GPO
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The GPO factories designed and produced switchboards and other equipment for the War
Department during World War 1 and World War 2. From the 1930's TMC
also produced some of these switchboards and GEC in the 1960's.
The pictures below also show the dates when the pictures were taken. POEEJ article about Post Office war signals designs - 1922. POEEJ article about WW2 switchboards - 1945 World War 1
Switchboard and concentrator - 5 panel, for GHQ in France (1918)
Switchboard, Magneto, Portable 40 Line in a suite of
three positions (1919)
Jack units in transportation crate
Switch-Unit, Buzzer 7+3 (1919)
Switch-Unit, Buzzer 7+3 and Switch-Unit, Magneto 10 Line (1919)
Switch-Unit, Magneto 10 line (1919)
Switchboard, Magneto, Portable 5 Line (1915)
Switchboard, Magneto, Portable 5 Line (1915)
Switchboard, Magneto, Portable 5 Line (1915)
Switch-Unit, Buzzer 7+3 and Switch-Unit, Magneto 10 Line (1919)
Switch-Unit, Buzzer 7+3 and Switch-Unit, Magneto 10 Line (1919)
Switch-Unit, Buzzer 7+3 and Switch-Unit, Magneto 10 Line (1919)
Switch-Unit, Buzzer 7+3 and Switch-Unit, Magneto 10 Line (1919)
Switch-Unit, Buzzer 7+3 and Switch-Unit, Magneto 10 Line (1919)
Switchboard, Magneto, Portable, Cordless, Buzzer 12 Line (1918)
Switchboard, Magneto, Portable, Cordless, Buzzer 12 Line (1918)
Switchboard, Magneto, Portable, Cordless, Buzzer 12 Line (1918)
Switchboard, Magneto, Portable, Cordless, Buzzer 12 Line (1918)
Switchboard, Magneto, Portable 20 Line (1918)
Switchboard, Magneto, Portable 20 Line (1918)
Switchboard, Magneto, Portable 20 Line (1918)
Switchboard, Magneto, Portable 20 Line (1918)
Switchboard, Magneto, Portable 20 Line (1918)
Switchboard, Magneto, Portable 20 Line (1918)
Switchboard, Magneto, Portable 20 Line (1918)
Switchboard, Magneto, Portable 40 Line (1918)
Switchboard, Magneto, Portable 40 Line (1918)
Switchboard, Magneto, Portable 40 Line (1918)
Switchboard, Magneto, Portable 40 Line (1918)
Switchboard, Magneto, Portable 40 Line (1918)
World War 2 Constructed on the unit principle, the Field and Fortress switchboard can be easily and quickly set up or dismantled and can be assembled to provide 20, 40 or 60 line capacities per operator's position. When required, a maximum of three switchboards can be erected side-by-side to form a non-multiple exchange accommodating 180 lines. The switchboard is of the magneto type but facilities are provided for connection of five lines per position to central battery (C.B.), central battery signalling (C.B.S.) or automatic telephone systems. Direct connections can be made to Army or civilian exchanges. The Mark 1 and Mark 1* switchboards are identical in design except for the method of connecting the external lines, and the Mark 2 switchboard differs from the Mark 1* in the way the external exchange lines are connected. The Switchboards Field and Fortess Mark 1* and Mark 2 are arranged to use cables terminating in connection strips while the Switchboard Field and Fortress Mark 1 is provided with line terminals. The switchboard is built up with separate units, the number varying according to the number of subscribers' lines to be accommodated. These are called Units. Unit A is a five line terminal unit with additional night
alarm and earth terminals.
Switchboard, Army, Field and Fortress, 5 Units (1944)
Switchboard, Army, Field and Fortress, 5 Units (1944)
Switchboard, Army, Field and Fortress, 2 Units (1944)
Switchboard, Army, Field and Fortress, 2 Units (1944)
Switchboard, Army, Field and Fortress, 5 Units (1944)
Switchboard AD1594 (1945)
Switchboard AD1594 (1945)
Switchboard AD1594 (1945)
Switchboard AD1594 (1945)
Keyboard Multiphone AD1316 (1944)
Keyboard Multiphone AD1316 (1944) Post WW2
Switchboard, Magneto, 10 Line This Switchboard uses magneto signalling but does not have a magneto in it as the magneto or oscillator fitted in the operator's field telephone connected to it is used to ring the remote station. It is equipped for 10 lines and designated YA6733 and has a heavy steel case fitted with a carrying strap. The drop flaps are secured by a flat bar during transit and the jacks stow in holes rather than standing upright on a shelf. In the picture jacks are inserted into operator's line and line 10 and the flap securing bar has not been released. Ten-Line Magneto Field Switchboard
Up to ten field phone extensions were connected to terminals on the rear of the switchboard. The field phone acted as the operators’ telephone circuit, enabling the operator to speak to and call an extension. Whilst generally used as an isolated system an extension circuit could be connected (as a magneto trunk) to a larger system if required. A calling extension would crank the hand generator, causing the associated indicator to drop a flap (and, if required, a buzzer to sound) to signal to the operator. The operator plugged his own circuit into the calling circuit jack to speak to the caller and reset the indicator. Having determined the required extension, the operator then plugged into the wanted extension circuit and cranked his magneto generator to call. After the called extension answered, the operator unplugged his circuit and instead connected the calling circuit plug into the called extension jack for the conversation to take place. On completion of the conversation both extensions ‘rang off’, both cranking their respective magneto generators and causing the associated indicators to drop. The operator removed the plug and plugged into each circuit to confirm the call had completed and to restore the indicators.
Keyshelf
Plug-in cord circuit with 2 cords, in cassettes
Operators key unit - with a blank plate where a dial would be fitted
A short article by Dave Fraley Field and Fortress Switchboard 40 /160 Somewhat more sophisticated than the 10-line magneto switchboard is the 40/160 switchboard, designed to be associated with the BRUIN switching nodes and serving military HQs. The title refers to the capability of a single switchboard to support up to 40 extensions or trunks, whilst four such switchboards, located side by side, could be interconnected to support up to 160 extensions / trunks for the very large HQs. It supported Magneto and Central Battery extensions as Automatic (loop disconnect 10pps) and Magneto Trunk operation for connection into both the BRUIN system and, where appropriate the local civilian telephone system. The unit operated by means of an external 24V DC battery (float charged where a power supply was available) but could operate as magneto only, if necessary, using a self-contained 6V battery (comprising four D cells). It was transported in three (heavy!) metal boxes, these also forming the structure of the unit. Starting from the floor upwards, the Spares box contains the cables and ancillaries and also the feet that secure to the underside of the box and provide stability on potentially uneven ground. The second box contains the keyboard unit and is secured by bolts on top of the Spares box. The cover protects the keyboard, secured vertically in the box. The keyboard is hinged at the top and secured into the box by bolts at the bottom. When these bolts are unscrewed, the keyboard swings out and is then braced into its fully open operating position. The third box contains the relay assemblies unit and this is bolted onto the top of the keyboard box. One cover protects the relay unit, containing the extension circuit jacks and indicators. A second cover protects the power distribution board and alarm board which, being hinged at the bottom, can be lifted into its vertical, operating position and secured with two arms at the rear. Cables from the Spares case connected the keyboard and relay unit, 24V DC power supply, patch panel and, where appropriate, adjacent switchboards. These cables mostly used screw-on Plessey-type plugs and sockets.
Keyboard The key functions are, from front to back, ‘Ring Call’, ‘Ring Answer’, ‘Monitor’ and ‘Speak’. The ring supply is provided by a ringing converter when operating on 24V DC and by the manual generator (located below the keyboard) when operating on 6V DC. Each cord circuit contains a call supervision status indicator each for the ‘Call’ and ‘Answer’ cords. Unlike most civilian designs the cords are not weighted, rather they retract into a container under spring tension. For magneto working the indicators need to lock in position when users ‘ring off’ (showing a yellow flag), therefore a mechanical linkage is provided so that on operating the ‘Monitor’ key to check the users have cleared, the indicators can be reset. The Operator’s telephone circuit similarly fits into the keyboard as a self-contained slice. Apart from the Dial and the ‘Night Alarm’ switch (to silence the call alert buzzer) there are four keys. ‘Dial Call’ connects the dial to the ‘Call’ cord in order for an automatically switched call to be established on behalf of users who do not have such facilities themselves. ‘Flash Call’ is used to signal a distant operator by momentarily breaking the ‘Call’ circuit to flash the distant calling indicator. ‘Speak Call’ and Speak Ans’, split the speech circuit, allowing the operator to speak privately to one termination or the other. Finally the ‘Couple Left’ and ‘Couple Right’ keys enable the operator circuit on the manned board to utilise the cord circuits on adjacent unmanned positions.
Relay Assemblies Unit Each calling indicator has an associated jack and function selector switch. The operation of each indicator is defined using a four-position switch with a slotted screw head. This enables the switchboard to be configured quickly for its working environment but the need for a screwdriver minimised the risk of ‘finger trouble’. The positions are labeled in clockwise order:- A – for connection to a civilian automatic exchange Depending on the position selected, the calling indicator is configured internally for that mode of operation. So, for example in the ‘M’ (magneto extension) position, the calling indicator yellow flag will become visible on receipt of a ring signal. It mechanically latches in that position until a plug is inserted into the associated jack, upon which the flag is released. In the ‘B’ (CB extension) position the calling indicator applies 24V battery and earth conditions to line and, on sensing line current, operates the yellow flag until the plug is inserted in the jack and disconnects the line current from the indicator relay. Connection to the cord circuit supervisory indicators is via the Sleeve connection of the plug for CB and Auto and via the Tip and Ring for Magneto and Trunk working. When connected to a civilian exchange, the calling indicator circuit (set to the ‘A’ position) will present the same conditions as a subscriber on that exchange would. To ensure compatibility with the various auto exchange signalling standards likely to have been encountered around the world at the time, there is provision for an Auxiliary Unit, supporting up to 4 lines, to be connected in circuit and configured to present the appropriate conditions. The blank space above the calling indicators was reserved for a multiple unit (carried in the spares box) to be installed. This was used when more than two switchboard positions were in use, each two positions sharing a multiple.At the top of the Relay Unit is the power and alarm panel (see Fig. 4). The main feature here is the 24V DC power distribution fuses supplying for the relay and keyboard units. There are two fuses for the relay units and two for the keyboard cord circuits to avoid full loss of service.These alarm fuses operate in a similar manner to the BPO Fuses No. 44A widely used in PO/BT telephone exchanges, but have a different form factor. A blown fuse is identified by a white bar, normally on the right, sliding to the left.The ‘battery’ side of the blown fuse connects with a fuse alarm strip to the rear lighting the red ‘Fuse Alarm’ lamp below. Below the FA lamp is a white ‘Pilot’ lamp, which alerts whenever a call signal is received. Additionally there are two 24V battery jacks and a Panel light (stored in the Spares box) for operation in the low lighting levels likely to prevail.A further significant feature was the capability to operate, self contained, on an internal 6V DC supply for the operators’ telephone circuit. This is provided by a tube of four D cells located just behind the notice board. This, along with the hand generator located below the keyboard, enabled a basic service to be provided for magneto extensions and trunks.Field Telephone Cable Distribution System Connectivity between the switchboard and the outside world was provided by a 30-line Patching Panel. The panel provides access to 30 circuits for disconnection, testing and patching. On larger installations several would be wall-mounted side by side. At the bottom of each panel up to 3 x 10-pair internal cables connect to the Relay Unit. Up to three 10-pair field cables plugged into the associated connectors for onward distribution. Each circuit is equipped with removable links, for patching/testing/disconnection and a lightning protector, emphasising the vulnerable nature of such field deployments. The ten-pair cable contains twisted pairs of stranded conductors for flexibility enclosed in a rubberised and extremely durable outer sheath. The cable is terminated in connectors that are identical at each end. Similar connectors are used on the patch panel, on the distribution unit and other items. Each connector is faced with 20 gold-plated pins such that engaging one connector into another will align the contacts face to face to complete the connection. The connectors are keyed so that the pins will align correctly and a screwed collar on one connector is tightened over the other to compress the pins together and ensure a waterproof and mechanically robust joint that can operate under water and tolerate vehicles being driven over the connection. Poor electrical connectivity was generally due to dirt on the face of one or more pins, so it was usual practice to wipe the faces with a clean cloth before connecting. The cable is issued in 30m coils, as shown here, or on quarter-mile drums. Distance can be increased by connecting cables together, in some cases over several miles. Land Rovers, known as line layers, were equipped with ACLs (Apparatus Carrier Line) to rapidly deploy and recover such cable. For shorter runs the ACL could be mounted on a cable barrow, from a fixed ground position or rarely, carried by two men. The Field Cable Distribution Unit (known by squaddies as the ‘hedgehog’) acted as a Distribution Point, terminating the ten-pair field cable and presenting each pair on screw terminals. Odd-pair terminals are located on one side, even-pair terminals on the other. The hedgehog normally sits inside its protective cover, held by a large knurled retaining screw bolt situated on the top. In this state the unit is fully weather resistant. Although sometimes lying on the ground they it would usually be secured vertically to a tree, wall, fence post, etc, above ground level and with the feed cable hanging from the connector at the bottom. For access the bolt is unscrewed and the distribution unit lowered until it hangs, fully exposed, from the cover. If required it can be fully separated from the cover. Single-pair cable, such as D10, connecting the field phone or similar, is fed through a grommeted hole in the bottom of the unit and up to the required terminals. There was no need to strip the insulation as the hole in the terminal post has teeth which pierce the insulation on tightening. Space was provided for circuit details to be written on associated labels. Following testing, the unit would be lifted into the cover and secured with the retaining bolt Whilst a variety of cable might be utilised for the ‘local tails’, ‘DON 10’ (or D10) was the standard choice for single-pair field telephone cable, being extremely robust. D10 consists of a twisted-pair of conductors, each conductor comprising 4 copper wires and 3 galvanised steel wires stranded together and insulated with black nylon covered in a polyethylene outer sheath. It could be laid along the ground, over tracks or hedges or strung from trees and pole crossings and then rapidly recovered, time and time again. D10 was issued in 800m lightweight canvas dispenser packs or on drums, both shown in Fig. 9. Also just visible next to the hedgehog terminals are copper sleeves used to quickly joint two D10 cables together by simply stripping off the insulation and inserting a conductor into each end of the sleeve and then compressing using a crimping tool. The result was a mechanically and electrically sound and weatherproof joint.
BRUIN
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