All-wave Receiver RA17C-12; manufactured by Racal Engineering Ltd., GB - Bracknell.
The Racal RA17C-12, the military variant of the RA-17L equipped with US tubes, has also been used by the Swiss Army, at least in small numbers.
The receiver, its front pabel is painted in the strange turquoise - blue - gray colour, the typical „light battleship gray“ of the British naval equipment, has a weight of 30.5 kg in the 19„ rack version, it was also delivered in a steel case and the weight increased to 44 kg. After the dust cover is been lifted off, you can access the shielded units of the internal frequency synthesizer.
The front panel is dominated by the frequency dial window. In the 15 cm wide dial, there is the 1 kHz dial covering 100 kHz and the MHz-dial with 1 MHz dial markers. One megahertz band is covered on a 1.4 m long backlit film dial, sliding behind the dial window, when the big kHz dial knob at the left hand is rotated. The 1 kHz lines are separated by approximately 1.5 mm, so a frequency can be read with an accuracy of around 0,5 kHz. The dial pointer line can be adjusted with after calibration. The second large rotary knob on the right below the frequency dial window is the MHz selector. Between the large tuning knobs is the mechanical tuning lock, which protects the receiver against unintentional detuning when used for point to point operation. On the top right is the signal strength meter, which is calibrated in S units (the british sets have a meter calibrated in microamps), a switch underneath is used to set it to display the NF output level.
Below the meter, the controls of the preselector (ANT.TUNE) are located; you find a fine tuning and a coarse preselector range knob; in the position „wide band“, the preselector is bypassed.
In the middle of the bottom row of controls, you will find the mode switch, to select manual or automatic gain control, to activate the crystal calibrator which provides a signal every 100 kHz, and a signal for calibrating the BFO. Next to the right is the power switch, and the „AF Gain“, the volume control. Under a cover with the description „AFLevel“, the level of a secondary 600 Ohms output can be set, for example to drive a teletype converter. At the far right, the antenna attenuator with 5 positions is located.
At the left of the mode switch, the switches for the noise blanker and the BFO are located. The BFO control provides fine tuning and is located at the left of the front panel. In the bottom row of controls, further to the left, the AVC speed and the bandwidth controls are located. Crystal filters are provided in the 100 Hz and 300 Hz positions, coil filters in the wider positions. In the top left corner, the monitor speaker and its corresponding switch as well as the RF gain control are located.
To operate the receiver, connect it to mains (make sure, the voltage is correct, most Swiss sets are set to 220 Volts) and hook up an antenna. The „POWER“ switch turns the receiver on, the mode switch is set to AVC for AM reception, AVC decay time to Slow, the bandwidth to 6.5 kc, the „RF Gain“ turned fully up. For the sake of simplicity, the pre-selector can be switched off with the „ANT Range“ control on „Wideband“. With the monitor speaker switched on, the „AF gain“ volume control in middle position, tune to the desired MHz range with the right-hand „Megacycles“ tuning knob; in the range of the correct MHz mark, tune to maximum hiss (VFO 1). Then use the „Kilocycles“ tuning knob to tune the receiver within the 1 MHz range, on the film dial, you can read the kHz-digits of the desired frequency - this tunign procedure is typical for all receivers using the „Wadley Loop“ circuitry.
To calibrate the dial, the crystal calibrator is switched on and a marker signal will appear every 100 kHz.
In difficult reception situations (many strong stations near the desired signal leading to overloading of the front stage, the preselector is switched on by selecting the desired frequency range with the ANT RANGE control and the signal is adjusted to maximum with the „ANT Tune“ control (meter switch in position „RFLevel“).
On the rear, there is the voltage selector switch and the mains connector and a terminal strip with different available outputs included the AVC control voltage in order to use the receiver for diversity reception. At the right of the antenna socket, the 100 kHz intermediate frequency output is located.
You will recognize the Wadley loop circuitry when you look at the block diagram: the signal from the antenna input will pass through a low pass filter. After an amplifier stage (V3) and the manual preselector, it is fed through another low pass filter to the first mixer. The master oscillator (VFO 1, V1) oscillates at 1 MHz, its harmonics up to 32 MHz are selected by a low pass filter. After mixing with a fixed frequency of 37.5 MHz, the frequencies from 40.5 to 69.5 MHz can each be selected in integer MHz values with the MHz tuning. The first mixer (V7) provides the first intermediate frequency of 40 ± 0.5 MHz, which is filtered in a corresponding band pass. After subtracting the signal of 37.5 MHz, the second intermediate frequency of 2.5 ± 0.5 MHz is applied. The second VFO, used for kHz tuning and coupled to the film dial, gives linear coverage of the range of 2.1 - 3.1 MHz. After another subtraction, the third intermediate frequency of 100 kHz is generated in the third mixer stage, the signal then will after amplification pass through the filter bank. In addition to two narrow - band crystal filters for the CW bandwidths, the RA 17 has 4 coil filters. At the low intermediate frequency of 100 kHz, a sufficient selectivity can be achieved with this cheaper solution. Even if the set comes without mechanical filters, the selectivity is surprisingly good.
After the diode detector, the AVC voltage is generated via a second diode, the signal passes to the AF amplifiers.
V1 (EF91/6AU6, crystal oscillator); V2 (EF91/6AU6, „harmonic generator“); V3 (ECC189/6ES8, RF preamplifier); V4 (6F33/6AS6, harmonic mixer); V5 (EF91/6AU6, VFO 1); V6 (EF91/6AU6, amplifier 37,5 MHz); V7 (E180F/6688, 1st mixer to generate IF1 of 40 MHz); V8 (EF91/6AU6, amplifier 37,5 MHz); V9 (E180F/6688, 2nd mixer to generate IF2 of 2 - 3 MHz); V10 (EF91/6AU6, amplifier 37,5 MHz); V11 (EK90/6BE6, 3rd mixer to generate IF3, 100 kHz); V12 (EF91/6AU6, VFO 2); V13 (EK90/6BE6, crystal calibrator); V14 (EF93/6BA6, 1st IF amplifier); V15 (EF93/6BA6, crystal calibrator); V16 (EF93/6BA6, 2nd IF amplifier); V17 (EF93/6BA6, IF output amplifier 100 kHz); V18 (EB91/6AL5, AVC amplifier); V19 (EF91/6AU6, BFO); V21 (EB91/6AL5, detector, noise limiter); V22 (EF91/6AQ5, AF preamplifier); V23 (EF91/12AT7, AF final amplifier); V24 (EA76/—).
V20 (GZ34, mains rectifier).
In the late fifties, the British company Racal Engineering Ltd. developed the receiver RA-17 based on the Wadley Loop drift cancelling circuit by Dr. TL Wadley. In the Wadley Loop, the oscillator frequency is obtained from 1 MHz harmonics of a single master oscillator, a selected harmonic is mixed with the signal of a linear VFO covering 0 - 999 kHz. Thus, a very good frequency accuracy and stability can be acquired for a comparably low price.
The basic version RA 17 was presented in 1958, the export version with US tubes RA 17C-12 was used by the Swiss Army in small numbers.