PAGE No. 30

New Bakelite Telephones
January 1936

In designing a modern telephone instrument several important points must be taken into consideration. It must have a high standard of electrical efficiency both in volume and purity of speech, a pleasing outward appearance combined with rigid construction, and, for reasons of economy, simplicity in maintenance and adjustment. The most effective combination of these qualities has been the ultimate aim of the Ericsson Company, whose wide experience in this section of telephone engineering has resulted in the production of new Bakelite table and wall telephones.

These instruments which are introduced as alternatives to the Bakelite telephones described in Bulletin No. 4, are intended for use with either automatic or central battery exchanges, and facilities are provided for easy conversion from one system to the other. Figs. 1 and 2 show the instruments equipped with a dial for automatic working. When required for central battery working the dial is replaced by a dial blank as shown in Figs. 3 and 4.

The importance of standardization and inter-changeability with respect to stocking and maintenance facilities has been closely considered, and in this connection the following standard British Post Office apparatus has been incorporated:

Dial Auto. No. 10 F.A.
Bell No. 59A.
Bell Gongs No. 2.
Coil induction No. 22.
Condenser M.C. No. 102.
Cradle springset, Tele. 162 Type.

At the first glance it would appear almost impossible to house the above apparatus conveniently in so small a space presented by the unique design of the casework, but a specially designed bracket arrangement of unit construction has made it possible to use not only standard British Post Office components but to mount them in such a manner that there is adequate spacing throughout. Moreover, these components, with the addition of the hand-microtelephone, are fully interchangeable on both the table and wall instruments.

The connections of the wall instrument are essentially the same with the exception of the desk terminal block and cord which are not fitted.

A high standard of transmission, reception and side tone reduction is assured by the use of the modern type of induction coil, whilst other Features are undistorted dial impulsing, and disconnection if the bell during dialling and speaking. Conversion From central battery to automatic working may be easily carried out by adding a dial and dial cord, and removing a metal strap fitted on the terminal block. Provision is made for the connection of an extension bell, and on the table instrument an extra conductor is included in the cord for this purpose.

The moulded Bakelite body of both instruments follows the same distinctive lines as the intercommunication telephones with exchange facilities described in Bulletin No. 5. The improved type of cradle is a noteworthy feature and is designed to withstand extremely rough treatment. The cradle is formed by a recess in the upper portion of the body moulding ; this recess is shaped to support the handset and guide it into position even when carelessly replaced. Two chromium plated plungers operate the switch movement, and these are specially shaped to reduce friction and prevent sticking troubles.

The Table Telephone N 1020
Accessibility is a characteristic feature of this instrument and has been obtained by mounting all the internal components on a separate frame, forming a unit which can be easily detached or replaced.

The frame, which is made from nickel plated metal, carries the ringer and switch on the upper side, and the bell domes,  induction coil, condenser and terminal block underneath. Extreme care has been taken in positioning this apparatus in order that each component may be removed without disturbing any other. A sectional view showing the unit fitted inside the case is shown in Fig. 6. The unit is held in position by three screws, and after removing the baseplate, which is secured by a single captive screw, these screws are exposed so that by loosening two and removing the third the unit can be withdrawn bodily from the case, as shown in Fig. 7.

Fig 6 - Sectional view of an auto table telephone Fig 7 - Auto table telephone with apparatus unit removed

The main features of the ringer are simplicity in construction, ease of adjustment and high sensitivity. The polarizing magnet is of cobalt steel. One end forms a bearing for the armature with hammer stem and the other end is clamped into the yoke by a single screw. If necessary, adjustment to the armature can be made by loosening this screw and sliding the magnet in or out of the yoke. By rotating the domes, their relative position can also be adjusted since the fixing hole in each is drilled slightly out of centre. Each coil has two soldering tags fitted in one of the Bakelite end cheeks and the resistance of each winding is 500 ohms, making the total resistance of the ringer 1000 ohms, i.e. with the coils connected in series.

The induction coil has five windings and embodies the latest anti-side-tone developments. The bobbin consists of a bakelised paper tube fitted with moulded Bakelite end cheeks. The tube is packed with soft iron core wire. Seven soldering tags are provided on the cheeks for making the external connections to the various windings.

The moulded hand micro-telephone, described in Bulletin No. 4, is the same as fitted on the N1010 type of instrument, and is now too well known to warrant any further description. A recent development, however, is the improved transmitter inset which, in addition to the advantages of being repairable, having a robust construction and a suitability for use under tropical and semi-tropical conditions, now

has a higher efficiency as regards volume, articulation and freedom from frying. The resistance has been made sufficiently low to enable it to be used on both central battery and local battery circuits, giving standard efficiency under either condition.

The connecting block is of moulded Bakelite and has 12 terminals with screws on one side for external or cord connections, and tags on the other side for the internal soldered connections. The end of the block protrudes through the outer case and is shaped to form an inlet for the handmicro and desk cords, the ends of which are securely held inside the set by a Bakelite clamping block, thus preventing any strain on the conductors.

The Wall Telephone N 1070
Although somewhat different in construction from the table set, the wall set embodies the same components and retains the same facilities for maintenance and adjustment. The Bakelite case is hinged to the back-plate and, when the set is fitted to the wall, the case can be swung downwards by loosening a single captive screw situated behind the handset grip. Thus any necessary adjustments to the ringer, switch etc., can be carried out from a position directly in front of the instrument. The picture to the right  shows the instrument with the case opened for inspection.

As in the table set, a form of unit construction has been employed and all the internal components are mounted on a flat nickel plated metal plate which is clamped to the back-plate by means of three screws. Slotted fixing holes enable the unit to be completely withdrawn by loosening these screws. The metal backplate, which is black enamelled, has three fixing holes enabling the instrument to be screwed to the wall; these holes are recessed so that, when fitted in position, the back 0f the instrument is spaced about 3/16 from the surface of the wall.

For use in tropical climates and where protection against damage by insects and moisture is required both the table and wall instruments can be furnished with a special non-corrosive finish on the metal parts, impregnated coil windings and suitably constructed cords, making them specially adapted for use under adverse climatic conditions.

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Last revised: October 14, 2019