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Larkspur Control Harness 'B'

This section has previously been published in 'Radio Bygones'

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A Larkspur equipped AFV usually had up to three radio sets as well as in intercom (IC) system. The crew, depending on the model of vehicle, could be anything from two to six men. Any member of the crew might need to use any of the communications systems at any time. The system of control boxes, distribution boxes and interconnection cables which enables this to be done is called the WIRELESS CONTROL HARNESS. Although principally intended for use in a vehicle, it can be used equally well in installations which have been dismounted and set up as a ground station. Incidentally, the Wireless Control Harness concept was not new as with the entrance of the very successful Wireless Set No. 19 in 1941, a similar harness was introduced controlling two radio sets and providing intercommunication in an AFV.

Two types of harness were in service for the Larkspur range of equipment:

• Wireless Control Harness type 'A' was of a rigid design which allowed strict use of all control units concerned. It was principally used in all tanks and Saracen ACV's with two or three-set installations. This harness was very expensive and not very flexible for use in other applications. The 'A' harness is essentially a small automatic switchboard fitted in the main JD9 box. To this box are connected up to three radio sets and all the control units, each of which is allotted to a crew member. The advantage of this system is that, in an emergency, the commander can push a button which will send an over-riding signal to the JD9 box, and all selected circuits automatically switch their crew members to intercom, irrespective of the setting of the crew member's control units. This system was necessary for tanks, but not for use in other vehicles. It was for this reason that the much cheaper 'B' harness was used elsewhere.
  

  The 'A' harness comprised of the following principal boxes and control units:
  Control Units Nos.: 30, 31 and 34, Junction Distribution No. 8 and 9. Various other boxes were later added e.g. to provide Clansman equipment to be connected to the existing Larkspur 'A' harness and visa versa.

• Wireless Control Harness type 'B' was designed for use in Fitted For Radio (FFR) 'B' type vehicles, ground stations, FV 432 vehicle series and in armoured cars. This harness was principally for the control of one or two sets, very flexible, variable in content and layout, and at the time the most common harness in use. The 'B' harness was made up of a series of junction boxes, additional control units and connector leads.

The units of the Wireless Control Harness type 'A' and 'B' are contained in fully sealed cast aluminium boxes and are capable of withstanding the severe conditions in the field. All leads interconnecting the boxes are Plessey type Mk. 4 multi-point plugs and sockets. A Commander's Headgear Assembly counts as a control box as it is virtually a Crew Member's Headgear Assembly and a Control Unit 'C' combined. Leads connecting headgear assemblies to boxes are 6-point. All controls are designed to enable switches and controls to be manipulated by operators even when wearing heavy arctic protective gloves.

Radio sets which may be controlled by the Wireless Control Harness type 'A' or type 'B' include the Larkspur range: Station Radio B47, B48, C42, C45, C11/R210, C12, C13 and by means of a special adapter connector other (older) sets such as Wireless Set No. 19 and Station Radio BE201.

It may be noted that within the Royal Armoured Corps the use of sets (as noted on the control panels of the harness boxes) controlled by a three set harness was standardized: the 'A' set for the forward net, the 'B' set for rearward net and the 'C' set for communication to supporting forces.

The Wireless Control Harness type 'B' requires a power supply of 24V or 12V for its operation, taking a current of about 1.5A at 24 V and 3A at 12V. Junction Box One Set 'J1', Two Set 'J2' and Control Unit 'R' must be selected for 24V or 12V operation according the voltage of the vehicle. The Intercom Amplifier 'I' and Amplifier Loudspeaker 'ALS' are only available in 24V version. All other units are suitable for operation on either voltage.

The wireless control harness and sets of the Larkspur range of equipment were designed for a nominal operating voltage of 24V or 12V DC. In practice, however, the actual voltages in a vehicle vary considerably and may be between 20.7 and 29 volts or 10.3 and 14.5 volts. To limit the variations in voltage used in the set, avoiding a serious loss in specifications, a close margin Voltage Control Relay in the Junction Box 'J1', 'J2' or 'J3' operates when the battery voltage rises above 25.5 volts or 12.75 volts and releases when the voltage drops to 23.5 volt or 11.75 volts. There is an overlap of about 2 volts on 24 volt installations and 1 volt on 12 volt installations.

If the battery voltage rises for example during float charging of the radio batteries in a vehicle with its engine running, the relay will operate and change the step-up ratio of the supply unit transformer, maintaining the output to the standard level. Note that when the Junction box is disconnected the supply unit is set to the high voltage tapping regardless of the actual battery voltage and the radio will not perform with maximum efficiency on low battery voltages.

Relay RLD in the power supply unit is normally in the high range battery position and operates only when the contact in the junction box is closed indicating low range battery level. Larkspur sets supporting this feature include the VHF FM series Station Radio B47, B48, C42 and C45. The HF sets such as SR C11/R210 and C13 have a voltage control relay (VCR) fitted in their power supply unit.

Vehicles marked 'Fitted For Radio' were all fitted to the same standard so that a radio set kit could be installed in any FFR vehicle or transferred from one to another without additional fittings. Every FFR vehicle included tables drilled ready to accept set carriers, brackets ready to accept aerial bases and aerial tuners. In addition, it had carriers ready for batteries or batteries already installed. Certain connectors which have to be clamped to the vehicle were already installed. In normal circumstances FFR vehicles required no further workshop action before the radio station could be installed. It must be noted the Fitted For Wireless (FFW) vehicles were furnished to different standards, just containing part of the installation and usually required workshop action before a radio station could be fitted. It is therefore not the same as a 'FFR' version.

Junction box  'J1' The Junction Box One Set 'J1' is the focal point of a one-set installation. The 'J1' or one-set harness enables several crew members to work with a set, intercom (IC) is available (provided the set is equipped with an IC amplifier) and remote control facilities are provided. The 'J1' harness also permits the set to be connected to another set. The Commander's Headgear Assembly, if used, connects to the 12-pt socket of the Control Unit 'C' or 'D'. The rest of the crew's headgear assemblies have a 6-pt plug and fit in the appropriate sockets. The Junction Box One Set 'J1' requires 24 V DC at the two-pin socket marked 'LT'. A 12 V version was produced but in limited numbers only. A fuse and indicator lamp are fitted. The LT connected to the equipment, nominally 24 V (12V), may vary considerably. A special voltage control relay is included in the Junction Box 'J1' which ensures that the set sees the sensibly constant voltage for large variations in LT supply. The remote control screw terminals allows connection of a Remote Control Telephone No. 1 or Remote Control unit 'K'. In addition it may be connected to the remote control terminals on a Junction Box 'J1' of another set (e.g. fitted in another vehicle) providing rebroadcast facilities without having to add any other components.

Block diagram of a typical one-set installation comprising Junction Box One Set 'J1' and complementary Control Units 'C' and 'D'. Note connection of Remote Control Telephone No. 1 on the line terminals of the 'J1' box.

Control unit 'C' Operators box The Control Unit Operator 'C' box is the operator's control unit for controlling one or two sets in a one or two-set installation using either Junction Box 'J1' or 'J2'. In addition it provides intercommunication and calling facilities. It has two 6-point headgear assembly sockets and connections to other Control Units 'C' or 'D'. When used in a one-set installation to provide an additional operating position, the 'A' set switch position cannot be used and was usually blanked off.

Control unit 'D' Drivers box Control Unit Driver 'D' is very similar to the operator's Control Unit 'C'. It is intended for use by the driver, employed in one-set or two-set installations. It provides listening facilities only on the radio sets 'A' and 'B', whilst retaining normal intercommunication facilities on intercom and 'call'.

Adapter unit 'E' box The two-set harness comprising Junction Box Two Set 'J2', Remote Control Unit 'R' and Rebroadcast Unit 'B' was complicated and expensive to make. As virtually all facilities were provided on a 'J1' box, the outputs of two 'J1' boxes were combined in the Adapter Unit 'E' . A two-set installation was then produced by standard units enabling two radios to be controlled by one operator through a Control Unit Operator 'C' or commander's two-set headgear assembly. A Manual Remote Rebroadcasting Box 'MRRB' may be fitted in this installation when rebroadcast facilities are required. The Adapter Unit 'E' is a simple interconnection box and has two 12-point sockets and one 12-point plug, the latter making it different from the 12-way 'T' box.

Block diagram of a typical two-set installation using two Junction Boxes One Set 'J1' and Adapter Unit 'E'. Rebroadcasting is produced by the addition of a 'MRRB' box, described below.

Junction box 'J2' Junction Box Two Set 'J2' was originally designed for two-set AFV installations for which emergency intercom and monitor facilities were not required. Although it was later replaced by the two-set installation using two Junction Boxes One Set 'J1' and Adapter Unit 'E', it still remained in general use, principally in wheeled AFV's. Control Units 'C' and 'D' may be connected via the lower 12-pt socket. The commander's headgear assembly is connected to the Junction Box Two Set 'J2' via the top left socket. The unit was made in 24 V (standard) and in a limited 12 V version.

Block diagram of typical two-set installation using Junction Box Two Set 'J2'. Remote Control Unit 'R' and Rebroadcast Unit 'B' are normally associated with this harness. The 'B' box was not always used.

Remote Control unit 'R' box Remote Control Unit 'R' is a combined local remote control unit and operator's unit for two-set installations using Junction Box Two Set 'J2'. It provides operation of both radio sets, intercom, remote control, remote rebroadcast and break-in switching. The unit has a 24V and a 12V version.

Rebroadcast unit 'B' box Local Rebroadcast Unit 'B' is part of a two-set installation and was principally used with the Junction Box Two Set 'J2' and Remote Control Unit 'R'. It has switch positions for local automatic or manual rebroadcast. A 25-point connector attaches it to a Remote Control Unit 'R' or directly to Junction Box Two Set 'J2'. The 'B' box is not needed when only remote rebroadcasting is required.

Junction box 'J3' The Junction Box Three Set 'J3', was developed exclusively for use in the Armoured Personnel Carrier FV 432, which enables three sets to work in one harness using standard Wireless Control Harness type 'B' components. A second Junction Box Three Set 'J3' was normally fitted at the rear of the vehicle, enabling the Infantry commander to stand at the rear of the APC and still retain all his communication facilities. The associated Commander's headgear assembly provides the selection of three sets by means of a switch fitted on the microphone.

Adapter unit 'A' box Adapter Unit Reception Set 'A' enables Reception Set R210 to be operated as a single receiver without Transmitter Radio C11. Connections are provided for two headsets, 24V supply and 12-pt lead to the receiver. A later version of this unit has a 'Phones out' insulated screw terminal fitted on top. Adapter Unit Headset 'T' may be used to connect additional headsets.

Block diagram of Reception Set R210 to be connected without its associated Transmitter Radio C11. If more than two headsets are required, an Adapter Unit Headset 'T' may be used.

Adapter unit 'T' box There are two versions of this junction box: the 6-point '6T' box, known as Adapter Unit Headset 'T', and 12-point '12T' Box. They are both simply junction boxes giving two outlets for one input. These units were used with either single set or two-set installations for various purposes e.g. connecting two 'C' boxes or connecting two Crew Member's Headgear Assemblies to one standard 6-way Mk. 4 outlet socket through an extension lead (see block diagram above).

Adapter unit 'O' box Adapter Unit One Set 'O' is a simple control box to enable two headgear assemblies (or Morse keys for CW operation) to be connected to a set without the complications of a harness. The Adapter Unit One Set 'O' provides a volume control as the minimum facility required to operate the set in an emergency and may be used instead of a Junction Box One Set 'J1'. It is particular useful to form a very simple ground station or training station. It has no voltage control relay, and whilst some sets (notably Station Radio B47, B48, C42 and C45) rely on this, the supply voltage must be kept to above 26 volts (13V) or the performance is reduced. Adapter Unit Headset 'T' can be connected to the either headset socket to enable additional headgear assemblies to be attached.

Connections of Adapter Unit One Set 'O'

Manual Rebroadcasting unit 'MRRB' With the Manual Remote Rebroadcasting Box 'MRRB' connected between two Junction Boxes One Set 'J1', a manually controlled rebroadcasting link could be operated between two HF sets (C11/R210, C12 or C13), or between an HF set and a VHF set (B47, B48, C42 or C45), or if necessary between two VHF sets. The Manual Rebroadcasting Box 'MRRB' is connected between the line terminals of two 'J1' boxes. Automatic rebroadcasting is only possible between VHF sets having squelch facilities. Operation is effective up to 1000 yards, using D10 field cable between both sets. The 'Auto-Manual' switch, in the drawing fitted on top of the box, was in a later version mounted on the left-hand side between the line terminals.

Block diagram of connecting the 'MRRB' box in two separate one-set installations. The 'MRRB' box may equally well be used in a two-set installation using two 'J1' boxes and an Adapter Unit 'E'

IC Amplifier 'I' box Intercom Amplifier Unit 'I' was for installations using a radio without intercom amplifier (e.g. Station Radio B47 and B48) and where IC facilities were required. Where fitted it is connected between the set and the set socket of the 'J1' box or the 'A' socket on 'J2' box of the main harness. The 'gain' control affects only the receiver output to the headgear connected to the 6-point socket. The gain of the amplifier is fixed. The intercom amplifier operates on 24V only.

Block diagram of the use of the Intercom Amplifier Unit 'I' box.

Remote Control unit 'K' box Remote Control Unit 'K' is provided for local and remote hand speed Morse keying for HF sets (for example SR C11/R210, C12, C13) and speech where an Remote Control Telephone No. 1 is not suitable. It has terminals for two-wire control cable, a call button and two special 5-pt sockets for 'Microphone and receiver headgear lightweight assembly No. 1A'. A pair of canvas straps are provided to allow the unit to be strapped on the operator's knee. The unit can be used remotely up to 1000 yards using D10 field cable. It is normally connected to the line terminals of the 'J1' box or to the 'R' box of a two-set harness. In Wireless Control Harness type 'A' it is connected to the line terminals of the 'JD9' box. The Remote Control Unit 'K' works in a similar fashion to remote handsets.

Remote Control Unit 'KV' (not shown here) was developed for installation in vehicles. It has two 6-pt standard Mk. 4 sockets instead of the special 5-point sockets fitted on the Remote Control Unit 'K', allowing the use of standard Crew Member's Headgear Assemblies, and no call button or straps are provided. Remote Control Unit 'K' Mk. 2 is similar to the 'KV' unit, but has two 6-pt standard sockets and can thus be used with a normal Crew Member's Headgear Assembly. With the 'K' Mk. 2 are supplied a 'Carrier Fixed Remote Control Unit 'K' Mk. 2' to mount it on the operating table and a 'Carrier Free Remote Control Unit 'K' Mk. 2' with two web straps.

Block diagram of connecting a Remote Control Unit 'K' to a one-set harness using a Junction Box One Set 'J1'. Shown is local keying (top) and remote keying (bottom) of a HF set.

Crew member's headset All headgear assemblies in the Wireless Control Harness type 'A' and 'B' use the same type of headphones (Receiver headgear SI Double No. 1A) but the microphone, cords, junction boxes and plugs differ. Note that the insets in the headset and microphone differ coloured green with white band for the earphones and red with white band for the microphone. The Crew Member's Headgear Assembly has a Microphone Hand SI No. 1 or No. 6, the latter being issued with a socket for a respirator connection. Separate connectors from the microphone and headphones are joined by a small junction box w hich normally hangs from the user's neck via a strap. A snatch plug fitted on the headphones and stud fasteners on the web neck strap allow the user to get easily free from the assembly in case of an emergency just by giving it a sharp pull. Selection of 'A' set, 'B' set or 'IC' is done on the box to which it is connected.

Remote Control Handset No. 1 The Telephone Hand SI Remote Control No. 1 is used only by an operator at the distant end for remote working of the 'A' or 'B' harness. The colour of the handset is green and it is very light. It is normally connected to the terminals of the Junction Box One Set 'J1' or 'R' box by up to 600 ft of D10 cable carried in a drum. See Junction box 'J1' block diagram for connecting. The Inset standard microphone carbon No. 1 is coloured red with a black band. The earphone is an Inset standard receiver E/W No. 1, coloured green with white band.

Commander's Microphone (2-set) The Commander's Headgear Assembly (two-set) can be considered as a portable Control Unit Operator 'C', because of its identical facilities. The headgear and quick release facilities are similar to the Crew Member's Headgear Assembly; the junction box, however, has a gain control fitted and the Commander's Microphone No. 7 has a selector switch below the mouthpiece enabling the headset to operate set 'A', 'B' or 'IC'. It includes a spring loaded 'Call' position. It may be connected to Junction Box One Set 'J1', Junction Box Two Set 'J2', Control Units 'C' or 'D' via the 12-point connector.

Apparatus loudspeaking 'ALS' The Apparatus Loudspeaking No. 19 'ALS' consists of an audio amplifier, two loudspeakers and associated connectors, which enables a radio set or intercom to be heard anywhere in the vehicle without the use of headgear. The loudspeakers are either 'Loudspeaker PM, 5-in 3-ohm' or as an alternative 'Loudspeaker SI 3¼-in, 9-ohms, general purpose, rubber housed'. The set to be operated from headgear must be connected directly to the 'ALS' (or via a Adapter Unit 'T6' if more than one set of headgear is required) in order to achieve muting of the amplifier during transmit. The amplifier unit operates from 24 V and uses valves, the HT being obtained from an internal vibrator unit.

Test unit 'M' box The Test Unit 'M' is not an operator's box, but was designed to enable Royal Signals radio technicians to carry out a limited number of tests. It is connected directly to the set (similarly to the Adapter Unit 'O') and provides the following test facilities: - A single normal headgear assembly output. - One headgear assembly output for testing the intercom amplifier. - A switch to operate, manually, the voltage control slave relay in the supply unit of the set by simulating high and low voltage conditions. - Terminals at which the limiter grid current of a receiver under test can be measured. - A gain control which affects the receiver output level of the set.

In all the Larkspur installations power is obtained from a separate radio battery. The 12V or 24V for the various units in the installation is distributed by separate LT interconnection boxes. There are three types of LT distribution boxes which were principally used with the Wireless Control Harness type 'B', each designed for a different purpose. These boxes are all similar in shape but sockets and input plugs differ. Although other LT boxes exist (notably Junction Box 4-way No. 5 and Junction Box 3-way No. 2), they were not common and believed to be used in early installations.

- Interconnection Box 4-way No. 4 was the primary LT distribution unit, issued to all set installation kits except for Station Radio C11-R210 and Reception Set R209. The radio batteries in the Rover Mk. 8 and 9, 1-ton Armoured truck 4X4 Humber and ¼-ton Austin CT truck ('Champ') could be float charged in parallel with the vehicle batteries. In the latter vehicle, the 'Power Take Off' junction box was normally already fitted in the vehicle. One of the inputs (4-point) of the LT box No. 4 was used for the power take-off connection in Austin CT only using a special 4/4-point connector. In the other vehicles a special 4/2-point connector was fitted between this input plug and vehicle battery supply (normally a Batten Terminal or Panel Distribution). The radio battery was connected to the opposite 2-point input plug on the LT box No. 4.

- Interconnection Box LT 4-way No. 1 was supplied to the Junction Box Two Set 'J2' kit and also used in two-set installations when a number of additional 2-point LT sockets were required.

- Interconnecting Box 4-way No. 1 was issued in the SR C11-R210 installation kit, the 4-point connectors being provided to carry the extra power taken by Transmitter Radio C11-R210.

Connections to radio battery and vehicle battery (Power Take-off Junction box) in Truck FFR ¼`-ton 4X4 Austin to alternative LT boxes. In other vehicles a special 4/2 point connector may be used to connect the LT Box 4-way No. 4 to the vehicle battery.