PAGE No. 54

July 1955

The telephone described has recently been selected by the British Post Office to be their future standard moulded wall instrument, Type No. 333.

The design caters for ordinary, Shared Service or Plan Number working in automatic or manual C.B. systems, Type N1073A being the straightforward automatic telephone without push buttons. The equivalent manual CB telephone is Type N1422A.

The Company’s Engineers have recently concentrated on improving the appearance of some of the types of telephones designed many years ago but which, though still in demand, have little aesthetic appeal today. Re-designed table and wall type intercommunication telephones, and a moulded table telephone for general use or adaptation for Plan Number working, have been available for some considerable time and have been described in a previous Bulletin.

Old Type (N1071A) New Type (N1073A)

Fig. 1 The Old and New type Telephones Compared

Another step in this logical development is marked by the introduction of an improved moulded wall telephone, which is now on the production lines. Both automatic and manual C.B. versions are being made, differing only in the substitution, in the manual set, of a dial dummy for the automatic dial and associated cord in the other.

The automatic telephone, Type N1073A, may be considered a companion to the type N1014B automatic table instrument, for both have similarly curved contours, identical handsets and basically the same construction, with a moulded one-piece case, and a metal base or backplate on which the internal apparatus is mounted.

Fig. 2 Interior view of the Telephone

For purposes of comparison, the new automatic wall telephone is shown in Fig. 1 beside its predecessor. It will be generally agreed that the designers have achieved their principal aim - improved appearance.

The primary quality demanded of any telephone is reliability in service, appearance being generally of less moment. Nevertheless, appearance may assume major importance when a telephone suitable for installation in elegant surroundings is being sought. It is a reasonable certainty that, of the two telephones shown in Fig. 1, Type N1073A would be preferred for the hall of a private residence or for a tastefully furnished office. It would also probably be preferred for warehouses and other places where robustness counts more than appearance : the new case, with a moulding free from sharp angles, is, if anything, stronger than its predecessor.

Comparison between the old and new sets on the grounds of reliability in service gives neither the advantage. Both have the same basic circuit and similar components, both utilize the practically frictionless roller-plunger movement which was evolved by the Company some years ago and has since proved highly efficient in service. The new sets are available - as were the old ones - in both standard and tropical versions ; thus the possibility that one type is more reliable than the other in tropical conditions does not arise.

Features of the earlier model that have been discontinued are, firstly, the removable equipment chassis; secondly, the gate-hinged case. It was considered that the advantage of the chassis design - which permits the removal of the equipment unit for servicing - was largely offset by its extra cost and was one which would be of real value only in the event of a major repair; a comparatively rare occurrence. Attention was therefore centred on devising a layout of the component parts on the backplate of the new telephone that would leave free access for servicing and obviate the need for a separate chassis. In this connection, one of the difficulties experienced with any type of wall telephone having a hinged case is that the case, when open, is apt to swing and impede access to the interior. The gate-hinged case offers an advantage in this respect, since it can be lifted off; but if the telephone were located in, say, a corridor, the case would probably have to be left on the floor and might inadvertently be kicked while maintenance was in progress.

Such drawbacks have been overcome in the new design by the attachment of the case to the bottom edge of the backplate by a strong webbing strap, so that it is suspended below and well clear of the apparatus, as shown in Fig. 2.

On the top edge of the case are two metal clips which engage with recesses in the edge of the backplate when the instrument is closed; a single captive screw at the bottom serves to lock the case in position.

A novel feature of the design is the dial mounting. Normally the dial fits direct into the aperture and is locked in position by one screw, the moulding round the aperture being shaped to form the seating. Improved seating is now achieved by the provision of a metal clamping ring (Fig. 3), which fits into the aperture and is fixed from the inside by three screws. The form of the ring is such that it is readily adjustable to the dial fixing lugs; thus the latter need not be bent to fit the aperture, and the risk of damage to the mechanism is eliminated. The ring is also designed to permit extraction of the dial without removal of its fixing screw.

Fig. 3 Dial Clamping Ring

The telephone backplate is a light alloy die casting to which four semi-resilient synthetic rubber feet are attached by tubular metal rivets which also serve to accommodate the telephone fixing screws. The feet are sufficiently flexible to compensate for any reasonable inequality of wall surface, so that distortion is avoided. When the instrument is installed, there is approximately half-an-inch clearance at the back for the issue of sound from the ringer and for the external wires. The latter enter the instrument through a hole in the bottom centre of the backplate. The handset cord is fed in through a synthetic rubber protector fixed in the bottom of the moulded case.

Of interest in the internal arrangement are the two brackets between which the cradle switch lever is pivoted; they are formed for the convenient mounting of the additional units required for shared service working. There are two such units, viz. British Post Office Adapter Shared Service No. 1 and No. 2 (Fig. 4). Each incorporates a flexible cord for connection to the main terminal block in the telephone, the layout of which facilitates this arrangement.

Adapter Shared Service No. 1 Adapter Shared Service No. 2 Typical 3-Plunger Plan No. Key Unit as used with Unit
No. 2 above

Fig. 4 Typical “Shared Service” and “Plan No.” Key Units

Adapter No. 1, comprising a Thermistor, copper oxide rectifier and a single plunger key with a make-before-break springset, is used for normal two-party conditions, the Thermistor being included to prevent bell tinkle during dialling, and the key to permit the recording on separate meters of the calls initiated by each party.

Fig. 5 Schematic of the Basic Circuit

Adapter No. 2 is provided when plan number working under shared service conditions is required. It is similar in most respects to Adapter No. 1, but as individual requirements differ, the key is replaced by a separate plunger key unit of appropriate type, such as that shown in Fig 4; the front of the telephone is drilled for one, two or three plungers as necessary any unused holes can be filled with an unobtrusive plastic dummy.

Fig. 5 is a schematic of the basic circuit. A slightly different arrangement obtains when shared service or plan number working is envisaged, the cradle switch springset then having an additional contact and the wires being terminated to provide convenient “break in” points for the leads from the key units, should these be required.

The internal connecting wires in both the standard and tropical models are insulated with p.v.c., as are the conductors in the handset cord, which is laid up circular and covered with a nylon cordonet braiding.

The new telephones are being produced in the usual range of colours, namely, black, ivory, Chinese red and jade green ; it is expected that this variety of finishes, combined with their pleasing appearance, will tend to make them more popular than the conventional wall telephones of more austere design that have long been the accepted standard.

A prototype of the new telephone was submitted to the British Post Office, which had for some time been contemplating the standardization of a moulded wall set and selected it from a number of designs as their future standard. Its further development was therefore carried out in collaboration with the Post Office Engineers, who have allocated to it the P.O. Type No. 333.

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