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Vintage Radio The Bush MB60 portable radio By Ian Batty We have previously described two Bush transistor radios: the early (1957) TR82C in the September 2013 issue and the VTR103 (1961) in August 2021. The MB60, also released in 1957, is the first valve-based Bush radio to grace these pages. T he Graham Amplion Company, founded in 1894, was well-known for loudspeakers from the early 1920s until their closure in 1932. The Bush radio company took over the remains of Amplion in 1932, deriving their name from their Shepherd’s Bush (London) facility. Initially trading as a subsidiary of the Gaumont British Picture Corporation, Bush became a subsidiary of the Rank Organisation. Bush was a major manufacturer of radios and merged with Murphy Radio in 1962. While their corporate history has been a roller-coaster, their products were among the best from England. Bush launched their popular DAC90A and DAC10 radios in 1950, followed by their distinctive TV22 television. David Ogle (MBE DSC) was a British industrial and car designer who founded Ogle Design in 1954. After the war, he studied industrial design at the Central School of Art and Design in London. He subsequently 92 Silicon Chip joined Murphy Radio, leaving Murphy in 1948 to join Bush Radio. While at Bush, he was responsible for the iconic design of the MB60 portable radio. The MB60 set a benchmark for style, well-matched by performance and sound quality. Valve lineup The MB60 uses Dx96-series directly heated valves. Released in 1940/41, the RMA/RETMA 1R5, 1T4, 1S5/1U5 and 1S4/3S4/3V4 series established the all-glass design that would continue almost until the end of receiving valve evolution, followed only by the short-lived Nuvistor and all-­ ceramic types. The initial release featured 1.4V filaments drawing 50mA (100mA for the 1S4/3S4/3V4 output pentodes). These appeared in the Mullard-Philips system as Dx91~93 releases. At 50mA per valve, a four-valve portable set would demand 250mA from the A battery. A compact set using a single ‘A’ cell Australia's electronics magazine would get less than ten hours of filament battery life. The Dx96 series halved the filament current consumption while giving near-identical performance, making portables more practical. The DK96 pentagrid converter differs from the familiar DK91/1R5. It’s the classic pentagrid, providing a committed oscillator anode. By comparison, the 1R5 inherited the dual screengrid design from the octal 6SA7. The DF96 pentode, DAF96 diode-pentode and DL96 power pentode use the same electrode structures as their predecessors, the 1T4, 1S5/1U5 and 1S4/3S4/3V4. Circuit details My redrawn (and hopefully clarified) circuit is shown in Fig.1. I am using the Bush’s own service manual circuit as my reference, as the Wireless and Electrical Trader 1403 version is impractical. I have preserved the component numbering but their strict siliconchip.com.au Fig.1: the circuit diagram for the Bush MB60. Note the extra IF amplifier (DF96, V3), making this set very sensitive. first-to-last numbering order has been upset by my aim of making the circuit more understandable. The MB60’s dual-band design (long wave and medium wave/broadcast band) is accommodated by a ferrite rod antenna with two windings, and an oscillator coil with just one. This design was reused in the follow-on TR82 that was mentioned in the introduction. The circuit parallels the ferrite rod’s two tuned windings for medium-­ wave reception. This gives a lower inductance than either winding by itself, allowing the antenna section of the tuning gang (VC1) to tune over 526~1605kHz for the medium wave/ broadcast band. Bush advises against adjusting the antenna coils for low-end alignment, so this is done by adjusting the oscillator coil for maximum sensitivity at 600kHz. Top-end alignment is performed using trimmer TC3. Revised antenna coupling The initial release’s antenna input/ car radio socket connects to the top of the tuned circuit via a 5.6pF capacitor. As noted below, this is not highly effective, and can put the antenna circuit off-resonance. The second issue of the MB60 uses the accepted design of a dedicated primary winding, as shown in Fig.2. The converter operates with zero bias and is gain-controlled from the AGC circuit via grid resistor R2. The oscillator section uses a secondary-­ tuned Armstrong circuit formed by transformer L4/L3. As the DK96 is a 6A8-style pentagrid, its oscillator anode (pin 3) is supplied from HT via resistor R5. Feedback is coupled to L4 via capacitor C13, while L4 couples inductively to the local oscillator (LO) coil’s tuned primary, L3. 515pF padder C11 ensures tracking between the antenna and oscillator circuits for medium-wave reception. L4/ L3 is slug-tuned to allow adjustment at the bottom end of the medium wave band. Trimmer TC4 provides top-end alignment, while the LO is tuned by the oscillator section of the gang, VC2. For long-wave tuning, the antenna circuit uses only the L1 winding on the ferrite rod, with L2 switched out of circuit. L1 alone, tuned by tuning capacitor VC1, now shunted by capacitor C3 (160pF) and the two trimcaps (TC1/ TC2), restricts the antenna circuit’s siliconchip.com.au Australia's electronics magazine March 2024  93 Fig.2: the dedicated primary winding of the revised MB60. tuning range to only 158~280kHz. A local oscillator’s tuning inductance is usually changed for different bands by switching in a different coil set, as changing tappings on one coil would modify the feedback ratio and affect the converter’s injection voltage. This is undesirable, as pentagrids must have a defined minimum injection voltage for optimal conversion gain. Instead, the MB60 switches extra capacitances into the circuit. C9 (450pF) is connected across tuned secondary winding L3, restricting the LO tuning range to around 630~750kHz. As C9 has a fixed value, low-end alignment and correct tracking rely on the adjustment of L4/L3’s ferrite core, which was set during the medium wave alignment. The LO’s top-end frequency is restricted by 33pF capacitor C10 and adjusted by trimcaps TC5/TC6. The converter drives the first intermediate frequency (IF) transformer IFT1’s primary. As with the other two IF transformers (IFTs), it has an untapped, inductance-tuned primary and secondary. The first IF amplifier operates with zero bias, with gain control via the first IFT primary. The second IF amplifier is similar, driving the third IF transformer, IFT3. Both stages get their screen supply via 33kW resistor R7, bypassed by 40nF capacitor C15. The secondary of IFT3 drives the DAF96’s demodulator/AGC diode. Demodulated audio develops across 500kW volume pot VR1, with the IF signal filtered out by 68pF capacitor C18 and 27kW resistor R10. The automatic gain control (AGC) signal is picked off via 2.7MW resistor R9, with filtering and voltage division by 40nF capacitor C16 and 2.7MW resistor R8. All controlled stages are fed with the same AGC voltage. The audio signal is conveyed to the first audio pentode section of the DAF96. This operates with low screen and anode voltages, as is common. The low anode current – which reduces the valve’s transconductance and thus its voltage gain – is compensated for by the high value of the 1MW anode load resistor, R13. The valve gets contact potential bias due to the action of 10MW grid resistor R12, allowing the grid to ‘drift’ weakly negative. The amplified audio signal is fed to the DL96 output valve’s grid via 3nF capacitor C24 and 330kW grid stopper R17. The DL96 gets about -5V bias via 1.8MW grid resistor R16 from the backbias developed across 560W resistor R18, filtered by 50μF capacitor C26. The DL96 valve drives the speaker via output transformer T1. The output transformer’s natural resonance is damped by 3nF capacitor C28. This is shunted by the tone control network of 10nF capacitor C27 and 100kW tone potentiometer VR2. Audio feedback is picked off from the loudspeaker and returned to the bottom end of 500kW volume control potentiometer VR1 via 10kW resistor R15, 40nF capacitor C23, 100nF capacitor C22 and 1kW resistor R11. My set is powered by a combined 1.5V/90V battery pack or from the mains. The later issue used two parallel D cells for the LT supply and a separate 90V B battery for HT. Mains transformer T2, with a multi-tapped primary, supplies full-wave rectifier MR2a/MR2b. After filtering by two-section pi low-pass filter C35/ R21/C34/R20/C33, it delivers about 1.35V to the filaments. The filament voltage from the mains supply is stabilised by shunt regulator MR2c. As the HT supply needs to deliver a lot less current, it is half-wave rectified by MR1 and filtered by C32/R19/C31. Mains/battery switching, via switch poles S2a/b/c/d, is performed by inserting or removing the mains plug. A quick glance had me puzzled. Was part of the battery HT+ wiring really going via the mains transformer primary’s wiring? Sure enough, it does, but only when the power plug is removed and S2 changes over to the battery position. This unusual connection effectively turns the set off via the On/Off switch in volume pot VR1: it cuts the mains input when on AC power and the HT supply when on battery. For battery operation, the LT supply is switched by S3a. Cleaning it up I got this set from a fellow HRSA member, happy to close the loop on Above: the controls for the Bush MB60 are located on the top of the cabinet. Right: a close-up showing the underside of the IF transformers with the added ceramic capacitors circled. Their values are in Table 1 shown opposite. 94 Silicon Chip Australia's electronics magazine siliconchip.com.au this line of distinctively designed English radios. It had been made to work, then smashed in transit. My friend and I divided the job – he would repair the case, and I would do the electronics. On receipt, it was working, but I reckoned it was a bit ‘deaf’ for a set with two IF stages. I recalled the Astor Aladdin (described in August 2016; siliconchip.au/Article/10049), which used a similar lineup. That set had only four valves but used two in the IF strip and employed one as a reflex stage for the first audio amplifier. Given the improvements in valve and component design, the MB60 should have been at least as good. The audio checked out OK, so it was on to the RF/IF section. All the IF transformer slugs were coated with white paint. A bit of gentle heating showed that I wouldn’t be able to soften it and free the slugs, a trick I had used on the Astor APN. What to do? At the converter grid, the IF responded best at 472kHz. So why did I get the best performance at 467kHz on the first IF grid and at 478kHz on the second IF? The bandwidth was wide enough to drive a truck through, confirming that, whatever the true intermediate frequency should have been, the various IF-tuned circuits disagreed. Also, it needed 20μV at the converter grid for 50mW of output, much worse than I expected. Believing that the manufacturer’s specification of 470kHz could be fiddled with a bit, I got a handful of 1~10pF trimmers, popped one across each tuned winding, and adjusted them for maximum gain. The final intermediate frequency of about 460kHz was lower than the specification, but the gain came up pretty well – see Table 1. A bit too well, in fact. I had been ready for IF oscillation with the trimcaps bodies hanging out of the circuit wiring, but expected that the feedback would be absent once I popped in small, fixed ceramics. It was stable but still a bit ‘chirpy’, so I dropped a 470kW resistor across the second IF primary. That did reduce the sensitivity at the converter grid from 6μV to 12μV, but the improved stability was preferable to instability. I then checked the antenna/LO alignment and found that the set working about as I expected. Having lived on a farm for around fifteen years, I reckon I know ‘agricultural’ when I see it. The LO coil looks like the designers forgot it, then just Table 1 – added capacitors IFT # Primary 1 Secondary 10pF 2 4.7pF 5.6pF 3 12pF 8.2pF threw it down, bolted it in place and told the assemblers to finish the set. Performance It is very good; more than just a standout example of 1960s design. For the standard 50mW output, it needed 60μV/m at 600kHz and 32μV/m at 1400kHz with signal+noise to noise (S+N:N) figures of 10dB and 11dB. For the standard 20dB S+N:N, the field strengths were 200μV/m and 150μV/m. Bandwidth for -3dB was under ±1kHz, implying some residual regeneration in the IF section. For -60dB, it was ±22kHz. Audio bandwidth, from volume control to speaker for -3dB was 140Hz to 10kHz, antenna to speaker about 130~1200Hz. Turning the Tone control to full cut brought the top end down to around 1kHz. The AGC was effective, needing a +40dB rise of input to give a +6dB increase in output. It would not overload even at 200mV/m field strength. The set went into clipping at 80mW, The front view of the Bush MB60 chassis which shows the ferrite rod antenna, permanent-magnet loudspeaker and controls. Nearly all the discrete components are mounted on this side. siliconchip.com.au Australia's electronics magazine March 2024  95 Tone Volume Bandchange A labelled photograph of the rear side of the chassis. In the service manual, they recommend an Ever Battery/Mains Ready type B147 Switch battery. Antenna socket 1st IF 1st IFT Oscillator coil Converter Output Transformer HT Rectifier 2nd IFT LT Rectifier 2nd IF 3rd IFT HT Filtering LT Filtering Demod/1st Audio Audio Out with 10% total harmonic distortion (THD). At 50mW, the THD was 7%, and 3% at 10mW out. Versions As noted above, the first release used capacitive coupling from the external antenna socket, while the follow-on used the conventional primary winding on the ferrite rod. The MB60 seems to have been released in just one colour scheme: a Mains Transformer grey case with a red perimeter band. There’s a moulded depression at the lower right of the rear cover in all three models. The VTR103 used it for the Tape Recorder output connector, but it was blank in the TR82. It was originally placed for the MB60’s mains connector plug. Mystery solved! Special handling Like the follow-on TR82 and This is a portable set running from 90/1.5V; you can see the battery plug and lead lying in the bottom of the cabinet. The cabinet was designed by David Ogle, who also designed the Ogle SX1000 car. 96 Silicon Chip Australia's electronics magazine Mains Socket VTR103, the tuning knob is a push fit. See the TR82 article (September 2013; siliconchip.au/Article/4404) for advice on safe removal. That said, I found finger pressure was adequate to withdraw the knob. Radiomuseum offers two online schematics (siliconchip.au/link/abrc). The Wireless & Electrical Trader 1403 version (like for the TR82 and VTR103) is difficult to understand: all switches are broken out into individual make/ break contacts. That demands that you get out a pencil and try to work out what is on (or off) for each band according to the description near the end of the article. It also takes some work to realise that mains voltage cannot connect through to the HT+ line. Pity the poor service technician. The other schematic, titled “Radio Servicing” is an extract from the Bush Radio Service Instructions MB60. This circuit is an improvement, except for the confusing power supply wiring. The manual contains extensive details and modification notes. I recommend the complete Bush original, which you can download from ElektroTanya (siliconchip.au/ link/abrb). They provide free original manuals, many from European equipment not hosted elsewhere. If you do visit them, consider uploading material they don’t have, or maybe just a donation. Radios like these come up on eBay, but you’ll also find them at auctions run by the Historical Radio Society of Australia (HRSA). SC siliconchip.com.au