PAGE No. 53

Improved Moulded Case Wall Telephones
July 1950

The Company has recently introduced improved wall type equivalents of their now familiar range of moulded case table telephones, in order to cater for all conditions of use. These wall sets were to have been available soon after the introduction of the table sets and were, in fact, in an advanced stage of development when the war caused work to be suspended. In consequence of this delay it was not until last year that full production of the new wall instruments was attained.

Fig. 1
Auto Wall Telephone - Ordinary Working (N1071)
Fig. 2
Auto Wall Telephone - Plan Number
Working (N1071D)
Fig 3
Magneto Wall Telephone (N2204)

Although there exist dual purpose telephones suitable for use on table or wall, careful consideration of the arguments for and against this arrangement led to the conclusion that entirely separate designs were to be preferred, but at the same time, components should be made interchangeable as far as possible.

Fig. 4
' Personal Call'

Instruments for ordinary or plan number working on automatic, manual C.B., or magneto systems have been developed and are typified by the illustrations in Figs. 1 to 3 which show, respectively, ordinary automatic, plan number automatic and ordinary magneto telephones. The C.B. instruments are similar in appearance to the automatic except that the dial impulse switch is replaced by a moulded disc, while the plan number telephones may have up to three buttons for operating various combinations of key switch units, the functions of the keys being indicated on a designation strip, as in Fig. 2.

From the illustrations it will be observed that the casework is a one-piece moulding with the top portion shaped to form a robust cradle for the micro-telephone. It is made of highly polished plastic material in any one of four fadeless colours - black, Chinese red, ivory or jade green - to suit individual taste, and external metal parts are bright chromium plated. The glossy finish and absence of ornamentation facilitate cleaning.

The front wings of the cradle are specially shaped to hold the micro-telephone in a vertical position when necessary, as in Fig. 4, to enable the user to go from the instrument during conversation, without leaving the handset dangling by the cord, to its detriment, or alternatively, replacing the handset and operating the gravity switch, a procedure which would release the connection on systems having automatic clearing.

Fig. 5
Interior of Magneto Telephone

With the exception of the gravity switch plunger mechanisms, push key plungers, and the automatic dial or moulded disc, all components are mounted on a metal chassis which is fixed inside the case by means of captive-type screws, while a metal baseplate on which the casework hinges is provided for mounting the telephone on the wall. The hinges, one at each left-hand corner, fit snugly into recesses in the case and each is held by one captive screw. This method of fixing provides a ready means of removing the instrument from its backplate when in situ, and facilitates maintenance, since it is necessary only to disconnect the external cable and unscrew one hinge in order to lift off the case with component chassis, leaving the backplate and fixings undisturbed. Conversely, during installation, the instrument can be more readily fastened to the wall if the weight of the case and chassis is thus temporarily removed. A captive screw in a deep recess in the side remote from the hinges is used to lock the case.

The backplate has three deep indentations forming feet which hold the telephone well away from the wall and ensure firm seating without distortion on uneven surfaces ; also, twin circles of holes to provide outlets for the sounds from the ringer gongs. At the bottom centre of the plate is a hole through which the external cable is brought in, looped through and tied to a 'U' plate on the chassis, then connected to the terminal block, in the manner shown in Fig. 5, so that the cable ends will swing with the case when it is opened and closed, a safeguard against the wires being strained or trapped between case and backplate. A diagram of the internal connections is fixed to the plate and preserved by a coating of clear varnish.

Fig. 6
Chassis of Magneto Wall Telephone

The need for making the components of the existing table telephones and the new wall telephones interchangeable as far as possible, was, for obvious reasons, deemed of paramount importance, therefore this objective was kept well in mind during development, with the result that it is possible to take a complete chassis assembly from an automatic or C.B. table telephone of either ordinary or plan number type, and fix it into the casework of the appropriate wall instrument without alteration of any kind, moreover, the metal plungers of the gravity switch, and the plan number key buttons and label used on the table set are identical to those on the wall set. This interchangeability is of undoubted benefit to telephone administrations concerned with the storage of replacement spares.

The positioning of components in a magneto table telephone follows, of necessity, a rather stereotyped form, and the interior space shape cannot be simulated in a wall set without sacrificing, to a certain extent, the external symmetry, therefore it was not considered necessary to have an interchangeable chassis for the magneto sets. The components on the wall instrument chassis (Fig. 6) are arranged in a manner similar to those on the common battery counterpart with, of course, the addition of the hand generator.

Fig. 7
Enlarged Interior View of Fig. 2 showing Plunger Quadrant Levers

Two types of magneto wall telephones are available; one for general use, fitted with a standard 'alnico' generator, and the other with a slightly larger alnico heavy duty generator for long or heavily loaded lines. The generator handle is at the lower right-hand side of the instrument and is sufficiently far forward from the wall to ensure ample clearance for the hand.

The gravity switch mechanism used in the new telephones is the Company's latest improved type designed for reliable working under adverse conditions. While being subjected to accelerated life tests in the laboratory the device completed more than one million operations without failure. The Metal plungers each have a recess in the head into which the bearing bush extends, thus a shroud is provided to prevent the entry of foreign matter (through the holes in the cradle) which might obstruct the switch movement. The switch operating plate lever travel is at, approximately, 900 to that of the plungers the motion of which is transferred to the plate by means of sliding bars freely pivoted to quadrant levers clearly seen in Fig. 7. These engage rollers on the wings of the switch operating lever, or plate, mounted on the chassis, (see Fig. 6) and by this means, friction is reduced to a minimum. This plate lever with rollers is now standardized for use on all the Company's moulded case table telephones, the arrangement being as illustrated in Fig. 8.
In view of the many types of instruments, no attempt is made to reproduce here the respective wiring diagrams, nor is much to be gained from illustrating a selection of them ; it may be stated, however, that ordinary or special conditions can be met, thus, according to the requirements of the various systems, provision can be made for the inclusion of condensers, for the connection of an extension bell, etc. An example of this is given in Fig. 9, a schematic diagram of a magneto instrument in which the internal wires are looped at the points marked X to enable condensers to be connected when necessary, and in which dotted lines show the wiring for an extension bell.

Fig. 8
Diagram of Roller Device in Table Telephone
Fig. 9
Schematic of Magneto Telephone Circuit

For automatic telephones, facilities are given for mounting on the instrument terminal block a radio interference suppressor, in the form of a small, compact filter unit which absorbs the high frequency component of the dial impulses so that interference with radio communication is reduced to a minimum. It should be mentioned, however, that a suppressor is very rarely necessary.

Instruments for use in temperate or tropical climates are available, the materials and finishes in the latter case being carefully selected and prescribed, coils impregnated and wiring P.V.C. insulated, also a dust cover for the dial mechanism can be fitted.

Every effort has been made to render the new instruments efficient, utilitarian and attractive while being reasonable in cost. It is confidently anticipated that they will prove to be as popular in their particular sphere as are their counterparts, the moulded case table telephones.


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