The Swiss Army acquired a number of Rohde & Schwarz Surveillance Receivers ESM-300 (BN 15074) in around 1960, the receiver is quite similar to the ESM-180 but covers a higher frequency range up to 300 MHz. It got the Swiss Army designation E-631.
The Rohde & Schwarz ESM-300 / E-631 is a quite heavy hollow state VHF surveillance receiver, in it's steel desktop cabinet, it's dimensions are 54 x 37,8 x 23,4 cm and it's weight 28 kg.
The receiver can be powered from mains 115 - 235 V and has an internal small monitor speaker.
The 60 ohms coaxial antenna connector is a so called Dezifix connector nowadays not common anymore, it is located in the left upper corner of the front panel.
At the left side, you find the sector shaped frequency dial, when the big waveband selector is turned, the respective dial is visible in a small window. The main tuning knob is located at the right hand slight further down.
At it's right, you find the main switch with the positions FM (frequency modulation) with deemphasis (50 usec.) active or off, AM (amplitude modulation) with AGC active or switched off; at it's right, you find the headphones jacks.
Two rotary controls act as RF gain control (active only in the position AM - no automatic control) and AF gain / volume control, when this one is pulled out, the monitor speaker is engaged.
For practical use of the receiver, connect a VHF antenna with the Dezifix antenna connector (the active pin is in the middle), set the main / operation switch to "FM o.E." if you want to control the emission of a military VHF transceiver in narrow band FM mode, set the bandwidth to 40 kHz, then select the frequency range and tune in to the signal. When the AF gain control switch is pulled out, you should hear some hiss in the monitor speaker.
To listen to a FM broadcast signal, select the mode "FM m.E." (Deemphasis 50 usec. active) and a bandwidth of 200 kHz.
To tune in to the weather report of the nearest airport, use the AM mode "AM geregelt" and look out for signals in the Air traffic band above 120 MHz. For special transmissions, the receiver provides a second heterodyne to tune in to CW transmissions.
The signal from the antenna socket is fed after a band pass filter to a first RF amplifier (with automatic gain control) (V1: EC80) and a second RF amplifier stage (V2: EF95) and then to the first mixer (V3: EF95), with the oscillator frequency (V4: EC81), the first intermediate frequency of 21,4 MHz is generated.
With the frequency calibration / second oscillator active, (V11: ECC81) (Switch located in the left lower corner of the front panel), the signal with it's harmonics every 21,4 MHz can be fed to the second tuned circuit, after mixing with the intermediate frequency, the main oscillator can be aligned by tuning to zero beat with the calibration control at the right next to the main frequency dial window.
After four I.F. amplifier stages (V5, V6, V7, V8: all EAF42), the signal is fed to the demodulator. In AM mode, this is a semiconductor diode (Gl 10: GK2261), im FM modes, V9 (PCF80) acts as limiter stage, in the FM small and wide demodulators, two semiconductor diodes (Gl 4,5 and GL 6,7: all GK2262) act as ratio detectors. V10 (EF42) amplifies the signal to feed an oscilloscope to visualize the signal.
When you switch from I.F. bandwidth 200 kHz to narrow band FM 40 kHz, V5 acts as 18 MHz oscillator (V5: EAF42), the I.F. of 21,4 MHz will be mixed to generate a second intermediate frequency of 3,4 MHz, for this lower I.F., there are narrower bandwith filters switched in the I.F. strip to achieve a FM narrow selectivity of 40 kHz. In addition, a 3,4 MHz oscillator (triode system of V11: ECC81) can be switched to act as BFO.
The AM signal is fed after demodulation in a semiconducter diode directly to the AF amplifier, in FM mode, the AF is generated - according to the I.F. bandwidth - in two separate ratio detectors. The audio signal is fed to an AF preamplifier stage (V14: EF42) and the AF final stage (V15: EL41), the set has an audio output of 3 W.
With the bandwidth switched to "narrow", the audio frequency response is reduced to 300 Hz - 3 kHz, in position "wide" frequency response is 30 Hz - 15 kHz.
In the power supply unit, two rectifier valves EZ80 (V16 / V17: EZ80) generate the plate voltage, all operation voltages are stabilised with valves V18 - V21 (PL81, PL81, EF40, stabilizer tube 85A2).