AM broadcast receiver as CB transceiver
Here is the circuit of a transverter shown, with which one could use an old tube AM broadcast receiver as a CB radio set. In the radios with a large wooden housing, could easily accommodate this circuit additional. For the additional power demand, the power supply unit of such receivers was usually sufficient. It was possible to achieve a transmission power of about 500 mW, as was permissible by German federal regulations.
Of course, the use of such devices was then inadmissible - and in the EU it is still today! Owners of an amateur radio license may natural make experiments in the 10m band.
With the oscillator signal of 28.5 MHz, the CB reception range of 26.880 to 27.985 MHz could be tuned in the medium wave range of 515 ... 1620 kHz. For example, with a standard 30 MHz crystal, also could be tuned the band from 28.380 to 29.485 MHz. For possibly necessary suppression of a local broadcast transmitter penetrating from the medium wave range, a suitably tuned blocking circuit had to be inserted at the transverter input (for example 648 kHz for the then BBC frequency). This problem does not exist anymore after switching off most medium wave transmitters. The rotary capacitor at the input had to be set to maximum reception volume on the respective tuned CB frequency. With an inserted wire loop in the anode line of the left triode system, which hat to be mounted closer to the input circuit, the input sensitivity could be significantly increased. In the course of this it was not even necessary to bring the so caused regenerative feedback until shortly before onset of oscilllation.
The transmitter could be operated by plug-in a suitable transmission crystal on any frequencies in this area. The in transmit mode switched-on rotary capacitor had to be set to maximum power respectively safe oscillate onset of the transmitter circuit. For modulating, the audio amplifier of the broadcast receiver (e.g., with EL84 or ECL86 power amplifier) could be used by a suitable wired switching. Since the signal of a dynamic microphone was not sufficient to drive enough the phono input of the amplifier part, a carbon microphone from a telephone receiver (speech capsule) had to be used, if additional circuit complexity should be avoided. About a small audio transformer, the voltage of a 9-volt battery was supplied to this. With sufficient transformation ratio (e.g., 1:10), the transmitter could be modulated aloud in this way.