gang wafer switch with most gangs having four poles, all shown in the Band 4 position. Most gangs are one-of-four selectors (eg, SWA/3) or three-of-four selectors (SWA/4). SWA/1 is like a one-of-four selector except it also shorts out C2 for Band 1. This side of the chassis is where the tuning gang and coil pack mount. Note the tightly packed inductors on the coil pack at right, and the use of a PCB for the components. transistor to exhibit constant oscillation to over 30MHz. TS3 operates with fixed combination bias (R4/R5/ R8). TS3’s base is bypassed to ground by 100nF capacitor C23; TS3 has a typical input impedance under 100W. TS3’s collector selects one of the L9-L12 LO transformers via SWA/12. The LO gang section, C38, connects to the tuned winding of the selected LO transformer via switch section SWA/10. As with other tuning connections, unused LO transformers are shorted out to prevent unwanted interaction, in this case by SWA/11. Each tuning range needs its LO frequency span reduced to guarantee tracking between the LO and aerial/ RF amplifier circuits. Broadcast band 4 uses C35. At 470pF, this is close to the value commonly used in broadcast-­ only superhets. As they need a wider frequency span (less restriction), Bands 3 and 2 use 1.5nF (C34) and 4.7nF (C33) capacitors, respectively. In theory, Band 1 can operate without padding – the required 455kHz offset is minimal compared to Band 1’s 14~30MHz tuning range and would cause only minor tracking errors. However, remembering that this band has a limited 1:2 frequency coverage, band spreading is applied by 180pF capacitor C32, the same value used to spread Band 1 in the aerial and RF amplifier circuits. The feedback for TS3 is taken from the low-impedance secondaries of LO transformers L9~L12. It’s common for oscillators to suffer frequency variations with variations in supply voltage. It’s mainly a problem with battery-operated equipment as the batteries run down. The PHA 520 ensures dial calibration by providing a stabilised LO supply using 4.7V zener diode ZD1 as a shunt regulator, supplied from the main battery voltage. Capacitors C11 (Band 1, 180pF) to C14 (Band 4, 8.2 nF) control the proportion of feedback needed for each band. SWA/9 selects these, in series with LO transformer feedback windings. IF section The rear view of the cabinet provides a good view of the major sections of the radio such as the Rola speaker and Panasonic battery pack at lower left. The converter’s signal is sent to the single winding of the first IF transformer, T1. This transformer has a single tuned winding, as it’s only needed to develop the 455kHz signal. T1 connects to T2 via 27pF capacitor C37, coupling the two tuned circuits. While 27pF seems like a low value, both T1 and T2, at resonance, will have impedances close to their loading resistors R11 (68kW) and R13 (68kW). C37’s reactance is only about 12kW but, considering it as part of each tuned circuit, it will convey the 455kHz signal from T1 to T2 with little practical loss. This part of the circuit acts as another bandpass filter. T2 accepts the 455kHz signal at its high-impedance Australia's electronics magazine siliconchip.com.au 96 Silicon Chip The other side of the radio chassis houses the components. tuned primary and conveys the IF signal to its low-impedance secondary. This feeds the base of first IF amplifier transistor TS4, an OC169. Like the transistors in the tuner, this is an alloy-diffused type. As it has low feedback capacitance, neutralisation/ unilateralisation is not needed, unlike transistors from the previous generation of alloyed-junction types. TS4 is gain-controlled by the DC voltage developed by demodulator diode MR1. In common with reverse gain-controlled stages, TS4 operates at a low collector current (around 0.3mA), allowing easy reduction of its stage gain on strong signals. TS4 feeds the tuned primary of T3, shunted by 18kW resistor R18. T3’s secondary feeds the base of second IF amplifier TS5, another OC169. This operates with fixed bias at a collector current of around 5mA, giving full gain with no AGC control. TS4 feeds the untapped, tuned primary of T4, whose secondary feeds demodulator diode MR1, a germanium OA90. MR1 demodulates the 455kHz IF signal, which is filtered principally by 22nF capacitor C52, with additional filtering by 100W resistor R25 and 22nF C54. MR1 also feeds the AGC line via 1.5kW resistor R24. The low-pass filter formed by R24 and back-to-back 25μF capacitors C47/C48 removes the audio signal. Electrolytic capacitors are ineffective at high frequencies, so a ceramic capacitor (22nF, C52) is added in parallel to C47/C48 to ensure complete filtering of the IF siliconchip.com.au signal and prevent feedback in the high-gain IF strip. The back-to-back connection of C47 and C48 allows for the AGC line (a negative voltage in most designs) going positive with very strong signals. A second filter section (1.5kW resistor R21 and 25μF capacitors C43/C44) supplies the AGC voltage to the first IF amplifier, TS3, and to RF amplifier transistor TS1, in the tuner section. Unusually, the PHA 520 does not have an AGC extension diode, as is common in high-quality domestic radios and near-universal in shortwave and communications sets. Both IF amplifiers use ‘single point’ Earthing. For example, TS4’s collector circuit bypass (C49, 100nF) and base circuit bypass (C40, 100nF) both return to TS4’s emitter rather than to ground, as in most designs. This gives more effective bypassing, with the advantage that no emitter bypass is needed. The demodulated audio is coupled via 25μF capacitor C56 to the base of audio driver transistor TS6. This drives transformer T5, which supplies push-pull audio to the output transistor pair, TS7/TS8. 8.2kW resistor R31 and 1nF capacitor C57 across T5 apply top cut (a reduction in treble). The output stage is biased into Class-B mode by bias supply divider R32 (3.9kW) and bias diode MR2 (AV2). This diode is effectively a This shows the other side of the coil pack. Australia's electronics magazine September 2025  97 Table 1 – Pye PHA 520 sensitivity vs frequency for 50mW output Table 2 – freq vs image rejection Frequency Input signal level S+N:N Signal level for 20dB S+N:N Frequency Image rejection 600kHz 7μV 10dB 25μV 1400kHz (band 4) 70dB 1400kHz 1μV 3dB 10μV 4.4MHz (band 3) 35dB 2MHz 2.5μV 3dB 20μV 14MHz (band 2) 31dB 4.4MHz 0.6μV 2dB 20μV 6MHz 4.8μV 3dB 15μV 14MHz 2μV 3dB 12μV 15.5MHz 7.5μV 10dB 9.6μV 28MHz 7μV 12dB 15μV circuit bandwidths increase, meaning less attenuation of the image signal. Table 2 shows the image rejection performance. IF bandwidth at -3dB is 5kHz, while at -60dB, it’s 25kHz. This relatively low figure would have made tuning easier for untrained operators, and it’s explained by the unusually high (nanofarad) values of IF tuning capacitors C34/39/42/50. Such capacitors are more commonly in the 200~300pF range. The AGC is effective. A signal increase of 78dB is needed for a 6dB rise in audio output. The set went into overload with an input signal of around 200mV. The audio response from the volume control to the speaker is 110Hz to 9kHz, while from the antenna to the speaker, it is 50Hz to 2.7kHz. Total harmonic distortion (THD) was only 2% for a 50mW output, and the same at 10mW, a sign that crossover distortion is well controlled. The maximum audio output at clipping is around 350mW. So, would I buy one? I would, if only to repeat Alby’s exercise of rescuing it from obscurity. If you come across one, I reckon you should, too! transistor with its base tied to its collector. This creates a low-voltage supply that delivers the correct bias for TS7/TS8. The AV1 has thermal characteristics identical to the base-­emitter junctions of the output transistors, giving accurate bias regulation with changes in ambient temperature. Feedback from the output terminals is applied to the emitter of audio driver TS6 via C59/R33/R30. The audio output can be directed to the internal speaker or muted, but it is always available at the 600W output connector. This allows the PHA 520 to be run at high volume as part The AVRS The Australian Vintage Radio Society is a not-for-profit organisation dedicated to preserving our radio and related electronic history. Members come from all walks of life and enjoy the company of persons with similar interests. Meetings are held on the first Saturday afternoon of the month; visitors and prospective members are most welcome. Most meetings include a talk by a presenter with experience in radio restoration or history, plus a display of radios and related equipment of the era. Advantages of AVRS membership include: ● Access to the Valve and Component Bank, where members can obtain valves and hard-to-get parts at reasonable prices. ● Access to the Circuit Diagram Service to assist members with their electrical restorations. ● Technical assistance. ● Restoration workshops. ● A bi-monthly newsletter. We meet at St Faith’s Anglican Church Hall at 4-8 Charles St, Glen Iris 3146, Victoria (Glen Iris is near Burwood). 98 Silicon Chip of a receiving or communications system without the nuisance of adding to noise in busy workplaces. The set was designed for simplicity of operation, with only three frontpanel controls: volume, band switching and tuning. It’s switched on by simply opening the cover, which actuates the lid switch, SWB. The set’s condition The set arrived in working condition, with all the electrolytic capacitors replaced. It had also been cleaned, so I didn’t have to do much; I just set about testing it. Unlike two other examples I am aware of, this set had black pointer knobs on the band change and volume controls, with a white ‘television’ knurled knob for tuning. The other examples used the white knobs for all controls. How good is it? It’s as good as commercial communications receivers of the day, lacking only such refinements as a signal meter and the beat frequency oscillator needed for Morse and single-sideband (SSB) reception. Given its purpose – receiving shortwave radio broadcasts rather than being part of a communication network – it’s perfect for its intended use. Operators were expected to have little previous radio experience. The straight-line dial makes tuning easy, especially towards the top end of the tuning range. Table 1 lists sensitivity figures for 50mW output (S+N:N is the signalplus-noise to noise ratio). As the RF amplifier adds an extra tuned circuit at the signal frequency, image response is improved over a converter-only front end. This improvement declines at higher frequencies, as the antenna and RF tuned Australia's electronics magazine Special handling It’s a robust set, made to operate anywhere, any time, by anyone. Just remember that it needs an external antenna to work. Conclusion Thank you to Ray Gillett of the HRSA for lending me his example, to HRSA member Alby Thomas for his research into the Colombo Plan, and to Kevin Poulter for his recollections of Pye manufacturing. I’d also like to thank Warwick Woods of the Australian Vintage Radio Society (AVRS) for the circuit diagram, parts list, parts layout diagrams and other assistance. For more information on these societies, check out the websites for the HRSA (https://hrsa.org.au) and AVRS (www.avrs.org.au). Also see the panel SC on the latter. siliconchip.com.au