|Line powered transistor amplifier, used as a waiting amplifier.
the Amp & LS No. 4.
Diagram SA 4173
ISSUE 3, DEC 66
AMPLIFIER AND LOUDSPEAKER No. 5 (SPEAKERSET 2)
Amplifier and Loudspeaker No.5 is intended to meet the demand for a small desk loudspeaker
for use with a telephone, giving reception of incoming signals only. It enables a
subscriber to relinquish the telephone handset after making or answering a call and still
maintain contact with the distant end. For sales purposes it is named Speaker-set 2.
2. Description and facilities
Amplifier and Loudspeaker No. 5 (black or grey) consists of a small self-contained unit in a black
plastic case approximately 4 in. x 4 in. x 4 in., together with a black 3-way cord. On the
upper face is a silver-grey recessed plate on which are mounted a black volume control and
an amber-coloured ON/OFF push-bar, which, in the ON position, is illuminated from within.
This colour scheme permits the unit to be associated with telephones of any type and
colour; further colours, e.g. ivory and two-tone grey, have been introduced to match
similarly coloured telephones.
Transfer of a call to loudspeaker reception is made by depressing the push-bar, which
connects the line to the unit. The handset is then either replaced on the telephone or
left on the desk. Restoration of the circuit is effected by picking up the handset and
again depressing the push-bar.
The unit is essentially for reception only and it is not possible for speech to be
transmitted during use. When the device is switched on, the telephone headset is
automatically disconnected, otherwise acoustic feedback from the loudspeaker to the
handset microphone would cause 'howling' unless the output of the loudspeaker were
drastically reduced. If both-way amplified transmission is required it will be necessary
to use a loudspeaking telephone (see F 1104).
An alternative method of operation is available in which the unit is interconnected with a
press-button handset (Handset No. 6). When the unit is switched on, the transmitter is
switched off but the receiver remains in circuit.
Pressing the handset button switches the unit off and restores the transmitter circuit.
Thus, if the button is used as a 'press-to-talk' control, a small group of persons nearby
may listen to remarks from the distant party over the loudspeaker. The unit is
automatically disconnected when the handset is replaced, even though the push-bar remains
depressed, and re-energised when the handset is picked up again. A further advantage is
that dialling is possible with the unit in operation. For normal handset use, the button
is locked down.
The unit is not recommended for providing broadcast reception in noisy or crowded
locations, for which an Amplifier and Loudspeaker No. 4 should be used.
In a quiet location an Amplifier and Loudspeaker No. 5 will perform satisfactorily as a
listening device for a small group of persons nearby.
The unit contains a transistor-type amplifier powered by line current. Although
principally designed for use on direct exchange lines on 50V systems (provided 30 ma
minimum line current is available) the unit will function also on P.B.X. extensions and on
the following subscribers' extension plans:-
Extension Plans 1 and 1A.
At main and extension
Extension Plans 5, 5A, 7, 7A, 105, 105A, 107, 107A.
At main, on exchange line only
Extension Plans 8, 8A, 9, 10, 12A.
At main and extension.
Provided the line current limits specified in par. 3 are maintained the unit may be
installed in non-standard circuits.
The complete circuit of the Amplifier and Loud-speaker No. 5 is given on Dgm. SA 4173.
Place the unit conveniently close to the telephone with which it is intended to work and
wire it in accordance with Dgm. N 4440 or, if the handset control facility is required,
Dgm. N 4441. Before connecting the unit measure the normal line current drawn by the
telephone and confirm that it is within the limits of 30-90 ma. If the measured current
exceeds 90ma (due, for instance, to the use of special transmission bridge arrangements)
wire a Resistor, Coil, No. 35A (preferably fitted in the terminal block) in series with
the green conductor of the instrument cord (connected to terminal AI of the unit), to
limit the current to this value.
Test the installation as follows:-
(a) Installations wired to Dgm. N 4440.
(i) Initiate a test call, preferably to a recorded announcement. If a suitable service is
not available locally, the assistance of. the test desk should be sought.
(ii) While listening to incoming speech on the handset, depress the push-bar on the unit.
Speech should then be heard from the loudspeaker and at the same time the lamp within the
push-bar should glow. The handset will be cut off. Rotate the volume control to give a
comfortable listening level. Note that the sensitivity of the unit can be adjusted for
wide variations in incoming signal level but it is necessary to guard against overloading
the amplifier. Overloading distorts the output and causes the brightness of the pilot lamp
(iii) Depress the push-bar again and note that incoming signals are restored to the
handset and that the pilot lamp is extinguished.
(iv) On shared service, with the unit in operation, depress the calling button on the
telephone. If this has an effect upon the loudness of the signal or the brightness of the
lamp, the installation is incorrectly wired and should be checked for line reversal
between the unit and the telephone.
(b) Installations wired to Dgm. N 4441
Proceed in accordance with (a) (i), (ii) and (iii) above, (iv) is not applicable, but note
that in (ii), when the push-bar is pressed, only the transmitter is cut off and not the
entire handset. Then press the handset button and note that this restores the transmitter
circuit and cuts off the amplifier.
To change the lamp, pull off the plastic push-bar cover and remove the lamp from its
bayonet-type holder. The correct replacement is Lamp No. 43A.
To change the cord, invert the unit and release the four small screws and washers from the
corners of the base. The base can then be removed with the internal assembly and the cord
detached. The correct replacement is Cord, Instrument, No. 3/108 AT, 72 in. (colour). The
following items may be changed if defective:-
Case - Part No. 1/DCA 112 (colour)
Base - Part No. 1/DBA 164 (colour)
Push-bar cover - Part No. 1/DBU 346 (colour)
Do not attempt to correct faults on the amplifier; change faulty units complete.
Circuit diagram - N4440.
Circuit diagram - N4441.