HANDSET No. 17


TELECOM INSTRUCTION
C MARKETING INSTALLATION
3 Internal
B2142
Issue 1, Aug 1981

HANDSET No. 17A (Conquest)

GENERAL
The Handset No. 17A provides amplified reception for deaf customers, it also incorporates a neon calling lamp.

The neon calling lamp is 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.

The amplified reception utilises an Amplifier No. 214A which fits in the cavity behind the receiver, with the volume control protruding through a slot just below the earcap.

The amplifier is line powered and needs a minimum current of 26ma to function. It is designed to limit the output power fed to the receiver on over- load, and thus safeguards the customer's hearing against excessive clicks or other disturbances. When maximum gain is required, the receiver should be held firmly to the ear to prevent howling, which is otherwise liable to occur due to acoustic feedback. For the same reason, the volume control should be turned down if the handset is laid on the desk during a call.

DESCRIPTION
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. The volume control of the amplifier protrudes through a slot just below the earcap.

The handset embodies an Amplifier No. 214A. This employs a single silicon planar transistor, and provides a maximum gain in excess of 20 dB. The volume control, which is mounted on the amplifier printed circuit, is of the rim operated type, protrudes through a slot in the handset body. The complete amplifier assembly is located in the cavity behind the receiver and is held in position by means of a rubber pad into which the Receiver Inset 4T is seated.

Lamp and Lamp Mounting
The Lamp is a neon gas discharge tube with wire connections which are soldered to a printed wire 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.

Cord, Transmitter and Receiver
The Handset No. 17A is supplied complete with a Cord, Instrument No. 6Y59AX (colour), a Transmitter inset No. 16 and a Receiver - inset No. 4T.

ASSOCIATED APPARATUS
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, 746 etc.

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 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 cord 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, 746 etc any 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 same 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.

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 50 Hz 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. 17A 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.

1. 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 1843 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.

2. 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. 17A 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 neon lamps and extension bells from a local a.c. supply. When several Handsets No. 17A 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.

3. Exchange Lines or PBX Extensions, where ringing is applied manually
The Handset No. 17A and a Lamp Signalling Unit No. 2 should be connected as shown in Dgm N 1901. The circuit is so arranged that the relay period of ringing, is then switched away from the line to local a.c. supply which also energises the neon lamps via The relay remains operated until the telephone handset is left connected to the line and may still be rung from the operates to the first be held operated from a the relay contacts. lifted, but the bell is switchboard.

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

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. 17A only; they are not suitable for ringing extension bells. Where Lamp Signalling Units No. 1 are used, a maximum of 50 Handsets No. 17A 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'.

INSTALLATION
The Handset No. 17A should be installed in accordance with Diagram N 1843. Associated equipment should be installed utilising Diagram N 1901.

PROCEDURE FOR NEW INSTALLATION
Where a Handset No. 17A is required at a new installation, a 700 type telephone should be requisitioned and the standard handset and cord on the telephone recovered and returned to the Supplies Dept if in excess of local requirements. The Handset No. 14A and any associated components should then be fitted in lieu.

Introduced in Mid 1981 and available in grey only.

Breakdown of major components

 
 
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