The amateur superhet 'Semiconda 68'
The Semiconda 68 was originally a do-it-yourself project suggested in the Semcoset catalog. Initially, the modules required to set up this device appeared in the delivery program, namely an RF input section for the classic five HF amateur bands 80m, 40m, 20m, 15m and 10m, an IF amplifier with demodulators for AM, SSB and CW as well as a suitable one audio amplifier with voltage stabilizer for the aforementioned assemblies. From the start the RF part had a converter input. With the housing that was offered soon afterwards, the Semiconda 68 receiver could be built with these pre-mounted and pre-adjusted assemblies without any prior knowledge. With significantly smaller dimensions and complete transistor equipped the performance comparable to that of the tube-equipped receivers from the Italian manufacturer Geloso, which was widely used in European amateur stations until the end of the 1960s, was achieved. The housing for the Semiconda 68 was also prepared for the installation of a 2m converter, which made it possible to obtain a five-band HF receiver with which it was also possible to receive the 2m band. Completely assembled versions of this device appeared later, but became not widely used. Because of this, most of these devices were assembled and wired by the user, taking individual requirements into account. As a result, there are hardly any exemplars left wich, like the device shown, correspond to the proposed original concept without any additional components and switches on the front panel.
In terms of circuitry, the RF and IF parts were a divided into two modules further development of the MB103 receiver module. This was designed exclusively to receive the 10m band and was used as a follow-up device for 2m converters, especially in portable 2m DIY transceivers. In the Semiconda 68, the RF part has now been expanded to include the other four HF amateur bands 15m, 20m, 40m and 80m. In addition, there was also a changeover to complete assembly with silicon transistors. With this device a small but fully-fledged amateur station receiver should be created, because at that time station equipping with separate transmitter and receiver was common. The target group of this device were mainly owners of the in Germany recently introduced license class C, which allowed amateur radio operations in the VHF range. Most of them only used the 2m band. The Semiconda 68 now made it possible for such VHF amateurs to also be able to listen to the shortwave.
As can be seen in the circuit diagram of the input module with the designation HFB3.0, the RF section is divided into a preamp, oscillator and mixer. Essentially, it has hardly any special circuitry features. What is interesting, however, is the design of the range switching taking into account the matching to the transistor impedances. This did not have to be taken into account in the case of pre-amplifier fitted superhets equipped with tubes. There it was sufficient to switch the resonant circuits, coupling windings for the antenna and, if necessary, for the oscillator feedback. The 80m coil in the input circuit now serves as a broadband transformer for the antenna on all other bands. The converter input acts via a coupling winding on the resonant circuit coil for 10m. Except in the 80m range, the resonant circuits between preamplifier and mixer are driven without tapping from the high-impedance output of the pre-stage transistor operated in a base grounded circuit. The matching to the mixer stage occurs via a capacitive voltage divider (47pF / 100pF). The AGC voltage is connected to that of the first 455 kHz IF stage. Incidentally, this assembly was also available under the marking HFB1.6 with a 1.6 MHz IF output, so that an ordinary medium-wave broadcast receiver could serve as as a follow-up device respectively as an IF part.
The intended for double conversion receivers IF part ZFB3.0 works exclusively with LC resonant circuits. In a mixer, the input frequency of 3 MHz is converted to the lower IF of 455 kHz. An oscillator without crystal control, which is adjusted to a fix frequency of 3.455 kHz, is used for conversion. In the two resonant circuits following the mixer stage, coils with a particularly high quality are used. The selectivity almost reaches that of cheap ceramic filters, which were hardly available at the time. The coupling and thus the IF bandwidth can be select by means of various capacitors, which are connected to ground via a rotary switch.
In the by Semco at the time also in other devices gladly used conception, the AM demodulator is followed by an AGC amplifier, through which a very effective gain control is achieved. The control voltage obtained there is fed to the AGC voltage connections of the RF pre-stage and the first IF stage via the RF gain potentiometer. Switching to manual control is also provided. The BFO for CW and SSB reception, which is also operated without crystal control, acts on a product detector. With the sideband switch, its frequency can be set either slightly above or below the IF center frequency.
The module NFB12-9 with the audio amplifier and the voltage stabilizer has hardly any special features. Only the complementary output stage is still built with germanium transistors.