Duotonal SV13

The SV13, among the sets produced in the ‘30s by Magnadyne, implements in the most complete way the Magnadyne philosophy concerning the possibilities offered to the user for adapting the quality of the reception to specific programs and receiving conditions. For this reason it was advertised as “The Stradivarius of radios”. The price, differently from other sets, was not reported in advertisements but making reference to other sets like the SV15, it was probably not less than 1300 Italian lire (equivalent, with good approximation, to 1300 Euro). The SV13 relies on a set of mixed European and American tubes (WE32/AK2 for frequency conversion and the classical set 78, 75, 42, 80 for all other functions) and uses the same group of antenna and oscillator coils as most other Magnadyne receivers of the same period, designed for long, medium and short waves (Figure 1).

Figure 1 – Some of the coils of the SV13. The alignment trimmers are located on the top.

Its schematic has never been available in the literature and the SV13 has often been indicated as a variation of other models (e.g. SV15, SV17). In fact the SV13 adopts the same conversion and intermediate frequency circuits of the SV74, the audio preamplifier of the SV17 and the power supply and audio power stage of the S35 (unicuique suum). The schematic deduced from the set #71471 has been now loaded on Radiomuseum.org.

Before some comments on the restoration of this fine receiver, it can be of some interest to discuss the peculiarities of its design.

A first feature, present also in other Magnadyne top receivers is the so-called Duotonal circuit (patented by Magnadyne);  it consists in a selectivity and audio bandwidth control performed acting, by means of a single knob, on both the coupling of the windings in the first I.F. transformer and on a traditional low-pass filter. Figure 2 shows the primary winding of the first I.F. transformer and  its large magnetic core to assure an high Q.  The rotation of the air trimmer is mechanically limited in order to use a part of its capacitance as a fixed parallel air capacitor; between the winding and the trimmer  an electrostatic shield has been inserted to prevents variations of the trimmer capacitance and consequent shifts in tuning when the coil is moved.

Figure 2 – Primary coil of the first I.F. transformer

The variation in the position of the coil determines variations of its coupling with the secondary coil and, consequently, a variation in the receiver selectivity. The same knob operates on a low-pass filter inserted on the plate of the 75 by means of the potentiometer shown in Figure 3 whose axis has a total length of 30 cm.

Figure 3 – The Duotonal control potentiometer of the SV13

The Duotonal is present also in other Magnadyne receivers, for instance in the SV3, but differently from the SV3, the selectivity and tone controls here are mechanically decoupled (the selectivity variation is performed only at the beginning and at the end of the rotation range and does not change returning back). As a consequence it is possible to select, with a single knob, arbitrary and independent selectivity and tone settings, indicated in the small window visible in Figure 4 where the area associated with low selectivity and full high tones settings is denoted as “High Fidelity”; the whole system reveals a clever mechanical design.

Figure 4 – Duotonal control indicators in the SV13

A separate bass tone control is also present, as can be seen in Figure 5; other circuits concerning the frequency response are the loudness-compensated volume control and the frequency-dependent load on the plate of the 75 constituted by the parallel connection of a 150 KΩ resistor with  a 100 KΩ resistor in series with a 10 nF capacitor.

Figure 5 – Detail of the SV13 schematic

Another feature inserted to facilitate optimal tuning is a shadow tuning indicator.

Some observations concerning the restoration of the SV13:

1) Wiring. Like in most other Magnadyne sets of the same period many wires have a rubber insulation and, in general, must be substituted. The silicone rubber coated wires produced by HEW-Kabel Heinz Eilentropp GmbH can match colors, size and look.

2) Decoupling capacitors. Many of them are 0.1 mF capacitors inserted in a single small box produced by Ducati and located under the board containing most other components, like in the Magnadyne S35. To access this box it is necessary to remove the board.

3) Potentiometers. The potentiometers were probably designed and produced by Magnadyne on the basis of the philosophy that led, eventually, also to the in-house production of vacuum tubes. Their resistive tracks can be completely dismounted without removing any rivet. Unfortunately they can be wiped out by solvent-based contact cleaners and increase even more than ten times their resistance when treated with other kinds of noise-reduction products. The substitution with other potentiometers is, in restoring, the last option; in the SV13 case it is also an almost impossible option for the Duotonal potentiometer reported in Figure 3 because of its peculiar mechanical structure. The solution that has been developed consists in the reconstruction of the resistive track as described in “Complete reconstruction of the resistive track in worn pots”. After reassembling the potentiometer with the new resistive element, its endurance has been tested by performing more than one thousand complete cycles; all that has been observed has been a modest decrease of the resistance after some hundreds of cycles because of the polishing of the surface.

4) Alignment. All the trimmers concerning the antenna and oscillator circuits are indicated on the schematic of the SV13 now available on Radiomuseum.org; their proper regulation is straightforward. It must just be remembered that the trimmers indicated as AM1 and AM2 in the schematic correspond, in fact, to the pairs of trimmers AM1a, AM1b and AM2a, AM2b that are connected in parallel (Figure 6).

Figure 6 – Allocation of the SV13 trimmers

The alignment of the I.F. channel is a bit more tricky since it could be made either in the maximal selectivity position (as it had been done with the SV3) or in the minimal selectivity one. Since the SV13 was designed for achieving a good quality in reproducing music, this last option has been selected and the alignment has been performed in order to obtain a response as flat as possible. The results, obtained by means of the measuring technique indicated in “Measuring the I.F. response in AM receivers” are reported in Figure 7 where the blue curve refers to the high selectivity setting and the red one to the wideband position.

Figure 7 – Frequency response of the I.F. channel in the SV13

5) Shadow tuning indicator. This component, produced by LESA, relies on a clever mechanical design that does not require any spring or electrical connections with moving parts so that it should be still perfectly working; a description can be found in the excellent article by Ernst Erb “History of Tuning Indicators: meters, graphs, Magic Eye, LED”. It can be easily opened  and its main components are shown in Figure 8.

Figure 8 – Main components of the LESA shadow tuning indicator mounted on the SV13

This tuning indicator is endowed with the small lever shown in Figure 9 for selecting the rest position of the moving cantilever and consequently the amplitude of the shadow band in absence of signal. It can be observed that this regulation acts only on the static magnetic field that keeps in equilibrium the moving cantilever when no current flows through the indicator coil.

Figure 9 – Lever for regulating the amplitude of the shadow band

The restored set has shown a remarkably uniform sensitivity over the whole extension of its bands and allows a decent reception even of the few remaining long wave stations, buried under extensive electromagnetic noise. The decoupling of the selectivity and tone controls allows a degree of flexibility in the reception of weak signals definetly superior to that offered by the Duotonal implementation of the SV3. Even today, using the SV13 is a pleasant experience that induces, in some cases, to forget its age.

Figure 10 – The restored SV13

Roberto Guidorzi, 03.May.14

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