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The Handset No. 4 superseded the Repeater, Telephonic No. 17A and Handset No. 2 .

Handset No. 4 was superseded by Handset No. 14.

ETL Part Numbers:-
Handset No. 4 - N9508D (Automatic, C.B. and Magneto).
Handset No. 5 - N9508E (Special LB situations).

Introduced in 1960.

3 Internal
Issue 2, April 1973

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 used.

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 Diagram SA4141. Loose leaf Diagram N1840 shows details of the connections of the handset for all telephone systems.

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:-

  1. It has a long shelf life of at least 1.5 years. In contrast, a Leclanché 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 Leclanché 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.

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.

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)

Close up of Volume Control

Handset No. 4

Handset No. 4 component pieces


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Last revised: September 23, 2021