|Amplified handset used with 700 type telephones.
The Handset No. 4 superseded the Repeater, Telephonic No. 17A
and Handset No. 2
Handset No. 4 was superseded by Handset No. 14.
Handset No. 4 - N9508D (Automatic, C.B. and Magneto).
Handset No. 5 - N9508E (Special LB situations).
Introduced in 1960.
Produced by Ericsson telephone Ltd (ETL).
C MARKETING INSTALLATION
Issue 2, April 1973
HANDSETS 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 30ma. Where the transmitter current is less than 30ma 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, Instrument 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 Diagram SA4141. Loose leaf Diagram 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 Diagram SA4158 and the connection of the handset and
battery for all telephone systems is shown in loose leaf Diagram N1841.
The following items should be requisitioned and assembled locally:-
Case No. 118A, Battery 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 No. 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:-
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 Nos. 4 and 5 will be superseded by Handset No. 14A
(C3 B2131 refers) for new work in due course.
(Formerly EI Telephones, Stations, F1102)
An extract from
THE POST OFFICE ELECTRICAL ENGINEERS' JOURNAL
Volume 53, Part 1 - Dated April 1960
Handset No. 4
A telephone handset containing a single-stage transistor
amplifier has been developed to provide amplified reception for deaf
Here are several thousands of telephone subscribers in this country who are
hard of hearing, and their need for amplified reception is catered for at
present by a valve amplifier (Repeater, Telephonic, No. 17). It is, however,
bulky and uses dry batteries, which necessarily require frequent
A new telephone, the Handset No. 4, has been developed which incorporates a
transistor amplifier drawing power from the telephone line current. All the
additional components required are contained within a standard
telephone-handset case. This new item will supersede the present valve
To illustrate more clearly the improvements provided by the new telephone, a
brief description is first given of the Repeater, Telephonic, No. 17.
REPEATER, TELEPHONIC No. 17
The Repeater, Telephonic No. 17 consists of a single-stage valve amplifier,
together with its l.t. and h.t. dry batteries, within a wooden cabinet 14in.
x 10in. x 5in. It is used in conjunction with a special desk telephone on
which is mounted a volume control and an on/off key. The maximum gain of the
amplifier is approximately 18db.
The valve amplifier has three main disadvantages:-
It is bulky and unsightly.
Frequent replacement of the dry batteries is
necessary and this results in high maintenance charges.
A special telephone instrument must be provided in
addition to the amplifier unit.
All of these difficulties have been overcome in the new
HANDSET NO. 4
The Handset No. 4 has been designed for use with the new standard Telephone
No. 706. No modifications are required to the telephone circuit and the
Handset No. 4 simply replaces the standard handset (Handset No. 3). The
external appearance of the new handset is identical to that of the standard
handset except for the volume control, which protrudes unobtrusively through
the body near the earpiece. In this position the control can be readily
adjusted by the subscriber and yet its rim cannot be damaged if the handset
is dropped on a flat surface. The control can be lifted out for maintenance
simply by unscrewing the handset ear-cap.
All the amplifier components, with the exception of the volume control, are
mounted on a strip of insulating material with printed wiring on the
underside. The strip is shown below. The amplifier is inserted into the
hollow handle of the handset via the cord entry hole and can be readily
withdrawn for maintenance. To permit this, the moulded block for the cord
entry hole is not cemented in position as in the Handset No. 3 but is held
in position by a removable spring clip within the transmitter cavity.
The handset cord is terminated on the amplifier by three
screw connexions, which are mounted on the tail of the amplifier strip.
These screws lie within the transmitter cavity when the amplifier is in
position inside the handset handle and are thus readily accessible for cord
The new instrument will be available in three colours - black, grey and
The circuit of the transistor amplifier consists of two parts: the
power-feeding section and the single-stage amplifier section.
The amplifier employs one transistor used in common-emitter configuration,
with the handset receiver as the collector load. Base bias is provided by
resistor R4. The incoming signal is fed to the base via a matching
transformer, series volume control, and coupling capacitor, C3.
The amplifier requires 3mA collector current with a 3-volt supply and,
assuming an x' of 50 for the transistor,* the maximum gain is approximately
25 db, whilst the maximum sound pressure developed by the receiver under
these conditions is about 200 dynes/cm squared. Severe limiting occurs if
the amplifier is driven beyond this point, giving protection to the user
against high-level transient signals.
Note - a' denotes the small-signal current gain of a
transistor used in common-emitter configuration.
With the volume control in the minimum position, the received signal is
approximately 6db below that of a standard telephone.
The amplifier performs satisfactorily over a temperature range of +10°C to
+50°C. When tested at 60°C the gain decreases considerably, but recovers
when the temperature is reduced.
D.C. power for the amplifier is drawn from the telephone fine, using a
non-linear resistor, MR1, in series with the telephone transmitter, and the
potential drop across MR1 provides a nominal 3volt supply for the amplifier.
The non-linear resistor provides automatic compensation for change of line
current with change of line length. For example, in a 50-volt exchange
network, the line current can vary from 30 to 100mA depending on the total
line resistance. The amplifier requires a minimum of 1-5 volts for
satisfactory operation and the components in the supply circuit introduce a
further potential drop of about 0*6 volt. Therefore 2T volts must be
available at the lowest line current of 30mA, which implies a value of 68
ohms if a fixed series resistor is used. With a line current of 100mA this
would give 6-8 volts, which is excessive for the amplifier. This wide
variation in voltage is reduced by using the non-linear resistor MR1 in
place of a fixed resistor. The forward current/voltage characteristic of MR1
is such that the amplifier supply voltage can be maintained between 2.8 and
3.8 volts for a line current range of 30-100mA.
TRANSISTOR AMPLIFIER CIRCUIT
MR1 consists of several small selenium rectifier plates
in series. To cater for line reversals two such units are used, connected as
shown above. MR1 is shunted by a 20uF capacitor to minimize transmission
The supply voltage to the amplifier is passed through a full-wave bridge
rectifier, MR2, to ensure correct voltage polarity at the amplifier
independent of line polarity. R3 and C2 form a filter network and decouple
the amplifier output from the line.
The Handset No. 4 has been designed as a hearing-aid handset for use on
50-volt exchange lines, but it can also be used on any common-battery
exchange where the line current is in the range 30 to 100mA. A second
version will be produced for use on installations where the line current is
less than 30mA.
The Handset No. 4 is less costly than the present valve amplifier and should
prove to be a robust and reliable item.
Acknowledgements are due to Ericsson Telephones Ltd., who were mainly
responsible for the design of the Handset No. 4.
|Close up of Volume Control
Handset No. 4
Handset No. 4 component pieces