The R109 Communications Receiver


 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.


 See the circuit diagram and the R109 Power Supply and Parts Lists



 Above, the R109 viewed from its right side/rear.

Below from the top showing the sockets for spare valves.



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.



Below..this is the rear/right side of the set.



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







 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.


Now see the commissioning of this receiver 

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