Zellweger developed the „Mobile Lightweight Radio Station“ in 1940, the transmitter FL40 S or S-300 is one of the most important components, which together with the Receiver "Uster" / E41 was used in various combinations as portable, mobile or motorized radio station.
The transmitter is powered by the rectifier with a 1250 V plate voltage and 300 V grid bias voltage, this is generated from the 3 x 50 V / 150 Hz three-phase current from the generator, which can be driven by an electric or petrol motor. The power supply of the transmitter is quite complex, it is worth get familiarized with the technical specifications of the transmitter before any voltages are connected.
The transmitter covers the longwave ranges 190-240 / 240-400 / 400-715 kHz as well as the shortwave ranges 1500-2300 / 2300-3300 / 3300-4500 kHz, a calibration crystal is used to calibrate the transmitter to a frequency of 4000 kHz marked red on the dial.
The large range switch is used to select the frequency band, an aperture in the frequency displays only the corresponding frequency dial and the log dial. The main tuning knob and the fine tuning control are coupled mechanically.
In the lower right corner of the front panel, there is the selector switch for the operation modes telephony, telegraphy I and II (A1, A2); the transmit / receive switch and the main switch are located in the middle between the connectors for the power supply cable and the morse key / headphones sockets. The output power can be reduced from 100 W (A1) or 30/25 W (A2 / A3) to 1/2 or 1/4.
On the units with serial numbers 1-20, the transmit / receive switch must be activated in all operation modes; units with a serial number higher then 21 have a push-to-talk button on the microphone. At the top of the front planel, above the round dial, the buttons for antenna coupling, the coarse and fine antenna tuning and the instrument, which can be set to display the antenna current, plate voltage and plate current, are located.