HANDSET No. 7

ETL Part - N9517D

TELECOMMUNICATIONS INSTRUCTION
C MARKETING INSTALLATION
3 Internal
B2141
Issue 2, Oct 1973

HANDSET No. 7
Description and Installation

SCOPE OF INSTRUCTION
This Instruction describes the 700-type telephone handset with a neon calling lamp known as the Handset No. 7 (colour), the associated miniature relay-units (Lamp Signalling-units Nos. 1 and 2) and plug-in transformers (Transformers No. 431A and 431B).
 

GENERAL
Where a number of telephones are installed in one room subscribers may find difficulty in determining which telephone is ringing. The Handset No. 7 provides a visual means of identifying the calling circuit by the neon lamp which is situated in the hollow handle behind a transparent section. The circuit arrangements of typical installations are shown in Dgm N 1901.

DESCRIPTION
(a) Handset Body
The body, is made with a transparent window moulded in the middle of the back of the handle, so that the external shape of the window follows the general contour of the handset.

(b) Lamp and Lamp Mounting
The lamp is a neon gas discharge tube with wire connections which are soldered to a printed wiring board. The lamp is secured to the board by a wire clip at the end remote from the connections. The printed wiring board is contained within the hollow handle and, when in place, the lamp is positioned symmetrically beneath the window. The board is held in place by a retaining clip, and also by a small SRBP strip, which slots over the screw terminals. The part of the printed wiring board visible through the window has a silvered finish.

(c) Cord, Transmitter and Receiver
The Handset No. 7 is supplied complete with a Cord, Instrument, No. 6/50AK (colour), a Transmitter-inset No. 16 and a Receiver-inset No. 4T. Early issues were supplied with a Transmitter-inset No. 13C in place of the No. 16.

Handset No. 7 broken down into component parts

ASSOCIATED APPARATUS
(a) Lamp Signalling-Units Nos. 1 and 2.  These are miniature relay-units which clip in between the uprights of the gravity-switch bracket of a Telephone No. 706, 710, 711, 740, 741, or 746.

The Lamp Signalling-unit No. 1 relay is connected in series with the bell and operates in unison with the incoming ring. The Lamp Signalling-unit No. 2 relay is connected initially in parallel with the bell and operates to the incoming ring. The relay is then disconnected from the bell circuit by its own contacts and is held operated by a local power supply until the telephone handset is lifted. In each case the relay is connected to a printed wiring board on which other circuit components are mounted.

A connection strip is provided for terminating extra card conductors and for mounting voltage dropping resistors when these are required. When more terminals are needed for extension plans with Telephone No. 706, 710, 740 or 746, an additional connection strip, Part 2/DST/836, may be fitted on top of the strip provided but mounted the other way round. When an additional connection strip is needed in a Telephone No. 711 or 741 it should be bolted directly on either of the gravity-switch brackets.

When fitted on some 74x type telephones where three capacitors 0.9pf are provided in the telephone circuit, it may be necessary to ease one capacitor slightly to one side to facilitate the fitting of a lamp Signalling-unit.

(b) Transformers Nos. 431A and 431B Each of these items consists of a transformer and mains plug combined into one unit which is designed for use with a 250v a.c. mains socket-outlet. The Transformer No. 431A has a 3-pin 15-amp mains plug with round pins; the Transformer No. 431B has a 3 pin 13-amp plug with flat pins. The output of the transformers is 75v 50Hz at a maximum current rating of 0.5 amp, and a lead is supplied already wired to the output terminals for connection to an adjacent terminal block. Both primary and secondary circuits are protected by Fuses No. 48/0.5 which are located inside the transformer covers. Access to the fuses is gained by removing the four screws securing the covers, and carefully removing the covers. The secondary circuit fuse is inside the back cover, while the primary circuit fuse is attached to the board on which the mains pins are mounted. The transformers do not contain spare fuses.

TYPICAL CIRCUIT ARRANGEMENTS
The Handset No. 7 is fitted in place of the Handset No. 3 on 700 type telephones, and connected in circuit as described in the following paragraphs. It should be noted that neon lamps cannot be operated in series with one another and that they do not operate satisfactorily when connected directly in parallel. Mis-operation of either of the relays to a surge on the line may be prevented by connecting a Thermistor No. 1A-1 in series.

(a) Exchange lines or PBX extensions where automatic ringing is derived from a ringing machine where there are no more than two bells and one lamp in series the lamp may be lit directly from the ringing supply, i.e. no local a.c. supply is required. As shown in Dgm N 1901 a resistor is connected in parallel with the lamp to extend its life by limiting the current flow and to restore the line capacitance condition for testing purposes.

(b) Exchange lines or PBX extensions where automatic ringing is derived from a vibrator source, or from a ringing machine where more than two bells and one lamp are required the Handset No. 7 and a Lamp Signalling-unit No. 1 should be connected as shown in Dgm N 1901. The relay operates in unison with the incoming ring to energise the rigor, lamps and extension bells from a local a.c. supply. When several Handsets No. 7 are required to be connected to a PBX with vibrator ringing, the GM (Eng. Divn) should consider whether it is more economical to replace the ringing vibrator by a ringing machine rather than provide individual lamp signalling units and transformers.

Lamp Signalling-unit No. 1	Lamp Signalling-unit No. 2

(c) Exchange lines or PBX extensions, where ringing is applied manually the Handset No. 7 and a Lamp Signalling-unit No. 2 should be connected as shown in Dgm N 1901. The circuit is so arranged that the relay operates to the first period of ringing, is then switched away from the line to be held operated from a local a.c. supply which also energises the neon lamps via the relay contacts. The relay remains operated until the telephone handset is lifted, but the bell is left connected to the line and may still be rung from the switchboard.

(d) Non-Standard Applications Where non-standard facilities are required particulars should be passed to GM (Sales Divn) for submission to THQ.

LOCAL RINGING OR AC SUPPLIES
An existing suitable local ringing supply e.g. a 75v ringing machine or Converter, Ringing, No. 4 or No. 5, should be used wherever Possible. Transformers No. 431A or B may be fitted to give a power supply for the neon lamps in the Handsets No. 7 only; they are not suitable for ringing extension bells. Where Lamp Signalling-units No. 1 are used, a maximum of 5 Handsets No. 7 can be supplied from one transformer, but it may be more economical to provide more transformers to avoid excessive cabling. The number of Lamp Signalling-units No. 2 being supplied from any one transformer should be limited to ten. Where it is proposed to use mains energised ringing converters or transformers, the subscriber should be given a copy of form A188 at the earliest opportunity. This explains that he should provide a BS 3-pin socket-outlet installed in accordance with the IEE 'Regulations for the Electrical Equipment of Buildings'.

Formerly EI Telephones, Stations, A1075

A 700-Type Telephone Handset Providing Lamp Calling Signals
Handset No. 7
by C. W. HOGBEN, B.Sc. (taken from POEEJ April 1959)

The Handset No. 7 incorporates a neon lamp visible through a clear window forming part of the handle. The lighting of the lamp provides a calling signal in addition to the sound of the telephone bell.

INTRODUCTION
HERE a number of telephones are used close together, the provision of a calling-signal lamp IN on each telephone enables an incoming call to be quickly associated with a particular instrument. If automatic ringing is applied, the light flashes in unison with the bell; with manual working the lamp is lit when ringing current is first applied and it remains alight until the handset is lifted. On 200-type and 300-type telephones the facility has been provided by a filament lamp mounted on a bracket attached to the telephone case. The lamp is lit from a 24v local power supply connected in circuit by a remotely mounted 3000-type relay.

HANDSET No. 7
Telephones No. 710 and No. 711, but not Telephone No. 706, have provision for the mounting of one or two lamps within the telephone case. These are used to indicate line-engaged conditions on extension plan arrangements, but are not suitable for calling signals as they can only be seen satisfactorily from the front. The handset of the 700-type telephones was therefore chosen to mount the calling lamp, because the handset is always prominently placed on the telephone. The hollow construction of the 700-type handset, utilised in the Handset No. 4 to contain amplifier components, provides a suitable housing for such a lamp. A clear Perspex window inserted in the back of the handle gives the lamp a wide range of visibility, and avoids the need for any brackets that would mar the appearance of the telephone.

The use of a filament lamp was rejected because of the possibility of damage by heat to the thermoplastic moulding and because of the fragility of the filament: a neon lamp is a gas-discharge tube and has neither of these limitations. Furthermore, neon lamps have a long life at a low current-rating and can, in many instances, be lit directly by the ringing current because they operate satisfactorily to a 60v or 75v a.c. supply.

The handle of the Handset No. 7 was originally made by moulding the body and window separately and then cementing the two together. This process proved unsuitable for mass production, and a further examination of the problem resulted in the double moulding technique now employed. By this method the body is made in coloured ABS (acrylonitrile-butadiene-styrene) with an aperture left for the window. The window is afterwards moulded in place over projections in the body moulding which ensure that the window is kept in position even when the handset is deliberately distorted.

The neon lamp is mounted on a printed-wiring board and fitted in the hollow handle so that the lamp is under the window, the printed-wiring board being inserted in the handle through the cord-entry hole. The moulded block that anchors the cord is normally cemented in place, but in the Handset No. 7 it is removable and is held in position by a spring clip contained in the transmitter cavity. This clip, in conjunction with the small synthetic-resin-bonded paper (s.r.b.p.) strip fitting over the terminals, also provides a location for the printed wiring board. Because an extra pair of conductors is always needed for the lamp connections a six-way extensible cord is supplied with the handset.

CIRCUIT ARRANGEMENTS
In the ideal arrangement the lamp is powered from the ringing current on the line and flashes until the call is either answered or abandoned. Such an arrangement can be achieved if automatic ringing is derived from a 75-volt ringing generator and the lamp is connected directly in series with the bell. A shunt resistor is used to prolong the life of the lamp by restricting the current flowing through it and also to retain the normal d.c. conditions for line testing, i.e. by providing an alternative connection to the telephone capacitor. The high value of the shunt-resistor ensures that a large proportion of the available ringing voltage is developed across the lamp.

In U.A.X. or P.A.B.X. systems in which vibrator ringers are employed, the voltage available may be insufficient to strike the lamp in a Handset No. 7. For such systems a relay wired in a full-wave bridge-rectifier network is connected in series with the bell. The relay operates to the ringing current and the lamp lights to a local power supply connected via the relay contacts. Because neon lamps do not operate satisfactorily in series or directly in parallel, this arrangement is also used where more than one telephone with a Handset No. 7 is connected to a single automatic exchange line or P.A.B.X. extension.

On manual systems the lamp lights when ringing current is first received, and it stays alight until the call is answered. This is effected by a relay connected across the bell; when ringing current is received the relay operates and is disconnected from the telephone circuit by its own contacts. Until the handset is lifted the relay is held operated to the local power supply, which also lights the lamp. In a similar way, the lamp-signalling units can be used to ring bells, if required.

AUXILIARY APPARATUS
To simplify the mounting and the wiring of the relay assemblies, miniature units have been designed to fit inside the telephone. These units are clipped between the gravity-switch supporting brackets at the rear of the regulator, and are known as Lamp Signalling Units No. 1 and 2. They include a Post Office Type 16 relay, a printed-wiring board with components, and a terminal strip. The terminal strip is secured by long fixing screws that can take an extra strip (Part 2/DST/836) if additional terminals are needed.

The local power supply used with the lamp signalling units may be provided by one of the following: a 75v 162 c/s or 25 c/s ringing generator; 60v 25 c/s ringing converter; a 75v 50 c/s plug-in a.c. mains transformer. The latter consists of a transformer combined with a standard size mains plug and is available in two versions, known as Transformers No. 431A and 431B. It should be noted, however, that 50 c/s current is not suitable for ringing extension bells.

The Transformer No. 431A has a 15amp plug with round pins, and the Transformer No. 431B has a 13-amp plug with flat pins; the large size of the plugs gives stability to the units when they are inserted in the appropriate mains outlet sockets. Both primary and secondary windings are protected by fuses and are separated by an earthed screen; the laminations are also earthed. The transformers are supplied with a lead-out pair already connected to the low voltage output terminals to avoid having to dismantle the unit during installation.

Last revised: December 10, 2019