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'Trabant KM' from Radio RIM

front view of original device RIM Trabant KM

As already mentioned in the simple 0V1 receiver, Heinz Richter introduced the circuit of the Trabant KM in one of his books. It was a transistorized SW plug-in coil receiver, which was sold by Radio RIM with board and housing from the late 1960s as a kit. I copied that circuit, but in the form of a self-construction on a wood board. This was my first self-made regenerative receiver with transistors that worked decently. With a blocking circuit in the antenna line, which was tuned to the very strong medium-wave local broadcast transmitter, usable reception could be achieved from about 2 MHz. So I could at least listen to it in the lower shortwave area working maritime radio stations (such as Norddeich Radio), which at that time still in AM sent. Medium-wave reception, however, despite blocking circuit due to my short distance to the local medium-wave broadcast station did not succeed, as it penetrated everywhere under 2 MHz. Anyhow could be received on all SW radio bands at that time almost countless radio stations. The regeneration was setting on smoothly and changed the tuning only a little so that the circuit could easily be adjusted for high sensitivity.
TRabant KM schematicThe then good experience with this circuit recently led me to build such a device again. I wanted to be largely based on the original model in order to judge that device from today's perspective. Consciously, I therefore abstained from a ball drive or parallel fine tuning rotary capacitor, as with the original. After I had everything assembled and wired together, the device shown below worked immediately, playing the program of "Radio China International" at a good volume.

Trabant KM 2018 front side

A little bit over the beginning of the oscillation turned, the receiver can be used in principle for the reception of telegraphy stations and even for SSB reception. Without fine drive or band spread, however, a clean setting hardly succeeds even with a lot of tact. The damping of the resonant circuit is greater, as in circuits with tubes or field effect transistors and the achievable selectivity is therefore not quite as good. At least, several radio stations in one and the same SW broadcasting radio band can usually be well separated. The selectivity is so good that exact tuning requires a lot of tact even at AM stations. Sometimes individual radio stations become so strong due to the propagation conditions that they can not be separated from neighboring stations. The problem with the breaking in of the local medium-wave broadcast transmitter, on the other hand, is eliminated today, because they have stopped their operation a while ago.

Trabant KM 2018 inside

Since the original circuit is designed for high-impedance communication headphones, as you hardly get it today, I have installed a matching transformer. It is a small push-pull driver transformer from an old clock radio, in which the one half of the winding, which was actually intended to drive one of the two output transistors, is connected to the headphone jack. This is wired so that a modern, low-impedance stereo headphones can be connected. The volume achieved in this way is so great that at strong stations, the volume control must be turned back significantly.

The plug-in coils are wound on 6 mm plastic rods, as they are e.g. remain when truncates poti axes. Those are sticked into DIN-plugs and fixed with adhesive. With the shown coil with 30 turns, the range of 7 to 12 MHz can be received with the 200pF variable capacitor I use.

Trabant KM 2018 backLong before I already had used to improve the circuit and design of the original in various ways. Among others I used a variable capacitor for VHF, whose disk packs I wired in parallel. This gave me a good band spread. As a result of the 1: 3 fine drive contained in the rotary capacitor, SSB stations could still be set with sufficient accuracy up to the 20m band. Usable AM reception was even possible beyond 30 MHz. CB radio stations, as well as radio amateurs, which made at 28.5 MHz AM operation usually with modified CB radio devices, I could receive with a in this way wired device in great quality. In addition, I supplemented the device with a power amplifier, so that also powerful speaker reception was possible. In this way, a complete device, built in a wooden housing, gave me as a schoolboy a lot of fun.

Encouraged by this, another device was created around such a circuit. I had designed this as a monoband receiver for the 80m band and equipped it with an RF preamplifier. As a result, the frequency stability and the recurrence accuracy improved considerably. In addition, I inserted between the receiving unit and its speaker amplifier a preamplifier, which mainly only the voice frequency parts increased in the range of 300 to 3000 Hz. Neighboring SSB and CW stations could be well suppressed with sufficient feedback dosage. One can also regard the SSB demodulation in the oscillating state of the regenerative receiver as a direct conversion receiver with self-oscillating mixing stage. Direct conversion receivers only reach their selection through NF filtering. So quite similar results are achieved for SSB and CW with the regenerative receiver. Similarly, so the unwanted sideband is not suppressed. In contrast to the direct conversion receiver with separate oscillator, however, the circuit of the transistorized regenerative receiver at high signal strengths is very prone to synchronize to the input signals, resulting in distortions. After a adjustment of the operating point of the preamplifier stage led to strong frequency alterations, I added a 5kΩ potentiometer in the antenna line as RF controller. Thus, the received signal could be well dosed in order to avoid synchronization distortions largely.

Taken into the summer vacation to northern Denmark, I listened with this into a wooden box built device to various SSB circles. It was a special experience when I was able to receive a QRP station from the north sea coast of the Netherlands in a legible manner. It was aboard a sailing yacht, transmitting in SSB with only one watt.
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