|Development of the combined Headset
C MARKETING INSTALLATION
Issue 2, Mar 1973
HEADSETS No. 1 AND No. 2
SCOPE OF INSTRUCTION
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
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 4oz, 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
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
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.
Internal view showing the receiver (right) and
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 as described in 3.3.
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
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 - shown to the right) 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
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
The receiver (Receiver-inset No. 3T - shown to the right) 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
PLUGS AND CORDS
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 inches or 72 inches. 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 inches is required, e.g. for subscribers' meter reading, one or
more Cords, Test, No. 4/108A, each being 109 inches long, should be connected in series with
the headset cord.
The receiver connecting cord (Cord, Inst, No. 2/133AB, 21inch) of the Headset No. 2 has
the conductor ends finished with bound loops. Two 3inch 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.
ADJUSTMENTS FOR USER
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 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)