The R109 Communications
Receiver
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This set, the "A"
version, has the serial number 6800 and must date from some time
after 1942 when the series was introduced into service. It uses
8 Mazda-Octal-based valves.
There were various versions
of this receiver (R109, R109A, B and C) covering different wavelengths
and carrying different controls and features. The A and B versions
used the ARP36 (SP61) as an RF stage. This has a 6.3 volt heater
and replaced the ARP12 (VP23) used in the R109 although reappearing
later in the R109C. The SP61 has a gm of about 8mA/V compared
with about 1ma/V for the VP23 and has a better large signal handling
capacity.
My example is the A model and
covers 2 to 12 MHz in two ranges and, like the other models,
is designed to work from 6 volts DC.
Strangely, it has "V3A"
marked on the chassis but has an ARP12 (with "Orig valve"
in pencil on the side), rather than the expected ARP36 fitted
in the socket. I'll need to find out why this is. |
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Above, the R109 viewed
from its right side/rear.
Below from the top showing the
sockets for spare valves. |
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Below: From the underside/front
you can see shock mounts for the (empty) spare valve tray (a
useful source of new MO valveholders). The vibrator and its spare
are both missing. |
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Below..this is the rear/right
side of the set. |
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Below, view from the left side/underside
showing power supply components. Initially I was puzzled by the
J25 rectifier (centre), but I discovered that the R109A variant
and others were modified for improved CW reception by doing away
with the AVC circuit and removing and rewiring the volume control
(R1a) in the audio amplifier to act as a bias control to control
RF gain, aided by a grid bias voltage applied to the IF amplifier.
I've added extra diagrams to show these mods below... W3a is
the J25 rectifier. V3a is the SP61 |
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Detail of the valve test
panel (note the mod for V3a), and below the outer case. This
is nothing like the original because mine must have been lost
by the previous owner however, given an old computer case one
can always knock together something suitable... after all I did
get GCE "O Level" Metalwork in 1958. |
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