PAGE No. 44

New Table Telephones for Export
July 1953

When the Company decided to develop special table telephones of the moulded plastic type for export, it was anticipated that, taking the current standard British Post Office types as a basis, substantial redesigning with some reduction in the facilities provided might be necessary, in order to hold costs at a competitive level and at the same time provide instruments of the highest quality. But detailed examination of the matter showed that, in view of new production methods and in the light of experience of overseas requirements, the existing and new designs could have many features in common.

Two instruments have been developed the N1014B type, for automatic working, and the N1373B type for manual C.B. working, the only difference between the two being that N1373B has a dial dummy, in lieu of the dial, dial cord and dial terminal block fitted on N1014B. Exterior views of the instruments (which are normally black, but can also be supplied in ivory, red or green) are shown in Figs. 1 and 2 respectively.

Fig. 1
N1014B Type Auto Table Telephone
Fig. 2
N1373B Type CB Table Telephone

The primary consideration governing the choice of types of components to be used was reliability ; but another important
factor was the ever-rising cost of raw materials, which made it necessary to reduce manufacturing costs correspondingly where ever possible. Standard B.P.O. type components of proved efficiency and in regular production were the obvious choice, especially as certain Administrations abroad hold stocks of them for use as maintenance spares, and would be justifiably prejudiced against any new design for which they would have to stock a further range of spare parts.

The main components and basic circuits of the new sets are similar to those of the equivalent standard B.P.O. instruments which have been described in considerable detail in a previous article, and are in any case well-known. A detailed description of the new types is therefore unnecessary, and it will be sufficient to compare the standard and new types, emphasizing points of difference.

Fig. 3
Telephone, Fig. 1 with Case Removed

It was decided at the outset to dispense with a separate chassis and mount the internal apparatus on the baseplate. A virtue of the chassis design is that it provides easy accessibility to the parts for maintenance purposes; but the advantage of accessibility has not been sacrificed, as the neat arrangement of the components (Fig. 3) clearly shows. The change has led to considerable savings in material and in manufacturing costs.

Experience has shown that comparatively few of our customers abroad require a pullout directory tray in the table telephone it was therefore decided, in the interests of standardization and further reduction in cost, to eliminate this feature.

Once these major questions of design had been settled, modifications in the shape of the moulded body had to be considered. All the proposals put forward favoured a reduction in its overall height as compared with that of the standard instrument. It was clear that access to the components must not be jeopardised by congestion, and that no facilities that could reasonably be given should be sacrificed. The chosen design satisfies both these conditions, for although the height has been reduced by half an inch, there is still ample room in the interior, even for the inclusion of the largest 3-button key unit used for Plan Number working.

The rounded contours of the case are not only pleasing to the eye, but also heighten the impression of the instrument’s smallness, an effect intensified by the curved shape of the handset. The elimination of sharp corners and ledges has reduced the risk of breakage and enables the highly polished surface of the instrument to be easily cleaned.

Four threaded metal inserts are moulded into the underside of the body to form a very secure fixing for the metal baseplate, while the cradle-switch plungers are held captive in the body by circlips. The construction of the plunger mechanism allows adequate clearance to ensure easy action, and special care has been taken to avoid horizontal ledges on which dust might collect and prevent smooth operation. The ends of the plungers engage rollers on the wings of a switch operating plate mounted on the base of the instrument. This arrangement is now standard in all the Company’s moulded-case telephones; it is so efficient that, before its introduction in 1948, it completed one million operations without failure during accelerated tests in the laboratory, and it has since proved very satisfactory in service.

Fig. 4
Push Button Springset in position

In addition to the cradle-switch springs and the associated operating plate, the baseplate of the telephone carries a ringer, induction coil, capacitor, terminals for the external connections and, for auto working, a dial terminal block. Of these items, the only one which differs from those in the standard B.P.O. instrument is the terminal block for external connections. This comprises a moulded ”L“ shaped block having counter-bored slots on the shorter side for accommodating two fixing screws, and six terminals which provide screw connections for the external wires and solder tags for the internal wires. Either one or two of these blocks can be provided, as required.

Perforations in the baseplate, beneath the ringer gongs, ensure audibility of the calling signals. For tropical conditions these perforations are covered by a fine mesh gauze, in the conventional manner, to prevent the entry of insects.

The telephone rests on four non-slip feet, moulded from special rubber, which are forced into holes in the baseplate, instead of being fixed by screws and nuts as in the standard sets. This innovation effects a small saving in cost without impairing efficiency, as the feet are securely held. Two of them are visible in Fig. 3.

The new telephone, as has been noted, has accommodation for the largest available combination of push button springsets it thus has a range of application equivalent to that of the standard instruments, being suitable for party-line, secretarial, plan number and other operating schemes. The springsets are of orthodox twin-contact type, suitably tensioned for reliable operation and fitted as shown typically in Fig. 4. Any standard combination of springsets operated from one, two or three buttons can be provided, the body being drilled for the appropriate number of plungers. A telephone fitted with three buttons is illustrated in Fig. 5.

Fig. 5
A Typical 3-Button Instrument (N1017A)

The micro-telephone (Handset) is of a new design, having a curved shape in keeping with the general lines of the instrument. A new method of moulding has made it possible to leave a small diameter hollow core in the handle, so that a cord can be threaded through; (the standard design has moulded-in connections between the transmitter and receiver). Threaded metal inserts are moulded in each end and provide a means of securing the receiver and the contact springs for the transmitter. Both the receiver and the transmitter are of the highly efficient inset type in regular production for the standard micro-telephone. The moulded earpiece is screwed on to the receiver case and clamps the diaphragm, whilst the mouthpiece is secured by a bayonet clip, for which a key is provided; both these methods of assembly conform to normal practice.

The standard B.P.O. type microtelephone may be used with the new instruments ; this arrangement was, in fact, accepted in the earlier stages of development, in order to avoid delay in the completion of certain orders, when circumstances beyond our control prevented production of the new micro-telephone by the planned date. The interim types then produced were designated N1014A (auto) and N1373A (C.B.), and are now superseded.

The automatic dial on the N1014B instrument is of the latest B.P.O. standard, “trigger” type, and the cords for both instruments are also those in current production, but a new terminal block for desk or wall mounting has been designed, as it was felt that the standard rectangular block was not of suitable shape. The new, small, circular type has been designed to accommodate three screw terminals for cable and cord connections, has a cover fixed by a centre screw, and is of moulded plastic coloured to match the telephone.

Instruments intended for use in the tropics have the special protective finishes which we have evolved through our extensive knowledge of the requirements for such conditions.

In conclusion, we may claim that the new instruments compare favourably with existing standard types, in that they have the same range of application and use the same types of components. Any slight disadvantage occasioned by the omission of the directory tray fitment and the interior equipment chassis is counter-balanced by the improved external appearance. With this refinement, our new Telephones for Export will come to be considered an integral part of the tasteful appointments of the home or office, rather than a strictly utilitarian fitting. We are, moreover, confident that they will uphold the high reputation for reliability which our products as a whole have won.


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