3 Internal
Issue 2, Jan 1973

Amplifying Telephone Handset

Handset No. 14A provides amplified reception for deaf subscribers and is designed to be a direct replacement for the Handset No. 3 on 700 type telephones. As soon as the field trial is completed, Handset No. 14A will supersede Handset No. 5.

It will also supersede Handset No. 4 when stocks of the latter are exhausted.

The Handset No. 14A incorporates 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 normally powered by the line current, and in these circumstances no local power source is needed. In cases where the line current is insufficient to operate the amplifier, the circuit may be re-arranged as shown in Diagram N1842, and two mercury cells used to provide the necessary power. The Handset No. 14A should be used as issued, that is, powered from the line, provided that the transmitter current with the amplified handset in circuit is not less than 26 ma.

The amplifier is designed to limit the output power fed to the receiver on overload, and thus safeguards the subscriber'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.

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, and protrudes through a slot in the handset body. The complete amplifier assembly is located in the cavity behind the receiver and held in position by means of a rubber pad into which the Receiver Inset 4T is seated.

A terminal plate is provided in the transmitter cavity, and serves to permit circuit re-arrangements, as described earlier. The standard 4 way handset cord (Cord Inst No. 4/88AK or 4/118A x 200 mm) is provided.

Handset No 14A, complete with cord is available in black, grey and ivory.

The circuit of the handset, including the Amplifier No 214A, is given in Diagram SA 4267. Loose leaf Diagram N 1842 shows details of the connections for all telephone systems.

When the transmitter current is insufficient to permit the amplifier to be powered from the line, two mercury cells must be provided. A Case No. 118A, and the appropriate auxiliary gravity switch should be requisitioned separately. The cells, which are obtainable at chemists and radio shops, should be purchased locally.

It is essential that the battery is the Type ZM9 made by Mallory Batteries Ltd. This is a mercuric oxide/potassium hydroxide/zinc cell and is suitable, because of the following properties:-

1. It has a long shelf life of at least 1.5 years. In contrast, a Leclanche type battery of similar size may have a shelf life of only 3 to 6 months.

2. It is leak-proof and therefore will not exude chemicals and cause corrosion inside the telephone instrument.

3. The voltage remains nearly constant at 1.3 volts per cell over the complete discharge period.

4. It is extremely stable over a wide range of temperature and humidity conditions.

It is important to note that the mercury-type cell differs from the Leclanche type in that the negative terminal is the central pole and care should therefore be taken to ensure that the cell is correctly inserted in the Case No. 118A, otherwise the transistor amplifier will be damaged. The polarity of the terminals on the Case No. 118A and on the cells are clearly marked.

Difficulty will be experienced if an attempt is made to insert the battery whilst the container is in the vertical position on the telephone cradle switch frame. The container should therefore be removed from the telephone and placed horizontally on a desk and one cell inserted in each half of the container.
Check that the cell polarities are correct then slide together the two halves of the container, ensuring that the locking spring is outside the container. The assembly should then be replaced in the telephone.

where a Handset No. 14A 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.

Diagram SA 4267

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Last revised: December 18, 2010