PAGE No. 69

W. SINCLAIR - Apparatus Engineering Department
July 1960

A natural corollary of the recently designed and world-wide accepted Etelphone (described in detail, Bulletin No. 38, Jan. 1959), has been the demand for a telephone instrument having the same pleasing contemporary appearance, wide colour range, and efficient transmission performance, but with incorporated features to meet the varying requirements of customers desiring extension facilities, including those of the recognized extension plan arrangements.

The Company therefore has introduced this new telephone - the Plan-Etelphone. This instrument, now being produced, has a flexibility in application that enables all extension requirements to be confidently fulfilled. The article describes the mechanical features of the telephone, and the auxiliary components-utilized in various extension plans.

Fig. 1
General view of the Plan-Etelphone instrument

In the late nineteen-thirties, in conjunction with the British Post Office, the Company developed key-switching units suitable for plan-working application, and since this time has been closely identified with plan-type instruments. The knowledge gained in this field, through the years, has now made possible the introduction of a new telephone - the Plan-Etelphone. This instrument, incorporating the best of accumulated ideas and experience, and satisfying the demand for a modern ‘plan’ telephone of high performance, is shown in Fig. 1.

To avoid the necessity for a multiplicity of different types of instruments to cater for the variety of extension plans, and yet provide a versatile and economical unit suitable over a wide range of application, maximum flexibility in use is essential. This has been achieved by the provision of a basic instrument that can be readily adapted to meet any facility by the use of suitable ‘addon’ units.

In order to maintain the established layout of the ringer, the dial, capacitor and induction coil, as in the Etelphone instrument, and also provide accommodation for, and the highest degree of accessibility to, other components within the dimensional limits imposed, maximum space utilization has been necessary. This objective has been realized by the use wherever possible of components of small bulk, without degradation of performance and reliability.

Fig. 2
A typical 4-button key-switching unit complete with associated wiring and terminal strips

To keep the variety of component parts to the minimum, a springset assembly, of a new miniature form, has been adopted for cradle-switch application and also for use in the key-switching units to be described later.

These assemblies, comprising springsets of a type similar to that employed in our new B.P.O. approved cordless switchboard and other equipments, have suffered no loss in reliability or performance through size reduction. Special attention has been devoted in the design of the twin-contact phosphor bronze springs to ensure their satisfactory operation within safety margins comparable with larger springsets of earlier design. High contact pressures have been maintained, insulation between springs increased, and uniform movement of springs made possible by comb-plate action.
Protection to all springs against dust and damage is afforded by transparent covers.

These consist of springsets, and spring-loaded plungers with or without latching devices, mounted on a slotted metal plate forming the front of the fixed main bracket.

A key-switching unit has capacity for 56 springs arranged in eight banks of seven springs. The banks can be disposed in single or double units, the springset assembly itself being secured to the plate by means of a single screw.

Where a large number of contact springs is employed in a predetermined switching arrangement, as typically shown in the key-switching unit illustrated in Fig. 2, the springs are permanently wired to auxiliary terminal strips for connection to the telephone circuit. For simple switching requirements, however, the flexible connecting leads are terminated with spade tags to permit single or double bank springsets to be connected in circuits requirements demand, using spare terminals provided in the base of the instrument for this purpose.

Normally the springsets are evenly distributed between plungers, but various plunger extension arms can be added to increase the number of springs operated by any single plunger.

Plungers can be locked in the operated position and also extended by again pressing the associated press button. This overpress facility enables secondary switching functions to be accomplished without additional press-buttons (i.e. ringing etc.). Release of locked plungers is effected on the operation of another button or by replacing the handset.

A feature, novel as well as advantageous and typifying the versatility of the key-switching units, is the provision made for a fifth plunger in the central position of a 4-button switching unit. This extra plunger can be operated by either or both of the central press-buttons when a common or late switching operation is necessary.

By arrangement of individual latch plates a sequence of locking can be obtained, the second button operated being released when further pressure to the first button is applied.

If it is desired to rearrange switching needs in situ, it is convenient to use a complete latch-plate assembly as shown in Fig. 3. It consists of a number of stainless steel latch-plates for engaging the plungers and

 is mounted on a main plate, which pivots at its lower end across the front of the fixed main bracket. On loosening each screw and by moving these latch-plates into any of three different positions, various switching functions can be obtained. The positions of the latch-plates (identified by numbers in the illustration) give the following conditions:-

1. Locking, and releasing any other latched plunger.
2. Non-locking, and releasing any other latched plunger.
3. No action.
4. Locking, and releasing any other latched plunger (as in 1).
5. Cradle-switch release of any latched plunger.
6. No cradle-switch release.

One facility this type of assembly cannot provide, is the consecutive locking of plungers described above.

The high complement of springs permissible in the key units necessitates termination points other than those provided in the basic instrument. Either two or three 6-way terminal strips may be fitted. When requirements are met by two terminal strips, these are fixed either side of the fixed bracket. If the number of springs employed justifies the use of three, these are accommodated together between the arms of the bracket.

Fig. 3
Alternative latch-plate assembly
Fig. 4
Combination of ordinary and independent-action buttons

The press-buttons, of contrasting colour to the instrument case, shaped at their finger-ends to provide an easy finger action, are positioned in line between rectangular holes in the top of the instrument case. To hold a button captive, yet not restrict its sliding movement, a metal pin is passed through a vertical elongated hole in the button, and pressed at its ends into grooves inside the case.

Another type of button is illustrated in Fig. 4. It provides a locking action independent of the internal mechanism. Held in position by a metal pin as before, this button - as may clearly be seen from the part-transparent model illustrated - incorporates a hooked latch, this being controlled by a sliding cap in the top of the button. When the button is pressed down, a movement of the cap to the ‘on’ position causes the latch to engage the fixing pin and prevent the button returning to the normal position. Release is effected by again pressing the button and switching the cap to ‘off’.

For simple switching requirements (i.e. non-locking ring buttons) two buttons can be accommodated in the space occupied by the normal button, thereby enabling twice the number of switches to be individually operated. A typical key arrangement showing the ‘two-in-one’ button is given in Fig. 5.

When less than four press-buttons are required the rectangular holes are closed by ‘snap in’ dummies.

In the instrument, provision is made for mounting two miniature screw-lamp fittings, with white flexible reflectors, on extension arms of the cradle-switch springset brackets.

When low-consumption lamps are fitted, effective illumination is obtained through transparent lenses inserted in holes pierced in the face of the instrument near its upper corners.

The translucent quality of the material forming the case has enabled an alternative indicator to be designed for lamps of higher wattage. This indicator is shown in Fig. 6. Its simple construction has been achieved by

reducing the thickness of a circular section of the case to a degree sufficient to ensure adequate penetration of light in the immediate area facing the lamp. Because of heavy pigmentation, black instrument cases cannot be provided with this facility.
It is of interest to note here, that the lamp unit positions can be alternatively used to accommodate press-buttons should space for this purpose be required (i.e. operator recall, etc.).

Fig. 5
Typical press button arrangement showing ‘two in one’ buttons
Fig. 6
New lamp indicator (on a CB telephone)

This adaptor, used for automatic separate-metering shared service lines, is usually mounted under any of four key-unit press-buttons. Alternatively, it may be mounted in place of either of the lamp units.

An adaptor, consisting essentially of a choke coil, may be added in the instrument (without dial), to convert it to a local-battery instrument. An additional cradle-switch contact unit is included as part of the unit.

Both of these assemblies may be mounted at the rear of the fixed bracket. The buzzer, small but robust, is mounted on an extension of the cord-securing bracket and secured by means of two screws.

The transistor-oscillator (operational details are given in Bulletin No. 40, Jan. 1960) is also mounted on the cord-securing bracket. The frame of the oscillator unit has two extensions that clip inside and against the walls of the instrument base to prevent any movement of the assembly.

Because of the large number of components required in the various extension plans, it is clearly impracticable to mount them all in an instrument of reasonable dimensions. In some extension plans either an additional bell or buzzer, or both are necessary, and these are, therefore, mounted externally. Fig. 7 shows both components housed in a plastic-moulded case.

Fig. 7
Ringer and A.C. buzzer in plastic-moulded case
Fig. 8
Desk terminal block and cord terminations plate

The ringer is of the latest type, and the same as used in the instrument. To permit ringing discrimination between the two, different sets of tone gongs are provided.

On a normal single-line instrument with two or three cord-termination points, disconnection or connection of the cord is a simple matter, but with the addition of more complex switching the number of conductors obviously increases. Therefore, with ten, twelve or more connections to be made in extension plan arrangements it is important that some arrangement be made to ensure the correct location of the terminations. To this end a new design of connecting tag has been developed for the desk cord conductors. The tags, together with their associated screws, are made captive on a plate of insulating material to maintain a permanent relationship between connections. The screws clamp the tags to metal bushes in the terminal block, (Fig. 8) thus holding the insulating plate in position and also providing high contact pressures and reliable low - resistance connections This method of termination permits the removal or replacement of an instrument without disturbing the local wiring or the need for skilled labour.

The Plan-Etelphone is treated to withstand wide extremes of temperature and humidity.

The facility to lock the cradle-switch actuating bracket in the ‘handset on’ position by means of a latch spring on the fixed bracket, is included in the instrument (a feature of the Etelphone).

The Plan-Etelphone is undoubtedly a considerable improvement on previous types, and will, it is expected, have wide appeal. It is extremely compact yet easy access to all components has been maintained. The mechanical design ensures a high standard of operational reliability, greater convenience in use, and reduction in maintenance costs. A further contribution by the design is that no unsightly appendages are necessary to meet particular applications; all extension plan requirements can be met without alteration of the instrument’s pleasing lines.

A subsequent article will detail facilities and circuit arrangements relating to the extension plans.

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