PAGE No. 34

July 1957

Cordless PBX switchboards of the c.b. and auto types are used by telephone administrations and business houses all over the world. High standards of reliability were established many years ago, and consequently designs have remained unchanged over a long period; this stability of design was founded on simplicity of circuit and construction. Whilst utilizing new materials and production techniques the new range of switchboards retains the basic simplicity and reliability of the older types.

ALL designs are controlled to some extent by materials and manufacturing methods. In the past timber has been employed for switchboard construction and this has imposed limitations on the shapes which could be used. The development of moulding materials and new forming processes has widened the scope of the designer in this respect, and the new materials can be coloured in many attractive shades. As can be seen from the coloured illustration, the new switchboards will harmonize with modern office settings. Design foibles which date the styling have been avoided and in accord with the long functional life provided it is expected that the appearance will prove pleasing for many years to come.

Fig. 1—12-line Switchboard with Cover off

Fig. 1 shows a switchboard of the new type with the drop-on cover removed from the unit formed by the base and component mounting plates. Models with two exchange and four extension lines or three exchange and nine extension lines can be accommodated within the case illustrated, which is approximately 13” long and 13” wide with a maximum height of 8.5”. The key and indicator plate and the relay mounting plate pivot on the base (see Fig. 2) to give access to all components and wiring ; this speeds production and simplifies maintenance. Spring washers on the plates act as brakes to prevent violent movement, and the plates are arranged to support each other so that there is no need for separate chassis members. Careful consideration has been given to the positioning of the four captive cover-fixing screws in order to ensure close seating on the key and indicator mounting plate (Fig. 3). Rubber feet are let into the base of the switchboard to prevent damage to the finish of the desk or shelf on which it stands.

Two pages are missing at this point 35 & 36.

Fig. 2
Switchboard Fully Opened
Fig. 4
Cover Construction
The alternative auto and C.B. front sections are shown

The cover is assembled from several parts (see Fig. 4) and has considerable strength. The cover ends are moulded in a thermo-setting material with a durable finish and good resistance to surface abrasion. For the c.b. type the front and rear centre sections are formed from aluminium sheet and the fixing screws, captive in the front section, are covered by a push-on facia strip.

On auto types the front section is an aluminium alloy casting providing a dial mounting. The metal sections are finished in mottled enamel and two-tone colour schemes have been used for the complete assembly. Where c.b. type covers are supplied initially, subsequent conversion to auto is easily arranged by changing the front section and fitting a dial. Dial connections are made by a plug and do not obstruct removal of the cover.

The base is shell moulded from an aluminium alloy and shaped to form a rigid platform for the complete switchboard.

Fig. 6
A Key Unit Removed to Expose Contact Springs
Fig. 5
Partially Assembled Key Unit

To reduce the size of the switchboard new designs for some components have been introduced; major contributions in this respect are made by the new keys and indicators.

The keys are assembled in units (Fig. 5), the component parts of the two or three keys in each vertical row being fitted on a common mounting which is secured to the key panel by two nuts. As shown in Fig. 6, access to the contact springs for cleaning or adjustment can easily be obtained. Careful choice of materials and of the shapes of springs and frames has enabled a considerable reduction in size compared with the previous type of lever key to be made, yet large contact clearances with adequate spring travel and spacing are maintained, whilst contact pressures are more precisely controlled than in previous designs.

Fig. 7
Key Operation

The key unit frame is formed from steel sheet in a shape that ensures complete rigidity. The contact springs are mounted between moulded insulating blocks and secured on either side of the frame. Twin contacts are fitted and the length of the springs is the minimum necessary to withstand the operating stresses for an extended period of use without fatigue or loss of contact pressure. The cam operates the
springs by means of a moulded comb lifting plate and has two pivot pins which centralize it in the un-operated position, thus preventing contacts operating on overthrow when restoring to normal. The twin pivots limit the arc of movement and establish a relationship between pivot and cam lifting faces which virtually eliminates wear of the moulded comb.

Locking is achieved by a toggle action with a compression spring housed in a central bush holding against the load of the contact springs. For keys which are non-locking on one side a special toggle plate is used to change the direction of the forces so that the cam returns to normal on release. Where a completely non-locking key is required there is no need for the toggle action and the cam is centralized by a small compression spring fitted between cam and frame. Fig. 7 shows the various types of action.

The keys have moulded push-on handles giving a large area for the finger. The shape of the cam and the resilience of the handle material together ensure security of fixing even when handles have been removed and replaced many times.

  1. Operated Exchange Line Indicator. Release is effected by pressing down projection above window.
  2. Extension Line Indicator with window and cover removed.
  3. Complete Extension Line Indicator, which is of non-locking
Fig. 8—The New Indicators

At the apex of the handle there is an insert of a contrasting colour which serves to show clearly whether or not the key is operated. The small angular movement of the handle permits keys to be mounted at the close vertical spacing of 1.5”, and the horizontal separation may be as little as 5/8”.

Keys are lubricated initially with a high viscosity grease containing molybdenum disulphide.

Indicators for exchange and extension lines, shown in Fig. 8, are again of a new design which saves space. As far as possible standardized parts are used for both types, and the size of the mechanism has been kept to the minimum to leave room for a large coil, in order to obtain maximum sensitivity.

The two iron shutters are pivoted within a U-shaped iron frame and held in the un-operated position by a leaf spring; the spring tension is adjusted by a screw on the underside of the indicator. A centre pole piece on which the coil is wound projects between the shutters and when the coil is energized the magnetic forces open the shutters horizontally so that a curved white plate occupying the whole frontal area of the indicator is exposed behind the push-on plastic window. The Exchange line indicator locks in the fully operated position and is released by pressing the front cover downwards; a moulded projection is provided for this purpose. The return spring adjustment together with the inertia and wide angle of movement of the shutters ensure discrimination between ringing current and dial pulses. The indicator will operate and lock on receipt of ringing current over a 1,000 ohm line, but will not lock during dialling, even with maximum exchange battery voltage and a low resistance line.

On both types of indicator a contact is provided which makes in the operated condition and can be used to complete an audible alarm circuit. The contacts can be seen without removing the indicators from the mounting plate.

A B.P.O. type 12 standard trigger dial is normally fitted but non-standard dial numbering or impulsing can be catered for.

The hand-generator is of the rotating magnet type described in Bulletin No. 34, and standard 3000 type relays and retards are used.

Fig. 10—Fixing of Designation Strip

An inter-Services sealed fuse holder carrying a miniature glass cartridge fuse is fitted on the side of the base of the switchboard. A fuse can be replaced without removing the switchboard cover.

The paper designation strip below the indicators is protected by a moulded transparent window held in a metal frame. As shown in Fig. 10, the frame is in two halves which can be separated to release the spring clips securing the designation strip to the key plate.

P.V.C. covered wire, with its advantages of high insulation, small bulk and bright identification colours, is used for the connections.

The operator’s handset (B.P.O. type No. 1) is connected to the switchboard by a miniature plug, B.P.O. type 420 MK1, and jack B.P.O. type 84A MK1. The circuit is similar to that of the new type 1000 telephone described elsewhere in this Bulletin and provides for high grade transmission over lines of up to 1000 ohms loop resistance.

The switchboard provides the usual facilities for connecting and supervising calls between local extensions and between exchange lines and local extensions. Each line is equipped with keys arranged in vertical rows beneath the associated call indicator, and the key handles can be moved up or down, each position operating in effect a separate key. With the key units mounted side by side the key handles also form horizontal rows. To connect two lines together a key for each line on the same horizontal row is operated in the same direction.

Fig. 11
Operation of Keys and Indicators under four typical conditions

The keys at the extreme right are associated with the operator’s telephone circuit and have no indicator above them. By moving one of these keys and a line key on the same horizontal row in the same direction the operator is enabled to speak on the line.

The vertical row on the extreme left contains the night extension keys. These are used to connect an exchange line to an extension when the switchboard is unattended, for example at night. Through connection without supervision at the switchboard is obtained when the night extension keys are operated together with the exchange and selected extension keys in the same horizontal row.

The downward position of the bottom horizontal row of line keys has a different function. In the case of the exchange lines it enables the line to be held while the operator connects or speaks to the required extension. On the extension lines the lowest key position is for ringing the extension. The keys are self-restoring from this position.

Different coloured handles are used to indicate the various functions of the vertical rows of keys. Fig. 11 shows typical operating conditions. A calling line is identified by the indicator showing white. Exchange line indicators are operated by incoming ringing current, while extension indicators operate over the extension telephone loop when the handset is lifted. Extension indicators also provide clearing signals when the extension handset is replaced. Contacts on all indicators are connected via the night service keys to a buzzer for audible signalling of incoming calls.

Fig 12
The Switchboard Circuits

Fig. 12 is a schematic circuit diagram of the new switchboard. During extension-to-extension calls the retard R provides battery current via relay S to both parties in parallel. The connecting keys when operated change the indicators from the line to a local circuit depending on the S relay contacts. During conversation relay S is operated by the microphone current of the two extensions, and it releases at the end of the call when both handsets have been replaced. S contacts then apply earth to operate both extension indicators via the connector keys and so give a clearing signal, on receipt of which the operator restores the connector keys.

On exchange-to-extension calls retard R is disconnected and the battery feed to the extension is supplied by the exchange, again via relay S in the connecting circuit. This relay is slugged so that it holds during dialling. The exchange indicator, which is connected across the line in series with a capacitor, does not fully respond to dial impulses but is operated by the ringing current. On restoration of the extension handset relay S releases and its contact operates the extension indicator. In this type of connection only one indicator operates as a clearing signal, the exchange line indicator remaining connected across the line to function as a follow-on call signal. Thus if an exchange call is received after an extension has cleared but before the operator has restored the keys to normal, the ringing current will operate the exchange indicator. Ringing current also passes through the connecting circuit to ring the bell at the extension, so that if the operator is absent the call can be answered at the extension. If the operator is present when a follow on call is signalled, the connector keys should be restored to normal and the call answered in the usual way.

The new switchboards are operated from a nominal 24 volts supply, and typical busy-hour loads are 0.4A for the 2+4 size and 0.6A for the 3+9.

The line loop resistance between the main exchange and the most distant extension should not exceed 1000 ohms if the extension telephones are of the new 1000 type, or 600 ohms if older instruments are used. If the exchange-to-switchboard line has less than about 360 ohms 1oop resistance it may be necessary to remove the strap to which Note 2 in Fig. 12 refers. The extension lines may have loop resistance up to about 500 ohms. If the loop resistances of the extensions vary widely, it may be advisable to pad the short lines to avoid the possibility of a short line shunting too much current from a long line to which it is connected, since a common feed is employed for both extensions engaged in a call. In normal installations this problem does not arise as extension lines are usually of less than 200 ohms resistance.

Fig. 13
Switchboard Connected for Functional Testing

The new cordless switchboard is manufactured under product group conditions and assembly and wiring are completed on a line assembly unit. Components such as lever keys, indicators and relays are tested before reaching the line, but inspectors are situated at suitable points to check equipment details, quality of joints and position of wires. The final position in the line is used for the application of bridging, meggering and full functional tests. The functional tests cover all conditions, including incoming and outgoing calls on main exchange lines, connection of each extension to each exchange line, and interconnection of all extension lines. The equipment (Fig. 13) used for these tests applies each condition automatically by means of a sequence switch and enables the inspector to complete all tests in from five to eight minutes, depending on the size of the board concerned, in spite of the large number of manual operations necessary to check the correct functioning of all the switchboard lever keys. The transmission of the speaking bridges is checked by means of a power level meter.

While retaining the features of simplicity and reliability inherent in earlier designs, the new switchboard, in comparison, occupies less space and is improved in appearance and efficiency. The experience of many years, supported by stringent laboratory tests, has been applied to the design of components and the selection of materials and finishes which ensure long life and reliable working under all climatic conditions.

A lamp signalling version of this switchboard is in process of development.

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Last revised: October 15, 2019