C MARKETING INSTALLATION
Issue 2, Jan 1973
HANDSET No. 14A
Amplifying Telephone 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
It will also supersede Handset No. 4 when stocks of the latter are
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.
DESCRIPTION OF THE HANDSET
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.
USE OF HANDSET NO 14A WITH LOCAL POWER SOURCE
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
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
3. The voltage remains nearly constant at 1.3 volts per cell over the complete discharge
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.
PROCEDURE FOR NEW INSTALLATION
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