HANDSET No. 7
|Handset with neon light (Lamp No. 44A) for indication of incoming calls. Used with 700
Fitted with Cord Instrument No 6/50AK.
Colours: Black, Grey and Ivory.
Diagram N 1901
ETL Part - N9517D
HANDSET No. 7
SCOPE OF INSTRUCTION
(b) Lamp and Lamp Mounting
(c) Cord, Transmitter and Receiver
Handset No. 7 broken down into component parts
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
(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 Diagram 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 Diagram 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.
(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 Diagram 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
Formerly EI Telephones, Stations, A1075
A 700-Type Telephone Handset Providing Lamp Calling
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.
HANDSET No. 7
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.
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.
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: January 03, 2021