THE UNIVERSAL TELEPHONE TYPE 300
THE TELECOMMUNICATION JOURNAL OF AUSTRALIA
A. W. McPherson
During the last 10-15 years many interesting developments have occurred in the design of telephone instruments. These developments followed the desire of subscribers generally for greater comfort when using a telephone, and
of engineers for greater efficiency in the transmission of the human voice.
Such advancements in telephone design have resulted in the replacement of pedestal type telephones by instruments of the handset pattern, of which the 332 type shown in Fig. 1 received in Australia in 1939, was then the latest British model. The magneto telephone in this series is known as the 334 type with built-in Alnico generator, and was
developed following certain suggestions from this Administration.
The shape of the instrument made it suitable for table use only, and the need for a specially designed wall telephone was still apparent. Associated with the development of an instrument for this purpose was the consideration of a Universal Handset Telephone
to meet the following requirements:-
(1) Modern appearance.
(2) Provision for wall or table mounting.
(3) Maximum transmission efficiency and side tone control.
(4) Simplicity and ready interchangeability of component parts.
(5) Economy in manufacture and ease of maintenance.
(6) Standardisation of circuits.
A universal telephone has now been developed, and the features incorporated therein form the subject of this article. The design caters for separate phenol formaldehyde moulded cases for wall and table telephones, and each is available for C.B., Automatic and Magneto
working. There are thus six distinct telephones of the universal type, viz., AT, AW, CBT, CBW, MT, MW, and to distinguish these from earlier handsets, the identification number 300 has been allotted to them. Typical completed instruments are shown in Fig. 2. Whilst the general appearance of the table telephone is similar to that of the 332 type, the construction and lay-out -of the component parts is such that greater facilities for interchangeability are offered by the 300 type. The wall sets present a neat and modern appearance, and should prove popular with
subscribers who desire a wall-mounted instrument.
The following apparatus is common to each of the six Universal telephones:-
Switch bracket Assembly.
Fig. 3 shows a base plate with all of the above component parts mounted, with the exception of the plunger assemblies, which are associated with the wall or table case. For the sake of simplicity of illustration the handset and
cord have also been omitted. The assembly of an automatic table telephone requires the following additional
Dial No. 10.
Dial Retaining Plate.
Dial Cord - 8 in.
External Terminal Block.
The complete assembly ready for screwing the base plate to the case is shown in Fig. 4. The securing of four captive screws attached to the base plate completes the telephone. The assembly of a C.B. table telephone is identical except that a dial dummy is used in lieu of the dial, and the dial cord is not required. The necessary strapping is accomplished by means of special links on the terminal block.
The assembly of the automatic wall telephone requires thee same components as
for a table
set, less the line cord, external terminal block, rubber feet and table case. A 15-inch dial cord is necessary to enable the telephone to be opened sufficiently for maintenance. The common base plate containing the components is secured to a
back plate by means of four screws, and the back plate in turn is hinged to the wall case moulding. Fig. 5 shows the back plate and moulding ready to receive the components, and Fig. 6 is a view of the complete telephone
before thee hinged portion is closed. It will be noted that the moving arm of the
switch bracket assembly is mounted in a position at right angles
to that occupied when fitted to a table telephone. This is necessary because of the different relationship of the base plate with respect to thee plunger assemblies. The method of
assembling the switch bracket in the two positions is shown in Fig 7.
One common circuit diagram shown in Fig. 8 is used for C.B. and automatic table or wall telephones, and the
characteristics individual to each instrument are indicated in the accompanying notes. The circuit is basically the same as that for the 332 telephone described in Vol. 2, No. 2, October, 1938, the only difference being that a B.P.O. No. 27 induction coil, which supersedes the No. 22 coil, is now used. The
operation of each is identical, but the No. 27 coil has different values of resistance and turns for the windings, and was developed to provide higher efficiency and slightly different side-tone characteristics to the No. 22 coil with which it is interchangeable.
The assembly of the magneto telephones is similar to that of the C.B. and automatic instruments, except that a built-in generator is provided in the position normally occupied by the dial or dial dummy. Flexible cords 8 in. and 15 in. in length respectively for the table and wall telephones provide
the means of connection between the generator and terminal block. The generator is designed specially for the purpose and the constructional details and electrical
performance will be published in a future issue. The circuit diagram for the magneto wall or table telephone is shown in Fig 9, and the operation
is fundamentally ,the same as that for the 233 M.W. telephone described in Vol. 2, No. 3, February, 1939, except that a condenser is placed in series with the signalling circuit, and the generator springset opens the bell circuit during an outgoing ring. It will be noted also that the cradle switch springs in the 300 type M.T. and M.W. conform with the standard arrangements for handset telephones, and the connection to the batteries from the table instrument is made by means of a three-conductor cord which also serves as the line connecting cord.
(1) The simplicity of the design of the plunger assemblies will be observed from Fig. 10. In earlier telephones the plungers were mounted in metal sleeves fixed in
position by means of screws, whereas the 300 type plungers operate in correctly dimensioned holes in the moulded case, and the friction sue-faces are metal to bakelite instead of metal to metal. Tests conducted to date indicate that the simplified plunger assembly will be just as satisfactory in service as the more elaborate metal insert type.
(2) The switch bracket assembly is provided with extension pieces as shown in Fig. 7
to facilitate the closing of the wall case. These projecting lugs engage with the bottom of the plungers and raise the latter gradually to their operating position. This obviates the necessity for holding the plungers in the upper position whilst the case is being
(3) The base plate of 14 gauge mild steel performs the dual purpose in the table telephone of providing a base for the instrument, as well as a means of mounting the component parts.
(4) The table case can be used for a 332 telephone.
(5) An additional ebonite plunger is provided on the switch bracket assembly so that a second springset can be fitted if special facilities are required in the telephone. The
switch bracket is drilled and tapped for two springsets.
(6) Means for the mounting of press buttons, etc., for miscellaneous purposes are provided by two mounting holes at the top of the
switch bracket assembly.
(7) The terminal block containing 19 terminals provides for the maximum flexibility by the use of connecting links between terminals.
(8) A two-conductor line cord is provided on C.B. and automatic telephones for straight services. A
three-conductor cord is fitted only when an extension bell is required.
(9) A 184 handset is provided. This type differs from the 164 handset previously employed on telephones in that the strap connection between one side of the receiver and transmitter
is removable and the four terminals are available if required.
(10) A ready means of locking the dial and dial dummy in position is provided by a retaining plate, and the method of fixture is shown
in Fig. 11.
(11) Press-in type rubber feet are used on the table instruments.
(12) The C.B. dial dummy is designed so that the label, celluloid disc and locking ring are inserted from the rear.
The entire telephone with the exception of the dial, dial cord, line cord and handset cord is made in Australia.
Owing to war-time conditions the colour of the instruments has been confined to black, but it is anticipated that other colours will be available in the near future.