SV3 Radio Magnadyne Radio

Description: Most models produced in the 1930s utilize similar circuits, particularly in the RF section, which follows a common LW-MW-SW scheme. Variations among different models primarily stem from the inclusion or exclusion of variable selectivity control, the sophistication of tone controls, and the output stage design. Some models employed European tubes (such as AK1, AF2, E444, E443H, 1561) while others used American tubes like 6A7, 6D6, 75, 42, and 80. Certain models, like the SV13, incorporated a combination of both European and American tubes (AK2, 78, 75, 42, and 80). The SV3 was a high-end model based on a European design, featuring a variable selectivity control mechanically linked to the tone control. The unit referenced in this note was largely original, with only one electrolytic capacitor and the power cord replaced. All other components, including the tubes, were original and bore lead seals indicating the payment of Italian taxes; they were also fully functional, with the exception of the E443H, which had an interrupted filament. Due to the high quality of the components, only a limited number of substitutions were anticipated, which turned out to be true. However, several aspects required significant attention, which are outlined in a list format for brevity.

1) Restoring the wiring: Unlike many contemporaneous receivers, most wiring in the SV3 was rubber-coated. The wires varied in type, with some being stranded and others solid, available in several colors: gray, green, blue, red, and yellow. Cloth-covered wires were used solely for tube filaments and connections between the chassis, field coil, and output transformer, which remained in excellent condition. All rubber-coated wires needed replacement. The closest alternative found was from HEW-Kabel Heinz Eilentropp GmbH, which offers flexible silicon rubber wires in various colors and diameters, also available from RS. Rigid rubber-coated wires could not be sourced, so cloth-covered wires from Radio Daze were used instead.

2) Electrolytic capacitors: The set features vertical tubular metallic electrolytic capacitors resembling modern designs. One original capacitor from Ducati remained intact and functional, revealing that it was connected to the chassis with two lateral 3MA screws, unlike current models that utilize a single large central nut. A metal casing can be adapted from a vertical electrolytic capacitor to recreate a similar base. The nominal values are 8 µF, and the negative terminal of the first capacitor must be insulated from the chassis due to the polarization circuit employed in this set.

3) Mica capacitors: The mica capacitors in the SV3 were manufactured by Ducati for demanding applications and are housed in robust metal cases.

The restoration of the SV3 model requires meticulous attention to detail, particularly in the replacement of aging components and the preservation of original wiring integrity. Each aspect of the restoration process should be approached with careful consideration of the model's historical significance and the specific requirements of the components involved.Most models produced in the 30s rely on very similar circuits; in particular the RF section of most sets relies on the same LW-MW-SW scheme while the differences between different sets are based on the presence or absence of the variable selectivity control, on more or less sophisticated tone controls and on the output stage. Some of these sets relied on European tubes (typically AK1, AF2, E444, E443H, 1561) or on American ones like 6A7, 6D6, 75, 42 and 80; some sets, like the SV13, used a mix of European and American tubes (AK2, 78, 75, 42 and 80). The SV3 was an high end model based on the European set and was endowed with a variable selectivity control mechanically coupled with the tone control.

The unit described in this note was almost untouched; the only substitutions concerned one electrolytic capacitor and the power cord. All other components were the original ones and also the tubes were original and characterized by the lead seal that testified the payment of Italian taxes; they were also perfectly efficient, with the exception of the E443H whose filament was interrupted.

Because of the high quality of all components it was expected that only a very limited number of substitutions would have been necessary; this was, in fact, the case but some other aspects called for a substantial amount of work. This note describes the points that, according to my experience, require more attention. To shorten as much as possible the length of the note, these points will be treated as separate items of a list.

1) Restoring the wiring. Differently from many other receivers produced in the same years, most of the SV3 wiring was based on rubber coated wires. Some wires were of the stranded type, others were solid and, to complete the picture, several colors were present for both types: gray, green, blue, red and yellow.

Cloth covered wires were used only for the filaments of the tubes and for the connection of the chassis with the field coil and the output transformer. These were in perfect conditions while it has been necessary to substitute all rubber coated ones. The most similar type of rubber coated wires that it has been possible to find on the market are those produced by the German firm HEW-Kabel Heinz Eilentropp GmbH.

Their extremely flexible silicon rubber wires are available in several colors and diameters and can be purchased also from RS. Since it has not been possible to find rubber coated rigid wires, these have been substituted with the cloth coated wires available, in a large variety of colors, from Radio Daze.

2) Electrolytic capacitors. The pictures of this set show vertical tubular metallic electrolytic capacitors that look like present day ones. One of the original capacitors (produced by Ducati) was still in place (and perfectly efficient too) and this allowed to observe that it was not connected to the chassis by means of a single large central nut as current ones but by means of two lateral 3MA screws.

It is not difficult, starting from the metal casing of a vertical electrolytic capacitor, to reconstruct a basis of this type. The nominal values are 8 uF and the negative termination of the first capacitor must be insulated from the chassis because of the polarization circuit adopted in this set.

3) Mica capacitors. The mica capacitors used in the SV3 were produced by Ducati for very requiring applications and are contained in robust metal cases (see one of these capacitors in Figure 14). Their