RF25B
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This unit is a plug-in part of the Gee Navigation
System
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During WW2 a method of assisting
pilots in blind landing, and later finding a target for bombing
was designed by ARE, at the time located in Swanage, later RSRE
to be located in Christchurch and then Malvern. The original
Gee system (named after the radio "grids" developed
by the transmitters) operated between 20 and 30MHz but the later
MkII used a wider range of frequencies, had anti-jamming and
internal detonators to destroy the equipment if it became necessary.
The four RF Units including
the RF25, were part of Gee MkII.
David Marshall from Brockenhurst
kindly donated an RF24B, the lower frequency
version and an RF26B, one of the higher
frequency versions (click to see these) |
Front view showing the 5 preset channels. The
RF24 was similar but the RF26 and 27 had slow motion tuning instead
of a switch
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Below: Low loss ceramic
coil formers and beehive trimmers
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Below: Three VR65/SP61 pentode valves, RF amplifier
mixer and oscillator. Although very common in WW2 equipment these
Mazda valves were quite advanced in their design.
Like the EF50, the SP61 was intended for use in TV
receivers, before WW2 put an end to that idea, or at least postponed
it.
They use an 8-pin Mazda octal base which at first
sight looks like a standard octal base but has slightly different
pin spacings.
Why would there be two different shaped valve envelopes?
Did Mazda sub-contract their manufacture? Interestingly the metal
coating is perfect on the outer two valves but about to drop
off the centre valve perhaps implying a different maker.
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Below: Very clear layout
with carbon composition resistors and moulded mica capacitors;
the IF output coil is mounted on the lower edge of the chassis
allowing it to be tuned for maximum output
IF output coax linked to the
Jones plug.
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Below:High quality ceramic
bandswitch and each of the five preset ranges has a set of three
beehive trimmers
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Below: Rear view with label showing no modifications
incorporated
Power is supplied from the R1355 chassis via the
6-pin Jones plug. Note the manufacturer's identification stamp
"F.R. 5 G. B." The serial number plate also carries
leading characters of "F.R." Is this Ferranti? Maybe
someone knows?
After WW2 the number of UK Defence Electronics contractors
gradually dwindled. In the field of military computers the three
largest being Plessey, GEC and Ferranti although there were lots
of smaller companies such as Elliott Automation. There were numerous
take-overs, mergers and bankruptcies. British Aerospace bought
into areas other than aircraft manufacture and absorbed the remnants
of Plessey.
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Below: View showing a
Pye plug which connects the RF amplifier coil to an aerial tuning
unit.
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Views of the label attached to the unit and original
packing.
The information indicates 14MU which is RAF Carlisle
and presumably the unit whas been removed from an aircraft or
another RAF station stores
I think Form 4509B indicates the item is up for disposal.
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A little about Gee
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There were sets of synchronised
transmitters sending trains of pulses received by the aircraft-bourne
Gee receiver and the distance between these and the aircraft
displayed as a periodic display on a horizontal trace on a Type
62 Unit.
Drive to the 62 Display Unit
came from an R1355 Receiver which carries the IF strip, handling
the output from the chosen RF unit, video amplifier and power
supply. The R1355 has a front slot into which one of the RF units
could be inserted.
The four equipments, RF24, RF25,
RF26 and RF27 were capable of reception of a specific band between
10MHz to 20 MHz wide as follows.
RF24: Tunes to one of five switchable
frequencies between 20-30 MHz.
1= 22MHz, 2 = 22.9MHz, 3 = 25.3MHz,
4 = 27.3MHz, 5 = 29.7MHz
RF25: Tunes to one of five switchable
frequencies between40-50 MHz
1= 43MHz, 2= 44.9MHz, 3= 46.79MHz,
4= 48.75MHz, 5= 50.5MHz
RF26: 50-65 MHz continuously
tunable
RF27: 65-85 MHz continuously
tunable
The receiver of choice was connected
to a simple whip aerial mounted on an aircraft, via a simple
tuning unit type LU51, and has an IF output of 7.5MHz which was
amplified in the R1355. As the aircraft had to be simultaneously
in range of a set of three transmitters it was possible to try
reception from 20 to 85 MHz by plugging in one of the receivers
listed above. This choice would help the system to work despite
the vagaries of skip and propagation.
As jamming was prevalent in
WW2 four preset options were available in the R1355 MkII to reduce
the effect.
The system proved to be so useful
it remained in service up to 1970. Whilst at camp with the Air
Training Corps at RAF Kinloss in 1956 we were given a task of
searching an area of heathland for debris from a crashed aircraft.
I remember finding a mangled piece of metal which I recognised
as the chassis of an R1355. Other than in adverts in magazines
for government surplus that is the only R1355 I've seen, although
I've had lots of the RF units which I used for parts.
Surplus RF units in the 1950s
were useful for receiving Band I TV sound. In Liverpool this
was available on 48.25MHz which fell neatly into the range of
the RF25 where judicious twiddling of the beehive trimmers magically
gave one TV on headphones. This could be done for 7/6d or about
37p when at the time a TV set cost an astronomic amount equating
to at least 2 months average wages. |
Testing the RF25B
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Out of interest I powered
up my latest purchase from a power supply delivering 250 volts
and 6.3 volts for the valve heaters. |
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Above, the circuit showing
an RF amplifier V1, Mixer V2, and local oscillator V3.
Results were excellent and are
shown below.
The pictures need a little explanation.
The first shows the local oscillator
signal on the right and indicated as 52.3MHz. This is only roughly
correct as it reflects the accuracy of the marker placement.
Take no notice of the indicated
signal levels as I'm using a special attenuator to protect the
analyser from high voltages.
See this explanation
On the left is a scan of the
response of the RF unit from its aerial socket to its IF output
connector as seen by the tracking generator which is sweeping
from 30MHz to 60MHz.
The peak of the response is
set by the tuning of the receiver and, in this particular case
indicates the setting of the beehive trimmers for Range 2 as
well as the tuning of a couple of IF coils.
Essentially you are looking
at the response of the unit to a signal at the local oscillator
frequency minus the IF.
Because there is no IF amplifier
the response is rather broad, but peaking at 7.5MHz below 52.3MHz
or 44.8MHz. As each horizontal division represents 3MHz the response
at the -60dB level is from 42MHz to 47MHz, centred at about 44.5MHz.
As the unit is switched to higher
ranges the response curve keeps its shape and moves to the right
tracking the oscillator by 7.5MHz. |
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In case you're not
familiar with this sort of picture, the centre of the display
above is set to 45MHz and the span is 30MHz (meaning that each
horizontal division is 3MHz apart) so the vertical divisions
are as follows:-
30MHz (left edge)-33MHz-36MHz-39MHz-42MHz-45MHz
(centre)-48MHz-51MHz-54MHz-57MHz-60MHz (right edge)
Amplitudes as shown are only
relevant in terms of differences in their heights because I'm
using an uncalibrated probe. Marker 1 frequency is indicated
on the screen.
The next picture has the
centre frequency set to 10MHz and the span 10MHz (ie each vertical
division is 1MHz apart) giving a sweep of 10MHz plus/minus 5MHz.
The second picture shows the
response at the IF without connecting the tracking generator,
but intead using a signal generator adjusted to approximately
peak the IF signal.
Although this is indicated as
8.33MHz it's true frequency is 7.5MHz and shows the response
of Range 2 of the RF25 to an incoming signal of about 44.8MHz
originating from my TF2008 signal generator.
The scan setting is 5MHz to
15MHz. I just tuned the generator to peak the curve hence there
will be a small error, although the settings of the beehive trimmers
and ageing of components will no doubt be responsible for some
of the 830KHz difference. |
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Below, offered on Ebay
some time ago, a rather tatty R1355 complete with an RF24
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Below, a picture of a
nice example of an Indicator Unit Type 62 for a Gee setup. This
version has a line of VR65 (SP61) valves but newer versions used
the VR91 (EF50). Close examination shows a valve positioned immediately
behind the 6-way plug. This is V12 a crystal oscillator whose
75KHz crystal should be fitted in the hole adjacent to the valve.
This is apparently missing. The 62A version of this indicator
has the crystal and V12 reversed.
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Gee was later developed
to the Mk3 system, below, in which the RF equipments were integrated
into the main receiver chassis, the R3673. |
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The R3673 Gee Mk3
Receiver
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The design of the receiver
is typical of other cold war airbourne electronic equipments.
The set can be extracted from its mounting leaving in place the
connecting box (361A) and harness. Tuning is carried out remotely
via a selsyn motor and the gearing seen below. |
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Above you can see a Jones
plug is missing which means I'll have to replace this if I wish
to get the receiver working. |
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It was common to see these
type of equipments dismantled and their various sub-assemblies
offered for sale. |
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Lower right below, a
previous owner has disconnected the two yellow wires to stop
the noisy blower motor which wouldn't be needed if one was experimenting
with the receiver. Below, the underside of the IF amplifier is
at the top of the picture. Centre left is the 80 volt high frequency
heater transformer T2 and adjacent to this is RL2 used to turn
on and off the channel selector motor. To the right of this relay
is the assembly carrying the motor, with its 625:1 reduction
gearbox, used for RF tuning. Lower left are two block condensers
C72 and C71 used for decoupling the 210VDC supply to the pulse
processing circuits. This HT supply is separate from the main
HT supply and needs to be especially clean in order to prevent
spurious noise affecting the display. |
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The receiver has a few problems
that will need fixing if it's to be powered up and tested. When
the chassis is stored vertically the pins on those multi-way
connectors are pressed out of position and will now require a
spot of superglue to anchor them in place. This example was manufactured before January
1952 as the mod record indicates. |
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Click the circuit to see
better detail.
Above, the RF front end of the
R3673 receiver and below, the IF strip and detector circuitry.
Thanks to Alf Fisher, G3WSD for kindly supplying the information
on this equipment. Choice of IF for this receiver is slightly
odd but presumably there must have been good reason to choose
7.5MHz.
As you can see from the above
circuit the receiver has a set of ten preset frequencies. There
are no complications of tracking because each stage can be tuned
to the desired fixed frequency. Channel selection is carried
out by the grounding of a specific contact at a remote selector
switch. A change at this switch activates relay RL2 which has
a 24 volt DC coil. It looks like the "Aerial loading unit"
carries a switch which senses when the desired channel has been
selected and at this point the channel selector motor M2 will
be deactivated. Gearing at the motor and in the RF unit ensures
the desired RF channel is selected at precisely the point at
which the motor stops. At first sight I'll need to wire a double
pole rotary switch across the connector at the rear of the equipment.
The power requirements for the
receiver, judged from the circuit above (to see a nice clear
picture click the circuit and an enlargeable PDF version will
appear), are as follows:- +HT 240VDC and +210VDC, Bias -100VDC,
+/-24VDC, and 80VAC at 400 to 2400Hz (this is for valve heaters).
Equipment chassis is the return for HT and bias supplies. Allowing
say 6 to 8mA per EF91/6J6/EL91 the 240V HT current will be around
80mA to100mA and for the 210V feed, about 15mA. The bias supply
current requirement would appear to be negligible, say no more
than 1mA. The 6.3VAC Valve heaters run via the high frequency
transformer T2 draw about 6A which puts the 80 volt supply requirement
at 0.5A. Although a little awkward to produce this for test purposes,
it would have been an ideal method in an aircraft because cable
size is significantly reduced and resistive losses would be minimal.
There's also a 24 VDC supply. This is used to feed the two motors
in the receiver, ie. that used for channel selection and the
other a blower for forced air cooling of the equipment. Without
the blower connected I imagine the current needed will be less
than 250mA.
From the overall layout of the
system (shown earlier) it would seem that the various DC power
rails are derived from the Waveform Generator equipment, but
fitted on the receiver chassis is a thermostat (TH1) rated at
87 degrees C which is wired to switch off the main 80 volt high
frequency mains feed to the complete system, if for example,
the cooling fan fails. |
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Click the circuit to see
better detail.
The majority of the valves used
in the R3673 are CV138 or EF91 and its equivalents. The local
oscillator is a CV858 or 6J6. Others are CV140, EB91 and CV136,
EL91 and a CV137, EAC91. |
The R3673 chassis carries
several interconnected units which can be removed for servicing.
The RF unit, replacing RF24-RF27 in earlier designs, is RF Unit
Type 148 and the IF strip, previously part of the R1355 is IF
Unit Type 125. There's also a Type 28 Synchronising Unit. Earlier
versions of GEE required the appropriate RF unit to be fitted
which was manually switched (RF24/RF25) or manually tuned (RF26/RF27)
but in the Mk3 GEE channels are pre-set and selection is carried
out using a motor controlled from a switch mounted in the aircraft
cockpit. This enables one of 10 channels to be selected although
usually only coils for 7 channels are fitted viz. 1,2,3,7, 8,
9 and 10. The functions of the receiver are handled by 19 valves
as follows.. I've included in the listing the safety thermostat
as this is a glass enclosed plug-in device.
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Circuit Ref. |
Valve Type |
Circuit Function |
Location |
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V1 |
CV138/EF91 |
RF Amplifier |
RF Unit Type 48 |
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V2 |
CV138/EF91 |
Mixer |
RF Unit Type 48 |
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V3 |
CV858/6J6 |
Local Oscillator |
RF Unit Type 48 |
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V4 |
CV138/EF91 |
IF Amplifier |
IF Unit Type 125 |
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V5 |
CV138/EF91 |
IF Amplifier |
IF Unit Type 125 |
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V6 |
CV138/EF91 |
IF Amplifier |
IF Unit Type 125 |
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V7 |
CV138/EF91 |
IF Amplifier |
IF Unit Type 125 |
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V8 |
CV138/EF91 |
IF Amplifier |
IF Unit Type 125 |
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V9A |
CV140/EB91 |
Detector |
IF Unit Type 125 |
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V9B |
CV140/EB91 |
Echo Suppressor |
IF Unit Type 125 |
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V10 |
CV138/EF91 |
Video Amplifier |
IF Unit Type 125 |
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V11 |
CV136/EL91 |
Video Amplifier |
IF Unit Type 125 |
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V12 |
CV138/EF91 |
A-Strobe Selector |
Synchronising Unit Type 28 |
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V13A |
CV140/EB91 |
A-Pulse Selector |
Synchronising Unit Type 28 |
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V13B |
CV140/EB91 |
Pulse Limiter |
Synchronising Unit Type 28 |
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V14 |
CV138/EF91 |
Part of Discriminator |
Synchronising Unit Type 28 |
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V15 |
CV138/EF91 |
Part of Discriminator |
Synchronising Unit Type 28 |
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V16 |
CV138/EF91 |
Part of Discriminator |
Synchronising Unit Type 28 |
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V17 |
CV140/EB91 |
Discriminator Pulse rectifiers |
Synchronising Unit Type 28 |
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V18 |
CV138/EF91 |
DC Amplifier |
Synchronising Unit Type 28 |
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V19A |
CV137/EAC91 |
Limiter on AFC Output |
Synchronising Unit Type 28 |
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V19B |
CV137/EAC91 |
Triode section not used |
Synchronising Unit Type 28 |
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TH1 |
B2/CLG |
87 Dec C Thermostat |
Main chassis |
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A complete aircraft system
would include two other main equipments, a Waveform Generator
Type 72 and a display, CRT Indicator Type 26, and in addition
to these equipments, a mounting frame includes junction boxes
for each of the three equipments to deal with power distribution,
aerial inputs, pulse distribution and output connections.
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