Restoring an R1155 Receiver (/cont.)

Now for the part of the R1155 that most people are unfortunately too eager to remove ... The D.F. Circuitry. Often this was removed to make way for an audio amplifier, usually in the form of a 6V6. I believe the R1155 was unique, being the only WW2 receiver specifically designed with integrated Direction Finding circuitry. This required an external Loop antenna and an ingenious Twin-Meter display unit which was quickly nick-named ‘The Drunken Men’ by aircrew. I remember being mystified by the settings on the Master Switch on my first R1155, not to mention the three controls referring to a meter, and just what on earth was Aural Sense and Switch Speed?

I will not go into the finer details of how the circuitry works since that can be found in great detail in the manual that is freely available from VMARS and other sources, but I can present a brief insight into what was involved. Firstly, the T1154 was required to be inhibited owing to

the delicate nature of the ‘Visual Indicator Type 1’. This was accomplished  by setting the ‘J-Switch’ to the ‘D.F.’ position. This also selected the shorter Fixed Antenna which ran from the ‘mast’ immediately behind the astrodome to one of the twin vertical stabilisers (in the case of the Lancaster). Once the receiver was tuned to the appropriate ‘beacon’, the operator (Wireless Operator) then set the Master Switch to ‘Balance’ and using the ‘Meter Balance’ control, set the needles to cross on the centre vertical line (see below). The Master Switch was then set to ‘Visual’ and the external Loop was rotated to re-centre the needles on the centre line.

With this accomplished, making note of the angle on the Loop Antenna mechanism, and knowing the current flight vector, the operator could then direct the pilot to vector the aircraft onto the correct heading. Since in the case of most aircraft, there was a duplicate Visual Indicator in the cockpit on top of the glare shield, the pilot simply had to keep the needles on the centre line. A task not necessarily as easy as it sounds, particularly if you are being hounded by night-fighters! The ‘Meter Amplitude’ and ‘Meter Deflec’ controls both influence the sensitivity of the Visual Indicator. The ‘Aural Sense’ switch was used for Aural ‘homing’ when the visual method was (for various reasons) not feasible. In this case, the Master Switch is set to the ‘figure eight’ setting. The two toggle switches are in a way related. The D.F. Circuitry employs a low frequency oscillator with two selectable speeds (80Hz and 30Hz). This results in a ‘hum’ which the ‘Filter’ switch can be used to reduce to some degree. Like I said above, this is a grossly simplified description. For a more in-depth description, refer to the appropriate manual, A.P. 2548A.    

It is actually very simple to construct a workable loop antenna. Details of which can be found here. I was absolutely amazed at how sensitive the system is to the loop direction with only a few degrees of azimuth sending ‘The Drunken Men’ from one extreme to the other. It is absolutely fascinating to watch the Visual Indicator working and a true testimony to Sir Christopher Cockerell’s team at Marconi. I have to remind myself that this is 1939 technology.

Two close-ups of the Visual Indicator Type 1. A very delicate instrument. I believe I was extremely lucky to get mine so soon after acquiring the receiver. Note the metal ‘sleeve’ into which the meter fits. I believe this to be mu-metal and thus serves to reduce magnetic influence on the sensitive movements.