Type 76 Receiver



This is quite a rare find. The receiver is part of a set of equipment used by the Fleet Air Arm mainly during WW2 and fitted in various aircraft (although I've only found a record of it being used in the Fairey Firefly, Fairey Barracuda and perhaps the Sunderland) to cover transmission and reception across the long and shortwave bands. This Type 76 receiver covers the former band, whilst the Type 78 covers short waves. The setup was referred to as "Four Square" after the shape of the rack and its contents, comprising pairs of receivers and transmitters Types 76/51 and 78/53. It's around the same vintage as the R1155 and post-dates the earlier R1116. As you can see below the case is made from aluminium and has suffered from being stored under damp conditions. Hopefully it'll respond to cleaning up and a re-spray. The "26V" refers to the voltage on which the system was designed to run (via a DC-DC motor converter) and is also the voltage used to run the valve heaters which are wired in series. Because this set didn't cover an amateur band it's not as common as the shortwave model.



The equipment was covered by document ARI5206, and the circuit diagram, kindly supplied by Andy Young, M0FYA, can be seen below. There's a similar sized and very famous US receiver, the BC453 known as the "Q5er" by radio hams (and used by many as an IF amplifier) which is roughly similar in size and function to the Type 76 receiver. The BC453 also has a matching transmitter.


Our local government surplus shop, Super Radio, in Liverpool always had a BC453 sitting on the counter, tuned to the Light Programme on 200Kc/s. I imagine this was either to keep the sales assistants from falling asleep when not very busy, or to publicise the fact that they had some for sale. I'm sure Liverpudlian radio enthusiasts, now pensioners, will remember this. 


  You'll see that the above schematic shows that only that the RF front-end is fitted into the case, the remainder including the IF amplifier is part of another box known as the modulator. You may spot the strange assembly "RT1", connected to the local oscillator. This is a device known as a "Desynn" which allows the receiver to be tuned from a remote location. A small motor within RT1 controlled by a remote similar part can be made to move in synchronism and, because the motor is connected to a small tuning condenser, the local oscillator frequency can be varied a little in order to provide precise tuning. Connections are made through Pins 7, 8 and 9 of the rear-mounted 10-way Jones plug. Below is an enlarged picture of the parts list then a series of pictures of the receiver.

A different method was used in the R1147 in aircraft like the Spitfire. This used a Bowden cable.











 Above you can see that many wires, but not all, have been disconnected from the Jones plug. Why?


 View showing the three gang tuning condenser. You may note the off-line tuning condenser coupler and a distorted chassis above the relay.


 Again, you may notice that the chassis is slightly bent from the angle of the tuning coupler.



 V2 is missing. This is a VR53 which is a common EF39 pentode. Also missing are its securing springs with bakelite top (like the one on V3). The lightning arrestor is missing its securing screws.  The relay, probably connected with transmit, has two energising coils, but the circuit diagram shows only one is used, maybe indicating two possible voltages could be used in different set-ups. The bent chassis and off-line coupler are again apparent.




 The front panel isn't in too bad a shape and may respond well to cleaning although the signwriting needs to be left untouched. The set has two wavebands, 150 to 270Kc/s (3000m to 1100m) and 270 to 505Kc/s (1100m to 594m)

 Part 10A/17259, which has been unbolted from its mounting holes above the wavechange switch, is a lightning arrestor.




  Behind the front panel you might spot more cut wires which I guess went to the dial lamp which is missing.

 Wires going to a missing dial lamp have been cut. Just peeking out from underneath the screened box is V1, a VR92 (an EA50 diode) which is wired to limit any positive voltage appearing in the aerial circuit. Was this to kill any radar signals getting into the MF aerial or just limit pickup from the associated transmitter?




 The lightning arrestor

  These are the aerial tuning coils. A screening can should be fitted but has been removed together with half the bakelite mount for the beehive trimmer, one of which is also missing.

 At this point I'll go over the slightly puzzling facts.

The centre section of the chassis is distorted and some parts are missing, V2 plus the can for the aerial coil, the beehive trimmer for one of the aerial coils and the dial lamp. The larger Jones plug is mostly disconnected as if someone started to remove it but never finished the job. At the rear I found a wire from the remote tuning motor had been cut off the wavechange switch together with one of the 10pF condensers. This must have been done to isolate the remote tuning circuitry. This may also explain the cut wires to the Jones plug, some of which are for remote tuning. In cutting off these wires an adjacent decoupling condenser has also been cut from its termination. A wire from the relay has also been cut, perhaps wire K1? What's the explanation for the above? There are two possibilities and neither seems to fully make sense. One is that the owner wanted the parts and removed them. Secondly, looking at the distorted chassis, the missing parts were damaged and were removed. The position of the distorted section of chassis implies a rather severe blow which may have broken the valve and badly dented the coil screening can and maybe broke the dial lamp? None of the latter would explain why the remote tuning circuitry and most of the pins to the Jones plug were disconnected. Also puzzling is the fact that the lightning arrestor was unscrewed from its mounting holes.




 Radio equipment for a Fairey Barracuda showing the Four Square radios.

 Another picture of the Four Square set-up

 See the transit case

 See its refurbishment

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