PAGE No. 24

January 1956

The loudspeaker telephone described in this article is designed for use as a master station instrument in intercommunication systems employing Types N. 1732/N. 1733 moulded cased side station telephones. A description of the latter and an illustration of a typical wall telephone appeared in Bulletin No. 27, July 1953.

The loudspeaker intercommunication telephone - designed for the managerial office - will afford the executive freedom of movement while he converses by telephone - a particularly welcome facility when it is necessary for him to consult records or make notes. The equipment includes a loudspeaker and a microphone which together enable the speaker to carry on a conversation without engaging either hand. The two equipment items mentioned have not always been incorporated in the instrument; one of the Company’s early models contained only the signalling and switching apparatus, the loudspeaker being housed in a separate cabinet, and the microphone in a specially constructed inkstand.

Fig. 1
Type N1747A Loudspeaker Telephone Equipped for a 15-line System

The Type N1747A loudspeaker telephone recently designed (Fig. 1), contains the loudspeaker, microphone and the switching and signalling components, the only extraneous items being the dry cells required for signalling and speech current.

It will be apparent from the illustration that whilst the design of the new telephone accords well with contemporary styles it has a dignity in keeping with more traditional types of office furnishings.

The wood of the casework, a warm coloured close grained hardwood semi-matt polished in the natural shade, was specially chosen to tone with the chocolate bronze colouring of the attractive plastic lattice grill and the key mounting plate. Distinction is given to the set by touches of contrasting colour in the form of a polished black strip at the base of the grill, a decorative red “engaged” lamp in the centre of the grill, and wedge shaped mid-Brunswick green key handles. For reasons which will be explained, the key at the left-hand end has a red handle, in certain circumstances.

A black moulded handset with an extensible cord rests on a bronze-finished cradle at the side of the telephone.

Fig. 2
Showing Key and Lamp Strip Assembly Removed

An incoming call is signalled by a buzzer in the telephone, and its origin is visually indicated by the glowing of the appropriate lamp behind the designation strip below the keys. This strip comprises a curved transparent plastic window, behind which is a frosted plastic strip on which the names of persons or departments are printed in black characters slightly magnified by the curvature of the window. The construction of the lamp and designation strip assembly is such that when any particular lamp glows, the printed characters in front of it are clearly outlined in a dark-edged rectangle of diffused white light which a plastic shroud screens from any other printing on the strip.

A valuable feature of the new instrument is the ease of maintenance which its design affords. This is exemplified in Fig. 2, which shows the key and lamp strip assembly drawn forward from the case to allow access to the key springs, etc. The assembly is fastened in the case by four screws incorporated in the key mounting plate. Should it be necessary to replace a lamp, the strip can be drawn forward independently of the key mounting. This is done by releasing the two captive knurled nuts at the ends of the window strip, and removing, successively, the window strip, the designation strip and the plastic shroud (Fig. 3), the last of which has a separate compartment for each lamp. The lamp strip can then be pulled forward.

Fig. 3
Showing Lamp Strip with Plastic Shroud and Window Removed

Access to other internal components is gained by releasing two captive screws and removing the rear portion of the case (Fig. 4). This exposes the instrument terminal block, which is held in position by four screws. When these are released, the terminal block can be withdrawn to reveal the buzzer, loudspeaker, transformer, “engaged” lamp jack and the wooden cover which encloses the microphone. The latter is protected by rubber from contact with the edges of the microphone aperture and is held in position by a tensioned spring.

A multi-conductor flexible cord connects the telephone to a desk terminal block (Fig. 5), which has a case of polished wood to match the instrument and contains screw terminals for the connection of the line wires, battery wires and, when required, a separate external loudspeaker.

The Type N1732/N1733 intercommunication side station telephones used with this new loudspeaker master station telephone have facilities permitting their connection to one or two master stations. With one master station there may be a maximum of 14 side stations with two, 13 side stations. It follows that the master station must be capable of connection either to 14 side stations or to 13 side stations and one other master station ; in each case, 14 keys are used. The circuit wiring can be so arranged that the key on the extreme left in the loudspeaker telephone may be used for an ordinary or master station line. The necessary wires are connected to the instrument terminal block, the type of connection required being determined solely by the interconnection of the appropriate terminals and the fitting of a 6-volt bulb for a master station line or a 4-volt bulb for an ordinary line. This arrangement offers the advantages, firstly, that only one type of loudspeaker telephone need be stocked, since it can be easily modified to suit the alternative system; secondly, that telephones already installed in a single - master system can be adapted in situ for two-master working. Identical keys are used in all the fourteen positions, except that a red handle is fitted on the first key when this is used for a master station line.

Fig. 4
Rear View with Cover Off

The circuit arrangements are such that calls can be originated or answered with the minimum of manual operation.

To make a call, the appropriate key is pressed to the full extent to operate the buzzer in the distant telephone. When the key is released it returns to an intermediate “conversational” position. To release the connection it is necessary only to restore the key to the normal position ; if this is not done and the person at the other end of the line re-calls, the latter can overhear any conversation taking place at the master station. But the “engaged” lamp glows whilst any key remains operated and serves as a reminder to release the connection.

If privacy of reception is desired, the telephone handset may be used, the action of lifting it from the cradle operates a switch which cuts out the loudspeaker.

Should there be a call to the master station when the latter is already engaged, it will not be audibly signalled but the appropriate call lamp will glow to warn the master that another station is calling.

Fig. 5
The Desk Terminal Block for the Loudspeaker Telephone

All calls to and from a master station, whether master-to-master or master-to-side station calls, are secret. Side station-to-side station conversations are non-secret to the extent that a side station calling a party to an established conversation can overhear.

When there are two master stations, one calls the other by pressing the red key. In the ensuing conversation the handset must be used by one of the two which shall do so may be pre-determined by mutual arrangement; alternatively the loudspeaker facility at one station may be restricted, being made available for all except master-to-master calls. Such a restriction is imposed by the removal of a strap from the instrument terminal block.

When the buzzer in the loudspeaker telephone is actuated by an incoming call, it returns a buzzing tone when the master station is free and is receiving the signal. No tone is returned if the master station is engaged.

Simplified elements of the loudspeaker telephone circuit are shown in Fig. 6 to clarify its working under the various conditions. A full schematic diagram may be seen in Fig. 7.

In Fig. 6A, the call lamp in No. 2 telephone is lit by current from the local microphone battery via the primary winding of the loudspeaker transformer at No. 1. When the key at No. 1 is pressed to the full extent, its springs K26-27 operate the buzzer at No. 2, from the “ringing” battery.

In Fig. 6B, the primary of the transformer provides a loop connection for the microphone in the No. 2 handset, the receiver of which completes a battery circuit for the microphone at No. 1. Also in Fig. 6B may be seen the terminals M and N, which in master station No. 2 are disconnected to cut out the cabinet microphone, for the restricted loudspeaker working mentioned above.

Fig. 6
Simplified Elements of the Loudspeaking Telephone Circuit

When the appropriate button at a side station is pressed to call the master station (Fig. 6C), current from the ringing battery, which is common to all the side stations, operates the master station buzzer, whilst the side station telephone receiver completes a circuit to light the call lamp from the master station ringing battery.

When the call is from master station to side station (Fig. 6D), it is the loudspeaker transformer winding, or the receiver of the handset, if that is in use, that completes the circuit for the master station lamp on the side station telephone, the key springs K26-27 applying current from the ringing battery to the buzzer over lead HL.

Fig. 8
Type N1747A Telephone Equipped for a10-line System

Under the conditions shown in Fig. 6E, the master station microphone operates in series with the side station receiver via the master station speech battery, and the side station microphone operates via the transformer winding and the same battery.

Figs. 6F and 6G, showing side station-to-side station connections, while not strictly relevant to this article, are included as being of interest.

Loudspeaker master station telephones greatly extend the facilities offered by the intercommunication system, and their usefulness as time-saving devices for the busy executive is being more and more widely appreciated. The Type N. 1747A instrument is likely to prove popular because of its simplicity of operation, convenience for maintenance and pleasing appearance.

The stock instruments are equipped with either the full complement of fourteen keys or with nine keys and six key dummies as in Fig. 8; the respective telephone Type Nos. for the specified systems are as follows:-

Type No.  Equipped Keys Type of System
N1747A1 9 10-line, with 1 master and 9 side stations.
N1 747A2 9 10-line, with 2 master and 8 side stations.
N 1747A3 14 15-line, with 1 master and 14 side stations.
N 1747A4 14 15-line, with 2 master and 13 side stations.
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