RF25B

 This unit is a plug-in part of the Gee Navigation System

 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

 

 Below: Low loss ceramic coil formers and beehive trimmers

 

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.

 

 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.

 

 Below:High quality ceramic bandswitch and each of the five preset ranges has a set of three beehive trimmers

 

 

 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.

 

 Below: View showing a Pye plug which connects the RF amplifier coil to an aerial tuning unit.

 

 

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.

 

 

 

 

 

 A little about Gee

 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.

See the R1355 circuit diagram

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

 Out of interest I powered up my latest purchase from a power supply delivering 250 volts and 6.3 volts for the valve heaters.

 

 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.

 

 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.

 

 Below, offered on Ebay some time ago, a rather tatty R1355 complete with an RF24

 

 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.

 

 Gee handbook click to see

Cossor plans to update Gee

 Gee was later developed to the Mk3 system, below, in which the RF equipments were integrated into the main receiver chassis, the R3673.

 

 The R3673 Gee Mk3 Receiver

 

 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.

 

 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.

 

 It was common to see these type of equipments dismantled and their various sub-assemblies offered for sale.

 

 

 

 

  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.

 

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.

 

 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.

 

 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.

 Circuit Ref.

 Valve Type

 Circuit Function

 Location

 V1

 CV138/EF91

 RF Amplifier

 RF Unit Type 48

 V2

 CV138/EF91

 Mixer

 RF Unit Type 48

 V3

 CV858/6J6

 Local Oscillator

 RF Unit Type 48

 V4

 CV138/EF91

 IF Amplifier

 IF Unit Type 125

 V5

 CV138/EF91

 IF Amplifier

 IF Unit Type 125

 V6

 CV138/EF91

 IF Amplifier

 IF Unit Type 125

 V7

 CV138/EF91

 IF Amplifier

 IF Unit Type 125

 V8

 CV138/EF91

 IF Amplifier

 IF Unit Type 125

 V9A

 CV140/EB91

 Detector

 IF Unit Type 125

 V9B

 CV140/EB91

 Echo Suppressor

 IF Unit Type 125

 V10

 CV138/EF91

 Video Amplifier

 IF Unit Type 125

 V11

 CV136/EL91

 Video Amplifier

 IF Unit Type 125

 V12

 CV138/EF91

 A-Strobe Selector

 Synchronising Unit Type 28

 V13A

 CV140/EB91

 A-Pulse Selector

 Synchronising Unit Type 28

 V13B

CV140/EB91 

 Pulse Limiter

 Synchronising Unit Type 28

 V14

 CV138/EF91

 Part of Discriminator

 Synchronising Unit Type 28

 V15

 CV138/EF91

 Part of Discriminator

 Synchronising Unit Type 28

 V16

 CV138/EF91

 Part of Discriminator

 Synchronising Unit Type 28

 V17

 CV140/EB91

 Discriminator Pulse rectifiers

 Synchronising Unit Type 28

 V18

 CV138/EF91

 DC Amplifier

 Synchronising Unit Type 28

 V19A

 CV137/EAC91

 Limiter on AFC Output

 Synchronising Unit Type 28

 V19B

CV137/EAC91 

 Triode section not used

 Synchronising Unit Type 28

 TH1

 B2/CLG

 87 Dec C Thermostat

 Main chassis

 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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