RX3A Dual Diversity Adaptor               RX6 Cathode Follower Kit

In 1952 the RAAF published Australian Air Publication 875 which detailed modifications to the RAAF AR7 Receiver. The RX3A Adaptor allows an AR7 to be used in a Dual Diversity Reception mode. The RX6 modification adds a cathode follower to the AR7 for use with the RX3 FSK Converter.

Dual Diversity. Two or more antennae are located well apart and fed to separate receivers. The output from the receivers is combined and the theory is that there will be less signal fading effect because while the signal in one antenna may fade the signal from the other one will not be affected at the same time. To be effective the IF signal from each receiver must be identical in frequency and to achieve this the RX3A Adaptor provides a common crystal controlled local oscillator signal to both receivers.

RX3A Dual Diversity Adaptor

The RX3A Adaptor was made by Pye Electronics and is a free standing or rack mounted box containing a power supply, crystal oscillator, buffer/multiplier and two cathode follower valves. It has comprehensive metering and there are 4 crystal positions allowing the receivers to be switched to different frequencies between 1.5 and 32 Mc/s.

In the rack mount configuration the RX3A is 20” wide x 5 ¼” high x 11” deep including connectors on the back (508mm W x 133mm H x 279mm D). It weighs 24 ½ Pds (11.1 Kg). In a free standing case with shock mounts, for mobile use, the height increased to 8 ¾” (222mm H) and weight to 29 Pds (13.2 Kg).

The valve compliment is:
                                    V1            6AU6               Crystal Oscillator
                                    V2            6AQ5               Buffer/Amplifier
                                    V3            6AQ5               Cathode Follower
                                    V4            6AQ5               Cathode Follower
                                    V5            5Y3GT             Rectifier

The Adaptor could run from 100-120v or 200-240v ac and consumed 52 watts.

To achieve the frequency range the Adaptor’s oscillator operates in fundamental, doubling, tripling or quadrupling mode.

                                    1.5 to   3 Mc/s                       Fundamental
                                    3    to 16 Mc/s                       Doubling
                                    16  to 24 Mc/s                       Tripling
                                    24  to 32 Mc/s                       Quadrupling

There are 4 crystal positions suitable for DC11 type crystals and for each one there is an associated variable trimmer capacitor to provide minor frequency adjustment.

There is also a 3 position switch for the three tuned output coils of the buffer/amplifier and a tuning capacitor with a vernier dial for tuning to the correct harmonic. To assist with tuning a scale reminiscent of that on AR7 coil boxes gives the tuning position for various frequencies.

The output from the Adaptor is via two cathode followers to co-axial sockets on the rear of the case.

The AR7 was modified by the installation of a co-axial socket on the rear of the chassis and an additional toggle switch on the front panel. A single pin socket was also mounted on the front panel and connected to the AVC line so that it could be metered.

The toggle switch was connected to the AVC line to earth it or leave it open. The existing wire from the AVC line to the AVC/BFO toggle switch was disconnected. Therefore it was now possible to have AVC on or off independent of the BFO on or off.

The wire from the HF oscillator coil box was disconnected and instead a co-ax cable connected from the new co-ax socket to C14, the LO signal feed capacitor for the mixer V3 (6K8G). By this means the LO signal from the RX3A Adaptor is fed to the mixers of each receiver to maintain them on the same crystal controlled frequency.

RX3A Hardware and RX6 Modification Kit.

This kit added a cathode follower to the AR7 to match the signal to the RX3 Frequency Shift Converter.

It consisted of a small metal box containing the circuit and a 6AU6 valve. It included a coaxial socket and 90 degree connector.

The box was simply attached with 4 nuts and bolts onto the back of the AR7 chassis. Four wires from the RX6 were soldered to earth, HT, filament and to the plate of the 2nd IF amplifier V5 (6U7). The 455 Kc/s signal was fed via co-ax cable to the FSK converter.

AR7 with RX6 Cathode Follower

The procedure for carrying out the modifications was outlined in RAAF Wireless Modification Order (Ground) No.11.

The shaded areas show the modifications.


Front View of AR7 Rear View of AR7 Note different IF cans
Homemade box for spare coils Another front view Rear view of chassis. Speaker Plug on Left, Earth, Aerial (2), Power Socket
AR7 Power Supply Rear of Power Supply AR7 Coil Boxes

Please note, this article is copyrighted.