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 designed by the R107 team and was initially
to have been designated the R108. It was then developed by AGI
(Aeronautical & General Instruments Ltd) founded in 1915
and as I write this in 2021 still operating in Poole having moved
from Croydon. The first design looked like
the R107 with its semi-circular dial but AGI changed this
to a drum, said to be for mechanical stabilty reasons, but clearly
not, because the slightly later R206 Mk2 went back to the original
dial design. Personally, I think it’s one of the best radios
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 as follows: -
Range 6: 0.55-1.1 Mc/s
Range 5: 1.1-2.2Mc/s
Range 4:, 2.2-4.8Mc/s
Range 3: 4.8-10.1Mc/s
Range 2: 10-20Mc/s
Range 1:20-30Mc/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
three-band long wave adapter for the receiver but this was long
since cannibalised when I built a transmitter in its case. |
