|Amplified handset, used with 700 type telephones and superseded by Handset No. 14 .
C MARKETING INSTALLATION
Issue 2, April 1973
HANDSET No's 4 AND 5
Amplifying Telephone Handsets
Handsets Nos. 4 and
5 provide amplified reception for deaf subscribers. The items supersede Repeaters,
Telephonic, Nos. 9A, 17A, 17B and 17C and are designed as a direct replacement of the
normal handset on the 700 type telephone. Both items embody a transistor amplifier built
into the handset handle but differ in the power feeding arrangement for the amplifier. The
amplifier of the Handset No. 4 draws its operating current from the transmitter current
whilst the Handset No. 5 requires a separate battery supply.
The amplifier is designed to limit the output power fed to the receiver on overload and
safeguards the subscriber's hearing against excessive clicks and other disturbances. When
maximum gain is required, the receiver should be held firmly against the ear to prevent
howling which is liable to occur due to acoustic feedback. For the same reason the volume
control should also be turned down if the handset is laid on a desk.
The Handset No. 4 may be used on all systems, provided that the transmitter current is not
less than 30 ma. Where the transmitter current is less than 30 the Handset No. 5 should be
HANDSET No. 4
The amplifier embodies a single germanium junction transistor and provides a maximum gain
in excess of 20 dB. The gain may be adjusted by a rim operated volume control which is
mounted in the receiver body. It is easily manipulated by thumb or finger whilst the
handset is held to the ear.
All components of the amplifier are mounted on a strip of insulating board, the wiring
being printed on the reverse side. The amplifier assembly is fitted inside the handle of
the handset and locked in position by means of a clip placed in the base of the
transmitter casing. The handset No. 4 includes a cord 4/89 AK 10" or 4/119 AX 200mm
and is available in black, grey and ivory.
The circuit of Handset No. 4 is given in Dgm SA4141. Loose leaf Dgm N1840 shows details of
the connections of the handset for all telephone systems.
HANDSET No. 5
The external appearance of Handset No. 5 is the same as the Handset No. 4. It requires a
separate battery for the transistor amplifier and this is provided by two small dry cells
of the hearing aid type. They are carried in a metal container (Case No. 118A) which has
to be fitted inside the body of the 700-type telephone. The method of fixing the container
is by clipping it to the frame of the cradle switch assembly. The handset, complete with
cord, is available in black, grey and ivory.
The circuit of Handset No. 5 is given in Dgm SA4158 and the connection of the handset and
battery for all telephone systems is shown in loose leaf Dgm N1841.
The following items should be requisitioned and assembled locally:-
Case No 118ABattery container
Part 1/DSP/1233, Battery on/off spring-set for Telephone No. 706
Part l/DSP/1242, Battery on/off spring-set for Telephone No. 710
Switch 19B-1, Battery on/off switch for Telephones 740 and 746
The two dry cells which are described above should be purchased locally.
It is essential that the battery to be used with Handset No. 5 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.
- It is leak-proof and therefore will not exude chemicals and cause corrosion inside the
- The voltage remains nearly constant at 1.3 volts per cell over the complete discharge
- 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. Batteries should be purchased locally and are obtainable from
chemists and radio shops.
FITTING THE BATTERY
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 horizontal 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. Replace the container in the telephone.
PROCEDURE FOR NEW INSTALLATION
Where a Handset No. 4 or No. 5 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. 4 or
Handset No. 5 with its associated components should then be fitted in lieu.
Both Handsets No's 4 and 5 will be superseded by Handset No .14A
(C3 B2131 refers) for new work in due course. (Formerly EI Telephones, Stations, F1102)
Diagram SA 4158
|Close up of Volume Control