Development of the combined Headset

3 Internal
Issue 2, Mar 1973


This Instruction describes the Headsets No. 1 and No. 2.. The Headset No. 1 is a one piece lightweight headset with small transmitter and receiver insets. The Headset No. 2 is a double receiver version of the Headset No. 1. Both headsets are available in black or grey.

The transmitter and single receiver of the Headset No. 1 are accommodated within a moulded case worn upon the head, in contrast to the previous PO headsets which have been of the 'head-and-breast set' variety. Sound from the mouth is conveyed to the transmitter by means of a horn and an acoustic channel. The weight of the headset without cord and plug is approximately 4 oz, achieved by the use of small transmitter and receiver, light alloys for the metal parts and thin sections for the moulded parts. Choice of nylon as the moulding material results in the headset being very robust. The headset is primarily for the use of switchboard operators and for identification purposes may be labelled by means of Labels No. 458.

This consists of the body moulding including the acoustic system, and the cover with the headband.

This is a nylon moulding, part of the exterior forming the earpiece and the interior providing seatings for the transmitter and receiver insets. A light alloy ring and plate are fastened within it to provide the necessary holes for fixing the transmitter, receiver and cover.

Acoustic system
This consists of a passage continuous from the transmitter to the horn (Part 1/DMO/66) via a transmitter-coupler (Part 1/DCO/118), a coupling-hole to the body, an elbow and a ball-joint with a stem. The acoustic system is held together by a spiral spring which runs inside the acoustic passage. The spring is in tension between a diecast detail at the end of the passage in the body and an anchoring washer at the end of the stem.

The stem on to which the horn pushes is surrounded by a soft plastic sleeve which in its uncompressed state is slightly longer than the stem. The tension of the spiral spring, exerted through the anchoring washer, causes the sleeve to take on a barrel shape and to open out into the horn to make a consistently tight fit and a good acoustic seal. It is important that there should not be a leak in the acoustic system and to prevent this a soft plastic washer is fitted under the head of the diecast detail, and the ball of the ball-joint turns in a conical soft plastic seating in the elbow. Special care is taken in manufacture to ensure that the mating surfaces of the elbow and body are flat and smooth. The elbow and body joint includes a V-cam arrangement giving two possible positions for the elbow corresponding a right or left-ear use of the headset. Movement about these two positions should be slight, merely enough to show that the flat surfaces are properly in contact; movement of the horn to suit the face of each individual user comes from the ball-joint. The elbow may be turned either way relative to the body any number of times; the diecast detail at the body end of the spring rotates to prevent turns being stored in the spring. The spring may be permanently damaged if the elbow is bent sharply backwards and a spur has been added to the elbow to reduce the risk of accidental damage to the spring in this manner. Dismantling and assembly of the acoustic system requires the use of special tools and must not be attempted.

This is a thin nylon moulding and is fastened to the body by two screws, captive within the cover, which screw into tapped holes in the light alloy ring in the body. A moulded stud in the body engages with an elongation of the cord-entry hole to provide additional fixing. On the inside of the cover a half socket is formed for holding a ball on the headband wire; the other half of the socket is formed by a small metal plate which is held by two self-tapping screws screwed into pillars in the moulding.

The headband is made up of the headband wire (Part 1/DHE/6), top pad (Part 1/DPA/6), side pad (Part 1/DPA/5) and end cap (Part 1/DCA/137). The headband wire is made of PVC covered stainless-steel with a phosphor-bronze ball attached to one end which is clamped in the socket.

The Headset No. 2 consists of the Headset No. 1 with a different headband, Headband No. 13, and an additional receiver and case (Part 1/DCA/99); a receiver connecting cord is also provided.

Headband No. 13 This is made up of a stainless-steel wire covered with PVC sleeving, two spring-loaded length adjusters, a central headband pad and two cord locators. The headband pad and cord locators are grooved to accommodate the receiver connecting cord. Phosphor-bronze balls are affixed to the extreme ends of the headband wire to act as pivot points for the receiver cases.

Additional receiver case (Part 1/DCA/99)
This is made of nylon and has a ball-socket formed in the rear section of the case in a similar manner to that described above. The rim of the Receiver-inset No. 3T seats on three moulded pillars in the rear section and a soft plastic ring, seated on a boss in the front cover, forms an acoustic seal when the complete case is assembled.

The receiver connecting cord enters the case via the pivot point hole.

The transmitter (Transmitter-inset No. 15) used in the Headsets No. 1 and No. 2 is a carbon granule unit designed for acoustic horn loading. The exponential horn of the headsets and transmitter are complementary, and together give a reasonably flat response over the most important part of the speech frequency range.

The transmitter, includes a soft plastic coupler (Part 1/DCO/118) which seals the sound inlet in the centre of the front plate to the acoustic channel in the headset body. The transmitter is held in position in the headset by a Y-shaped spring which is fastened by a screw, held captive in a loop in the spring and screwed into a metal plate in the headset body.

The life of the transmitter is increased when its feed current is restricted to 40 ma and, where the maximum sending efficiency is not required, e.g. in main exchanges, the current should be reduced to this value.

When the transmitter is used on low-voltage (e.g. 3 or 4.5V) low-resistance supplies it may suffer large fluctuations of current and, therefore, of sensitivity. To stabilise the current to some degree the resistance of the supply should be increased to at least 15 ohms.

The receiver (Receiver-inset No. 3T) used in the headsets is a rocking armature type of unit and the earpiece formed in the headset body moulding, forms part of its acoustic equalising system. The receiver has an impedance of 150 ohms at 1000 Hz (d.c. resistance 20 ohms). For special applications, receivers of other impedance's, denoted by different suffix letters, may be fitted locally.

The receiver seats in the headset body upon a soft plastic sealing ring (Part 1/DRI/30) and is held in contact with it by an insulated wire spring with two screws, captive in the spring looped ends, which are screwed into tapped holes in the body alloy ring.

When used continuously by operators on public exchange switchboards, the receiver may be too loud for comfort. Under these circumstances the sensitivity should be reduced by shunting the receiver with a Resistor, Carbon, No. 4, 150 ohms. When used at PBXs this shunt is not required since an 180 ohm series resistor is provided in the operator's circuit.

Headsets Nos. 1 and 2 are stocked without cords, which must be requisitioned separately. The normal cord is stocked complete with plug under the title Plug No. 420 with Cord No. 4/77AB, 54 in or 72 in. For maintenance purposes plugs and cords are stocked separately.

The headset end of the cord is fitted with a curved plastic grommet. This has a squared section which engages with ribs on the inside of the cover to prevent twisting. The cord-entry hole in the cover is split so that by bending the moulding, the cord may be fitted without threading it through the hole. The grommet must be fitted so that the edge of the hole seats in the groove provided in the grommet. The cord conductors connect directly to the terminals on the transmitter and receiver, blue and white to the transmitter, red and green to the receiver. The cord ends may be either spade tags or wire bound loops. Where they are bound loops, the blue conductor, which has a larger loop than the others, must be connected to the nut terminal on the transmitter.

When a cord longer than 72 in is required, e.g. for subscribers' meter reading, one or more Cords, Test, No. 4/108A, each being 109 in long, should be connected in series with the headset cord.

The receiver connecting cord (Cord, Instrument No. 2/133AB, 21in) of the Headset No. 2 has the conductor ends finished with bound loops. Two 3in 6BA screws and nuts are supplied with the cord; these are for use when series or separately connected receivers are required, the necessary tags being bolted together and the joints insulated with tape or sleeving. Generally, the two receiver insets should be connected in parallel.

The scope of adjustments is limited to that necessary to accommodate the natural range of head variations, thus preventing the user from degrading the transmission by excessive movements of the horn s stem. The Headset No. 1 has provision made for right or left ear use by a 180 degree swing of the elbow. The headband wire is shaped to suit the average user and the two headpads may be adjusted, by sliding, to the best position for comfort and stability to suit each individual user. The headband wire may be bent to suit, if it is not found possible to meet the requirements of a particular user by movements of the headpads. The Headset No. 2 can be adjusted to position the horn at either side of the mouth by means of the 180 degree swing of the elbow. Suitable positioning of the earpieces can be obtained by adjusting the variable length of the headband. The correct position for the horn is at the corner of the mouth. Two adjustments are provided to enable this position to be achieved, and it may be necessary to use a combination of these adjustments to obtain it. The adjustments are at the headband ball and socket joint and at the horn acoustic system ball and socket joint. The cone of movement at the latter should also be used to prevent the horn touching the face or mouth.

The following replacement parts are available:-

Cord, Instrument, No. 4/77AB, Grebe, 54 in or 72 in
Plug No. 420
Plug No. 420 with Cord No. 4/77AB, 54 in or 72 in Receiver-inset No. 3T
Resistor, Carbon, No. 4, 150 ohms
Transmitter-inset No. 15
Part 1/DMO/66, Colour (mouthpiece)
Part 1/DRI/30 (receiver-inset sealing ring)
Part 1/DCO/118 (transmitter-inset coupler)
Part 1/DHE/6, Colour (headband wire)
Part 1/DPA/5, Colour (side headpad)

Headset No. 1
Part 1/DPA/6, Colour (top headpad)
Part 1/DCA/137, Colour (headband cap)
Headband No. 13

(Formerly EI Telephones, Stations A1069)

Made originally by Standard Telephones and cables Ltd and later by A.P. Besson.


BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: June 21, 2023