HANDSET No. 4

ETL Part Numbers:-
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).

TELECOMMUNICATIONS INSTRUCTION
C MARKETING INSTALLATION
3 Internal
B2130
Issue 2, April 1973

HANDSETS No's 4 AND 5
Amplifying Telephone Handsets

GENERAL
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 used.

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.
or
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.

BATTERY
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.

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. 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.

SUPERSESSION PROPOSALS
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, April 1960

Handset No. 4

A telephone handset containing a single-stage transistor amplifier has been developed to provide amplified reception for deaf  subscribers.


INTRODUCTION
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 replacement.

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 amplifier.

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:-

1. It is bulky and unsightly.

2. Frequent replacement of the dry batteries is necessary and this results in high maintenance charges.

3. A special telephone instrument must be provided in addition to the amplifier unit.

All of these difficulties have been overcome in the new design.


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 replacement.

The new instrument will be available in three colours - black, grey and ivory.

Transistor Amplifier
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.

Power-Feeding Arrangements
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 losses.

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.


CONCLUSION
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
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

Last revised: June 17, 2023