Type 76 Receiver
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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. |
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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. |
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Above you can see that
many wires, but not all, have been disconnected from the Jones
plug. Why? |
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View showing the three
gang tuning condenser. You may note the off-line tuning condenser
coupler and a distorted chassis above the relay. |
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Again, you may notice
that the chassis is slightly bent from the angle of the tuning
coupler. |
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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. |
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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. |
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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? |
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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. |
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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. |
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Radio equipment for a
Fairey Barracuda showing the Four Square radios. |
Another picture of the
Four Square set-up |
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