|Introduced in 1963.
Issue 2, July 1973
LOUDSPEAKING TELEPHONE No. 3
Instruction gives details of the installation of the Loudspeaking Telephone No. 3.
Reference should be made to B2000 for information on the acoustic problems associated with
Loudspeaking Telephone No. 3 (Mark II) comprises the items listed below:-
Control-unit No. 10A
Loudspeaker Unit No. 5A
Amplifier No. 138B
Case No. 111A
Plates, Mounting, No. 3A
Plates, Mounting, No. 3A are only required when the loudspeaking telephone is
The separate items are supplied complete with cords and plugs for interconnection.
Control Unit No. 10A
Loudspeaker No. 5A
In most installations the control unit and the loudspeaker unit will be wall-mounted and
in these circumstances it is recommended that the control unit (containing the microphone)
should be located about 4 ft (1200 mm) above floor level and the loudspeaker unit 3 ft
(900 mm) above the control unit. The dimension of 4 ft (1200 m) is largely determined by
the requirements that the control unit must be at a convenient height for operation of the
buttons, that the speaker's mouth should not be more than 18 in (460 mm) away from the
microphone and yet the unit must not be mounted too high for a small person.
The loudspeaker unit must be at least 3 ft (900 mm) away from the control unit to overcome
acoustic feedback (howl) but there is no need to locate it vertically above the control
unit if another position, e.g. to one side, is preferred.
Special conditions obtain in hospital operating theatres, where it is possible for
inflammable gases to be present in the atmosphere and precautions are observed in the use
of switchgear. The hospital authority should be consulted in advance regarding the
positions of the units. Since the loudspeaker unit contains buzzers that might be
considered a potential risk, buzzer calling should not be provided in an operating theatre
and an alternative, e.g. lamp signalling, should be adopted. To simplify the installation
in such circumstances, wherever possible the theatre instrument should be connected as a
direct extension from the PBX manual switchboard.
Relay-switch No. 3201 NL 1-15 should be substituted in the circuit for the a.c. buzzer and
Dgm N1291 used as a guide for the connections. Since this arrangement will signal only
while ringing current is applied, the operator should be instructed to ring
intermittently. If the theatre instrument is an extension from a main instrument, e.g.
Extension Plan 107, and d.c. calling is used, a Relay-switch No. 3106 NL 1-15 should be
substituted in the circuit for the d.c. buzzer and Dgm N 1874 used as a guide for the
A Plate, Mounting, No. 3A, consisting of an enamelled metal plate with keyhole cut-outs,
is required for mounting the control unit and the loudspeaker unit (one each). The plates
should be secured in the agreed position by the 1.t in x No. 6 screws provided. Each plate
is fitted the same way up as shown in Fig 2. With each plate are also provided three
studs which should be screwed home into the base of the associated unit to enable it to
engage in the keyhole slots of the mounting plate. The studs should be screwed into the
appropriate holes in each unit so that the front panel of the control unit faces obliquely
upwards and the front panel of the loudspeaker unit obliquely downwards.
As an alternative to wall-mounting, the units may stand on a desk. Packed with each unit
is a self-adhesive rubber pad, Pad No. 5. The protective layer should be peeled off and
the pad stuck to the base of the unit so that it may rest securely without any tendency to
slip on the surface of the desk. The two units should be spaced 3 ft (900 mm) to 5 ft
(1500 mm) apart on the desk. The labels on the control unit should be unscrewed and
rotated through 1800.
Using the screws provided, the two case mounting bars should be secured to the back of the
Case No. 111A and the Amplifier No. 138B secured within the case. The case should be
mounted in a convenient position away from direct sunlight or heating appliances, using
the four 1.5 in x No. 12 Chromium-plated round-head screws, bearing in mind that the
maximum connector length to each unit is 12 ft (3.65 m). The connectors should be passed
through the entry holes of the case and the sealing flanges screwed on firmly, after which
the connectors should be plugged into their relative plug or socket in the amplifier and
anchored by their grommets in the two outer slots provided.
If the full length of connector is not required, it will be found possible to wind up to
three loops of each cable around the amplifier lid as shown in Fig 3, the sealing flanges
and cable clips being repositioned to suit.
A 3 ft length of 7-way cord is provided with the case and this should be passed through
the centre entry hole and the flange sealed. This cord is provided to connect the external
wiring to the internal terminal blocks of the amplifier. The other end of the cord should
be terminated on a suitable terminal block.
Three terminal blocks, one 4-way, one 5-way and one 7-way, numbered 1to16, are provided in
the Amplifier No. 138B . The straps on these blocks are set initially for a direct
exchange line installation as shown in Dgm N 4472 (for Loudspeaking Telephone No. 1) and
these should be rearranged to suit the particular installation required as given on Dgm N
Before connecting the line to the equipment the line current should be measured with a
Resistor Coil No. 55A - 300 ohms in place of the equipment. If the equipment is installed
on a PBX extension line with through clearing facilities at the PBX, two measurements of
line current should be made, firstly with the extension line connected through the PBX
switchboard to another extension, and then connected through to the main exchange.
The current measured must not exceed 90 ma otherwise the amplifier may be damaged. If this
value is exceeded (due, for instance, to the use of special speaking bridge arrangements)
then two equal Resistors, Coil, No. 35A should be inserted in the A and B lines to limit
the current to this value. At line currents lower than 30 ma there will be progressive
deterioration in the loudspeaking performance.
The correct gain setting of the 'send' amplifier should be checked by ensuring that the
strap is between terminals 12 and 13 for exchange lines riot exceeding 500 ohms and all
internal extensions, and between 13 and 14 for exchange lines exceeding 500 ohms.
After completing the wiring in accordance with Diagram N 4474 screw down the amplifier cover;
make sure that the neoprene gasket used for sealing the lid of Case No. 111A is properly
in place and secure the lid firmly. The whole installation should then be impervious to
the effects of spraying and washing down.
If conditions permit, the amplifier may be installed outside the area subject to moisture,
e.g. in an adjoining room; the Case No. 111A may then be dispensed with.
(a) Operate the ON/OFF button to call the controlling station, e.g. PBX switchboard or
main station of an extension plan, and check that the lamp inside the button glows. Ensure
that the VOLUME key is in its normal position.
(b) The continuously variable volume control in the amplifier (engraved REC VOL) must be
adjusted to a setting just below the 'howl' point under conditions of calling the
controlling station. To adjust the volume control, first slacken the locking screw visible
through the aperture, rotate the slotted wings of the control and finally retighten the
(c) Make several test calls, both local and junction, and check that the transmission and
loudspeaking reception are satisfactory when the user is approximately 18 in (460 mm) from
the microphone. After a call has been established and the line is therefore approximately
correctly terminated, it should be possible to press the VOLUME button to improve the
volume of reception. This button must be pressed again to restore it to its normal
position at the end of the conversation, otherwise the circuit will howl due to an
incorrect line impedance being presented to the hybrid transformer under incorrect
impedance matching conditions.
(d) Check the operation of the button marked MAX VOLUME RELEASE TO SPEAK. Operation should
provide maximum loudness of received speech and at the same time render the microphone
inoperative in order to obviate the howl that would otherwise occur.
(e) Apply functional tests to the a.c. and d.c. buzzers or calling lamp as relevant to the
particular installation, and check that these operate reliably and efficiently.
(f) Apply functional tests to the fourth control button (when used) and check that the
intended function in the particular installation is correctly performed.
Formerly EI Telephones, Stations, F3004