The R206 Communications Receiver

I bought the receiver depicted above in 1958, via a government surplus ad in Practical Wireless. I recall that it cost about £22:10:0 including carriage and it arrived in an enormous packing case on the back of a British Rail 3-wheeler truck. It’s an early version known to aficionados as the MkI. A friend saw it and ordered one but when it arrived it turned out to be a MkII which had a standard semicircular dial like its cousin the R107. As far as I know the R206 was developed by the Marconi Company from a much earlier radio but I’m pretty certain that Cossor made this particular model in 1940. Personally, I think it’s the best radio built in the period and has lots of intriguing design details. The valve line-up was the last word in what was available at the time and no expense was spared in construction to ensure stability. The front end uses a turret tuner like the ones that appeared in the first multi-channel TV sets. This example employs six sets of four wedge shaped copper boxes which fit together to make a massive drum. Each box carries a set of heavy rhodium plated contacts which mate up with springs on the back of the tuner module. The drum can be rotated by a large starting handle-like device on the front panel, which operates huge gearwheels via a chain similar to those found in Meccano sets of the same vintage. The six wavebands are approximately as follows: - 0.54-1.2 Mc/s, 1.2-2.4Mc/s, 2.4-4.8Mc/s, 4.8-10Mc/s, 10-20Mc/s and 20-30Mc/s. The tuning dial is a drum affair lit from an interior lamp and with its tuning condenser, which has specially shaped vanes, gives an excellent linear presentation of the bands. The two-speed tuning knobs have a built-in flywheel and carry a vernier scale giving superb resettability much like the HRO of the same period. Sensitivity is superb as the front end uses EF50 and EF37 RF stages, an ECH35 frequency changer and a separate EF50 oscillator. I must admit to never having studied an official circuit diagram but tracing the wiring shows that the RF oscillator has an earthed anode and employs the screen grid in the anode function. Both AF and RF gain controls are provided, and the latter is switched out when AGC is selected. The IF strip uses EF37s in three stages of amplification running at 465Kc/s and switchable crystal filters giving passbands of 8Kc/s, 2.5Kc/s and 900c/s. On CW or MCW further enhancements can be made using audio filters down to a couple of hundred cycles with heavy clipping and noise elimination; the clipper giving a mellowing of the narrow band audio. The BFO is tuneable and uses a separate EF37. The AM detector is an EB34 and also carries an AVC rectifier providing an input to a stage of amplified AGC. Audio output is via an EL32 to an internal loudspeaker carried in the matching power supply unit. Together the receiver and the power unit must weigh over a hundredweight although I must admit to never actually having weighed them. The oscillator has its own stabilised HT supply which seems to double as a tuning indicator, as it can be seen through a hole in the front panel of the power unit, and its neon glow changes with signal strength.

I found one weakness in the set which made me carry out modifications a few years after I bought it. There is strong pulling of the local oscillator when strong signals are received on the highest waveband. I found this out when I used the receiver as a back end to a nuvistor VHF receiver for 2 metres. I substituted a dual gate FET in place of the EF50 oscillator, which cured the problem. As the mod was done on an old EF50 base the new device merely plugged into the original valveholder so could easily be removed. Like all single-superhets of similar design, image rejection is poor because of the low IF frequency. I added an FM demodulator when narrow-band FM became the vogue on 2 meters and I removed a large plug-in plate, which carried the aerial terminals and made up a new plate with an S-meter and a new BNC style aerial socket. I once had a matching two-band long wave adapter for the receiver but this was long since cannibalised when I built a transmitter in its case.

R206 Hexagonal Turret Tuner Drum showing banks of 4 coil packs and the IF section with crystal filter module at rear

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