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Vintage Radio Tecnico Tecnico 1952 1952 Model Model 1259A 1259A The The Pacemaker Pacemaker By Associate Professor Graham Parslow Australian radios do not come much quirkier than this one. In this set, the entire dial flips up during use, exposing the speaker grille, which is normally hidden behind it. Unfortunately, this is a case of ‘style over substance’, as the sound quality suffers from this unusual configuration. The standard case for portables of the early 50s was a fabric-coated timber box with a flip-up or down panel that protected the dial and speaker. So this design is quite a deviation from the norm. Unfortunately, the relatively small area revealed when the dial is flipped up limits the useful size of the speaker that can be mounted behind it. In this case, it is a Rola 5C 5-inch speaker that provides inferior performance to other contemporary portables, which often incorporated 6in or 8in speakers. This set was available with other case colours, including red and grey. Most buyers were likely to choose a Pacemaker for style, rather than performance. Despite this, they went all-out with the circuit design, including fitting it with an RF amplification stage. It has both battery and mains power supply options. A visually identical battery-only model was also available, designated 1259B. The back panel of the radio clicks into place without retaining screws, so it is easy to move the mains power cord in and out of storage. My first impression on seeing the chassis from the rear is that everything is sturdy and comparable to most other portables of the time. A minor exception is the slim mains transformer, but it does not need to deliver high power, and there is limited space available for it. miniature valves have been used for a conventional lineup of functions for a superhet radio with RF amplification. The circuit for this set is therefore significantly different from the 1946 model 651 “Aristocrat” and 1950 model 1050 “Fortress” sets from the same manufacturer that we described previously. Those articles were featured in our February 2020 (siliconchip. com.au/Article/12350) & April 2020 (siliconchip.com.au/ Article/13817) issues, respectively. One innovative aspect of this radio is the use of a selenium rectifier stack, rather than a rectifier valve, which uses five selenium elements to produce DC from the mains transformer secondary. Selenium diodes could not withstand much more than 25V peak inverse voltage, so this stack of five can deliver 112V to C27 (50µF, 150V). The voltage dropping resistor R19 (1.5kW, 1W) lowers the HT to 94V. Toggle switch S2, at the rear of the chassis, provides an easy means to switch between battery and mains. In this case, the battery is a dual-output type in a single package (Eveready type Features In 1952, most old stock of full-size octal valves had been used up, and sets using all miniature valves were becoming the norm. In this case, five 7-pin siliconchip.com.au Australia’s electronics magazine 99 753), incorporating a 9V “A” battery and a 90V “B” battery. The filaments of these one-series valves are all carefully manufactured so that they draw 50mA at 1.5V, allowing them to be connected in series. That also applies to the 3V4 audio output pentode valve, which has two 1.5V filaments in series, so it can be driven by either 3V between pins 1 & 7, or 1.5V with pins 1 and 7 joined, plus a connection to pin 5 (the centre tap). So there are four valves in this set with 1.5V filaments and one with a 3V filament, giving 9V total (4 × 1.5V + 3V). The 3V4 is the output valve, so it needs higher electron emission from the filament to provide the meagre 250mW of audio output; hence, its filament consumes twice the power of the others, operating at the same current but with twice the voltage. When it comes to listening, it is decibels rather than watts that determines the acceptability of the listening experience, so the 3V4 is perfectly adequate for this radio. Construction As indicated on the circuit diagram, the loop antenna that forms the primary tuning coil is located behind the dial scale. The brass spring clip used to connect an external aerial is mounted on the side of the chassis adjacent to the “A” stencilled on the rear panel of the chassis. The other end of the chassis has a similar earth clip adjacent to the “E” marked on the chassis. The location of the loop antenna, elevated above the metal components within the case, means that there is no shielding blocking reception from any direction. In practice, the antenna by itself is adequate for receiving local stations, partly due to the extra amplification provided by the first 1T4 pentode valve, operating as an RF amplifier. The mechanism driving the flip-up lid is relatively simple, as illustrated in Fig.2 for removing the chassis from the cabinet. The dial string is driven by a second drum attached to the tuning capacitor shaft. The string passes through a hole in the right-hand pivot point of the dial. The flexibility of the string easily copes with the rotation of the lid through 180° without overly affecting its tension. Disassembly may seem like a fiendish task, but it is surprisingly easy. Removing two chassis clamping screws at the rear, then removing the knobs allows the chassis to slide out. The dial must be kept at 90° as the chassis is removed, so it passes smoothly The case is most likely made from PVC, not Bakelite. 100 Silicon Chip Australia’s electronics magazine through a slot in the case above the speaker grille. Circuit details The circuit is shown in Fig.1. It’s a relatively conventional superhet with an RF preamplifier. This preamplifier stage means that a three-gang tuning capacitor is needed, with one gang for tuning the aerial circuit, one for the local oscillator and one for the RF preamplifier. This ensures that only signals around the tuned station are amplified. The local oscillator (L4, L5 and C13) produces the appropriate difference frequency to feed to the oscillator grid (marked OG) on the 1R5. L4 provides positive feedback to the local oscillator to sustain oscillation. The 455kHz IF signal passes from the 1R5 mixer to the first IF transformer, for IF amplification by the second 1T4. The second IF transformer is coupled to the single diode in the 1S5 diode-pentode valve to demodulate the signal and also to generate the automatic gain control (AGC) voltage. This feeds back to the first two stages to lower gain for high strength signals via R10 (1MW). R12 (500kW) is the volume control potentiometer that passes the signal to the 1S5 audio preamplifier pentode grid. There is no tone control on this radio. The 3V4 output pentode grid gets audio from the 1S5 via 10nF capacitor C25. Grid bias is generated via the series filament connections. The 3V4 filaments are connected at the top of the 9V supply stack, and this is a directly-heated valve, so pin 5 is also the cathode connection. Its grid is DC biased to ground, so the grid is negative relative to the cathode. While using a directly-heated valve can complicate the design, it has the advantage of a near-instant turn on without a significant warm-up period. The selenium solid-state rectifier facilitates quick operation on mains; other contemporary mains/battery receivers that used a 6X4 valve rectifier with an indirectly heated cathode took some time for the HT supply to come up after switch-on. When powered from the mains, the filament current is derived from the full HT using series resistor R20, specified as 2kW, 5W. In practice, R20 is two 4kW resistors in parallel, both rated at 5W. This combination drops siliconchip.com.au Fig.1: the Tecnico 1259A came in two versions: a B variant which could only be powered via a battery (an Eveready 753); and the A variant which also included a mains plug and the necessary circuitry to allow the 240V AC 50/60Hz mains to supply the required 90V HT and 9V LT. Another difference is that in the 1259B, C18 (a 250µF 12V electrolytic capacitor connected to the filament of pentode 1T4) is instead rated at 25µF 40V. June 2020  101 Australia’s electronics magazine siliconchip.com.au 103V; the dissipation will be just over 5W, shared by the two resistors. Its operating mains power totals 11W. From a manufacturing perspective, the extra cost to provide a separate 9V supply, reducing mains power consumption (and waste heat) by around 5W, would be hard to justify. Power use does not change with audio volume because the output stage operates in Class-A mode. Restoration Fortunately, this radio presented with no component failures. However, before powering it up, I cleaned all the pins. Experience has shown me that many portables like this one develop oxide creep, which breaks the continuity of the filament connections. I initially powered it up from a dual-output bench supply. It drew 50mA from the 9V supply and 13mA from the 90V supply, for a total power consumption of 1.62W. These are spot-on, based on the manufacturers’ data, so all seemed well. Reception tests then proved that it was fully functional. That’s lucky because fault-finding on this radio would be difficult. The chassis is unusually thin as a result of the speaker being mounted well back into the body of the cabinet. Components under the chassis obscure all of the valve bases. So directly checking pin voltages is not possible. Physical restoration required replacing the carry handle, the Tecnico badge in the centre of the dial and the yellowed cellulose dial cover. The clear dial was reproduced using polycarbonate sheet that I cut to shape using tin snips. Luckily, the polycarbonate did not require heat moulding to fit because it is firmly retained in position by the screws holding the central Tecnico badge. The original badge was screenprinted with the Tecnico logo, and that printing had all but disappeared. Pictures of other radios allowed a reproduction to be created with graphics software and I then printed it on lightweight paper. This paper deformed evenly, so it glued smoothly onto the dome-shaped aluminium badge. Above you can see the rear view of the Tecnico 1259A’s chassis showing the five valves, power transformer, variable capacitor and Rola 5-inch speaker; the power switch is also visible at lower right. One nice feature, is that all valves are marked on the chassis. Right: the Tecnico badge is a ► reproduction printed on paper and glued onto an aluminium badge. The dial was made using a polycarbonate sheet cut to size. 102 Silicon Chip Australia’s electronics magazine siliconchip.com.au Conclusion As mentioned in passing earlier, I wrote up the Tecnico models 651 (Aristocrat) and 1050 (The Fortress) in previous Vintage Radio articles. This article on the Pacemaker completes the trilogy, covering the stand-out radio icons made by Tecnico. So who designed this unusual radio? It is most likely to be Zenith in the USA. A Zenith advertisement in the Saturday Evening Post of 1948 proclaims “The new Zenith Pacemaker is one great forward step in radionic engineering and modern styling”. The Pacemaker then appeared in New Zealand, manufactured by Collier and Beale in Wellington (see the book “Radio Days” by Peter Sheridan and Ritchie Singer, p247). Tecnico had associations with Collier and Beale as they had previously made and marketed Tecnico Aristocrat radios. Consequently, it seems that the right to use the Pacemaker design passed to Tecnico. The cabinet of the radio featured here is moulded with the attribution “Seco mould CAT. No. 700-1 C&B Ltd” and it seems likely that the cases were imported from the USA (other Tecnico models in my collection do not have this attribution). These are my own surmises, and they may be in error; any corrections from readers who know more would SC be welcome. The underside of the Tecnico 1259A’s chassis is absolutely packed with components connected via point-topoint wiring. This makes any form of testing quite difficult. The Zenith radio in question which has a near identical design to the Tecnico 1259A with the exception of the dial. ► siliconchip.com.au Australia’s electronics magazine Fig.2 (left): a diagram showing the dial cord arrangement and explaining how to remove the chassis. June 2020  103