|LST No. 4A - Introduced in 1966
LST No. 4D - Introduced in 1967
LST No. 4E - Introduced in 1976
LST SA4255 - Introduced in 1973
LST SA4256 - Introduced in 1970
LST No. 4D to work on the UK Plug and Socket system
C MARKETING INSTALLATION
Issue 3, July 1976
LOUDSPEAKING TELEPHONES 4A-1, 4C-1, 4D and 4E
Description and Installation
This instruction should be used in conjunction with Diagram N4475 when installing Loudspeaking
Telephones No. 4... (LST4). For general information on loudspeaking telephones see B2000.
LST4.... is a voice-switched amplified loudspeaking telephone with microphone and
loudspeaker contained in the same desk unit as the handset. As it is powered from a
separate 50v d.c. power supply it is suitable for use on long exchange lines etc where
LST1 may not be satisfactory.
It is essentially a 'prestige' type instrument and as such it is expected that it will be
installed in quiet and non-reverberant surroundings. If these conditions are not satisfied
then the LST4 may not function properly.
As the loudspeaker and microphone are close, to each other there is a certain amount of
acoustic coupling between them, i.e. a certain amount of the loudspeaker output is
reflected off the surrounding walls and furniture etc into the microphone. If this
acoustic coupling becomes excessive the loudspeaking telephone may mis-switch and
conversation will be impossible. If this trouble occurs it can sometimes be alleviated by
reducing the setting of the loudspeaker preset control situated on the amplifier (switch
on LST4A straps on LST4D & 4E).
If there is a high level of noise in the room the loudspeaking telephone will switch to
'send' very readily. This means that any slight murmur from the customer might cause the
incoming signal to be attenuated. It is therefore very important to keep the level of
noise in the room as low as possible.
In order for the loudspeaking telephone to discriminate between the users speech and room
noise it is essential that the speech level should be as high above noise level as
possible. The customer should therefore always speak directly into the microphone and at a
distance of not more than 500 mm from it. The receive volume should be set to give the
lowest comfortable listening level.
VARIANTS OF THE LOUDSPEAKING TELEPHONE No. 4
There are three basic versions of the Loudspeaking Telephone No. 4
1. Loudspeaking Telephone No. 4A-1
This is the original form of the Loudspeaking Telephone No. 4, it has a rotary dial and a
handset mounted on the back panel. There are four mark numbers of this type:-
1.1. Mark 1 - This can be identified by its five-way line cord and spring-set type of
1.2. Mark 2 - This is an updated version of the Mark 1 with a micro-switch type of gravity
1.3. Mark 2A - This is a Mark 1 loudspeaking telephone that has been updated to a Mark 2
version by the PO Factories Div. It can be identified by two dome headed studs on the
1.4. Mark 3 - This is a Mark 1A but without the dome headed studs and with increased tone
Marks 1A, 2 and 3 versions have a spare microswitch fitted above the minimum volume
button. All recently repaired items are updated to Mark 3.
2. Loudspeaking Telephone No 4C-1
This is a non-standard version of the Loudspeaking Telephone No 4A-1 for use on PABXs with
d.c. Code A push-button signalling.
3. Loudspeaking Telephone No. 4D (pictured above)
This is similar to the Loudspeaking Telephone No. 4A-1 Mark 2 but with two
additional facilities. There are Marks Nos. 1, 2 and 3 but the differences are minor. Mark
1 does not have the spare springset fitted to the on/off button as standard.
3.1. A variable tone-ringer is fitted on the back panel. The control has three positions:
loud, >, soft. The position marked > is a crescendo position.
3.2. It has an extension loudspeaker socket and an extension microphone socket both fitted
on the back panel. The microphone socket is behind the microphone and the loudspeaker
socket behind the loudspeaker.
4. Loudspeaking Telephone No. 4E (right)
This is an enhanced version of the LST4D.
It differs from the LST4D in that a Push Button unit replaces the dial and it has a wooden
trim finish. The push button electronic circuits are powered by rechargeable
nickel-cadmium batteries, Batteries Secondary No. 22. The charging current is derived from
the power unit used with the LST, N diagram N4477 refers. This model is loop dial only.
FIELD OF USE
The LST4.... may be used in any extension plan arrangement where a Telephone 706, 710, 746
or 740 could have been fitted with the following exceptions:-
The main of extension plan 105, 105A.
The main of extension plan 107, 107A.
It should be noted that a LST4.... must not have terminals T5 and T6 strapped. It also
requires two extra line cord conductors for the power supply, and therefore the line cord
may not be the same as shown on the N diagrams for the respective extension plans. The
wiring of the most common extension plans is shown in Diagram N4475.
For direct lines, PBX extensions and certain extension plans it is not necessary to open
the instrument, all connections being made on the block terminal. For other installations
it may be necessary to open the instrument; instructions for removing the lid can be found
on the base of the instrument. In order to fit certain accessories it may be necessary to
remove the centre partition. Instructions can be found on the inside of the lid.
A loudspeaker extension unit (Loudspeaker Unit No. 4B-1) and/or a microphone extension
unit (Microphone Unit 1B-1) may be provided on the LST4D only. The microphone unit is
provided with a microphone cut-off button which cuts off both the extension microphone and
the main microphone. Care should be taken in the siting of these units in order to avoid
excessive acoustic coupling between them.
Loudspeaker Unit No. 4A (right) and Microphone Unit No. 1A (Left)
Check the telephone part of the LST as for a Telephone 710 or 740. To check the
loud speaking part of the LST make several test calls, if possible to an officer
experienced in testing loud speaking telephones, to check that the instrument performs
satisfactorily on both transmission and reception. Calls should be made to check the
instrument on volume control positions one, two, and three. To check transmission
performance, speak clearly not more than 500 mm from the microphone and, after speaking,
wait for the distant party to reply. Apart from normal conversation, it may be useful to
ask the distant end to repeat groups of random numbers back over the line and check that
they have been received correctly.
When the volume control is set to positions one and two the voice-switch switches to
'send' on the user's first syllable. It remains in this condition and will not switch back
to 'receive' until the distant party utters his first syllable. On position three of the
volume control, the voice switch still switches to 'send' on the first syllable of each of
the user's speech bursts, but it reverts to receive as soon as the user pauses. Clipping
of speech will occur at each of these switching instants. In the tests, therefore, cheek
carefully that the clipping is not sufficient to interfere with conversation. Bear in mind
however that there will always be some clipping of initial syllables of speech bursts.
Instruction Card A3784 is packed with the LST 4 for both new and repaired instruments.
All LST4's need to be powered by a PSU No. 86B (50 volt DC) and this will need to be requisitioned
C MARKETING INSTALLATION
LST SA4255 FOR PABX's WITH DC CODE "C"
This is a loudspeaking telephone based on the LST4.... with the dial replaced by Push
Button Unit No. 3C. It can be used as a direct PABX extension or for providing extension
PUSH BUTTON UNIT NO. 3 ...
This unit is designed for use with the DC Code "C" PABX signalling system. The
push buttons are arranged in 3 columns of 4. The 11th and 12th buttons are marked * and
# respectively. The 12th button is used for enquiry and transfer. The 4 variants of the
Push-Button Unit No. 3 are as follows:-
PBU No. 3C is used in Loudspeaking Telephone SA4255. PBU No. 3D used in Push-Button
Units 1/SA4259 and 2/SA4259.
Loudspeaking telephone SA4255 is shown in Diagram SA(L)4255. Diagram SA(L)4252 also shows
the use of these telephones for providing extension plans. For operating instructions see
It is essential for the A and B conductors of the extension line to be connected to the
telephone in accordance with the appropriate diagram. A reversal of the A and B
connections will result in false operation of the Push-Button Unit No. 3.
Testers for DC Keyphones are to be held by Maintenance groups in Areas for the present,
and should be borrowed by Installation groups if necessary. Attempts should not be made to
insert dial labels from the back of the telephone case, the correct procedure is to remove
the Perspex cover at the front of the case.
Each telephone must be supplied with a signalling earth connection.
Correct line polarity must be observed.
Both wires of the line between the telephone terminals and the exchange must be kept
free from shunt paths, such as bell circuits, when the handset is off-hook to ensure a
clear line for dc keying signals.
Earthed-loop recall is provided by button # on the keypad. A separate recall button is
not therefore required.
TITLE AND CODE OF TELEPHONES AND PUSH-BUTTON UNITS
Rate Book Item Code
SA4255 Loudspeaking 410291
An extract from the TI
Issue 2, Aug 1976
LOUDSPEAKING TELEPHONES 4D, 4E, SA 4255 AND SA 4265
This Instruction describes the maintenance to be performed on Loudspeaking Telephones 4D,
4E, SA 4255 and SA 4265. These are all voice switched loudspeaking telephones. LST's 4E, SA
4255 and SA 4265 are keypad variants of the LST 4D, which is the normal dial version. The
LST 4E produces 10 pps loop disconnect pulsing (LD), whilst LST's SA 4255 and SA 4265 are for
use on proprietary type PABX's employing DC code C and multi-frequency signalling
The LST's 4D and 4E are described in C3 B2004.
||Dial No 21
||Amplifier SA 4286
||Cord 6/44A Grey 120in, (6/61AT Grey 3000mm)
||Cord Inst 4/120AX Grey 200mm
||Loudspeaker Unit 4B-1
||Microphone Unit 1B-1
||Tone Ringer 9A Mk II
||Lamp 41F (LES, 24 volt, 40ma)
|LST 4E Battery
||Battery Secondary No. 22 Red
|On/off Switch Mechanism
|Remote Handset Bracket
Any faults which cannot be rectified by replacing parts listed above should be dealt with
by changing the complete instrument.
For LST 4D see Diagram N4475. Diagram N4477 shows the connections for the LST 4E. The
diagrams for LST's SA 4255 and SA 4265 are SA(L) 4255 and SA(L) 4265 respectively. For
auxiliary fittings see Diagrams N4475 and N848.
DISMANTLING THE INSTRUMENT
(a) To Remove the Lid, press the wrists against the front edge and carefully insert the
fingertips into the gap between the lid and the back panel. To obtain maximum leverage
place the fingers 3 inches from either end. Prise upwards to free the concealed snap
fastener and draw forward. In cases of difficulty the tip of a Screwdriver Instrument No.
3 may be used, but care must be taken not to damage the lid.
(b) To Remove the front panel after removing the lid, take out the two screws visible
through holes in the runners under the instrument, and the top central screw on the panel
itself. Lift the panel clear, taking care not to strain the connecting leads to the
microphone, dial and loudspeaker. The front panel should only be removed if it is
necessary to add accessories or to change the dial.
(c) To Remove the back panel after removing the lid, take out the three screws along the
top of the back panel and lift clear taking care not to strain the connecting leads.
(d) To Remove the dividing panel remove the front and rear panels and release the two
screws securing the dividing panel to the base. They are located under the felt about one
inch from either end.
CHANGING THE AMPLIFIER
To remove the amplifier remove the back panel and the connections to the tag strip. Slide
the amplifier back on its runners until the plug-in connector is visible and can be
removed. The amplifier can then be carefully disengaged from the runners and the new unit
The original amplifier for the LST 4D was Amplifier SA 4260 (4 card type) Amplifier SA
4286 (2 card type) is an improved design amplifier which is being introduced on new
production and for maintenance replacement purposes. The amplifier is supplied strapped in
the high gain position which is suitable for most installations. Re-strapping is required
to select the other (low) gain position, which is used if mis-operation of the voice
switching occurs due to bad room acoustics.
Replace the panels in the reverse order. Ensure that all the screws are replaced and
tightened or mis-operation of the switches may occur.
These are standard types as used on Telephones 740 and are detailed in Diagram N848. The
latching arrangements are similar to the Tele 740 and are also illustrated in Diagram
N848. Remove the front and dividing panels to gain access to the switches.
To gain access to the Lamps No. 41F (only LP1 is normally fitted, the other bulb
holders are empty):-
- Stand the instrument on one end.
- Remove the two screws securing the small plate in the base below the lamps.
- Slacken the two brackets securing screws and slide the lamp bracket towards the rear
of the instrument to disengage it from the framework.
- Pull the assembly through the hole in the base.
To change the dial:-
- Release the front panel as detailed above.
- Loosen the dial clamp screw and slide the ring back along the leads.
- Draw the dial through the front panel.
- Unscrew the connecting leads and reconnect on the new dial.
- Reassemble the components in the reverse order. Ensure that the lug on the case
engages in the small cut-out in the retaining ring and that the dial finger stop engages
in the recess in the front panel ad this prevents the body of the dial from rotating.
- Tighten the ring.
NB: When a dial requires changing the dark grey number ring, if in serviceable condition,
should be transferred from the old to the new dial.
Make several test calls, if possible to officers experienced in testing loudspeaking
telephones, to check that the instrument performs satisfactorily on both transmission and
reception as a loudspeaker and as a handset telephone. Also, if possible, make calls over
different routes to check the instrument on all volume control positions.
To check transmission performances, speak clearly not more than 18 inches from the
microphone and, after speaking, wait for the distant party to reply. Ask the distant end
to repeat groups of random numbers over the line and check that they are received
When the volume control buttons 1 or 2 are operated, the instrument switches to 'send' on
the user's first syllable. It remains in this condition and will not switch back to
'receive' until the distant party utters his first syllable. When volume control button 3
is pressed the instrument still switches to 'send' on the first syllable of each of the
users' speech bursts, but it reverts to receive as soon as the user pauses. Clipping of
speech will occur at each of these switching instants. In the tests, therefore, cheek
carefully that the clipping is not sufficient to interfere too severely with conversation.
NOTE:- Unwanted switching can occur if too high a volume control position is used or if
excessive room and/or line noise is experienced.
Check other functions of the instrument such as the ON lamp, the tone caller and the
microphone cut-off button.
The importance of a quiet well-furnished room to ensure satisfactory performance of
loudspeaking telephones cannot be over-emphasised. Steady room noise biases the instrument
to 'send' making it more difficult for weak signals from the line to switch the set to
'receive', while impulsive noise will switch the set to 'send' in the same way as speech. Reverberation, or the reflection of sound back and forth between hard surfaces, gives rise
to the well known hollow effect at the distant end, and, by affecting the degree of
coupling between loudspeaker and microphone, hard surfaces can cause misoperation of the
The environment should also be electrically quiet, that is devoid of strong electric and
magnetic fields. Problems have occurred in the past with speech clipping which has been
found to be due to such sources of interference as microwave ovens and heavy electrical
machinery. Strong HF and VHF radio signals can also cause this problem.
Satisfactory results should be obtained in a room with a large carpet, thick curtains and
upholstered furniture and with the user seated close to the instrument. Rooms with either
extensive hard surfaces or where other people are working are not suitable.
NOTE:- The top of the LST must be kept clear; it should not be used for the storage of
papers or telephone or electrical equipment.
THE LOUDSPEAKING TELEPHONE 4E
This LST is an enhanced version of the 4D. It has a self-contained keypad in place of the dial and a wooden trim finish. The keypad,
electronic equipment and battery are identical to those used on Telephone 1/764 (see
B2830), but the Battery Secondary No. 22 is always charged from the Power Unit No. 86. The LST 4E is
described in C3 B2004.
In cases of wrong numbers or no pulsing, check the battery voltage (nominally 4.8 volts)
and check that a charging current is present. This is done by putting a meter in series
with the black lead on Terminal DA1 (see Diagram N4477). There should be a current of
around 3 ma flowing. Where the battery is suspected, a known good battery may be
substituted to see if this clears the fault. If the fault is shown to be due to the keypad
or its associated electronics the complete instrument must be changed.
To test every digit of the Push Button Unit for correct operation, a selection of test
numbers must be keyed which include all the digits 1 to 10.
The Loudspeaking Telephone 4E is scheduled to have a phased introduction into the field
commencing late 1976.
THE LOUDSPEAKING TELEPHONE SA 4255
This LST has a 12 button keypad and produces dc Code C signals. The Push Button Unit is
not available as a replacement part and therefore any faults which occur on the keypad
should be rectified by changing the complete telephone. For details of connections see
Diagram SA(L) 4255. This LST is repaired in the PO Factory (Enfield) Service Pool (see
A0015 for details).
LOUDSPEAKING TELEPHONE SA 4265
This LST has a 12 button keypad which produces multi-frequency signals (Signalling System
MF 4). Any faults which are proved to the push button unit should be rectified by changing
the complete instrument. For details of connections see Diagram SA(L) 4265. It should be
noted that no 'confidence tone' is heard when keying on this LST (see C3 B3010).
SOME KNOWN DIFFICULTIES
The fixed part of the handset rest has caused trouble in the past by becoming
loose. The nut holding the rest in place may be secured in place by applying Loctite Nut
Lock to the threads. Loctite may be purchased locally.
LST 4D on/off switch-Switch 31B
Switch 31A has been modified to overcome failure of the
latching mechanism. The new switch (Switch No. 31B) is available from London, Holloway Factory
on Form A1093. This problem has now largely been eliminated and therefore local stocks
should not be required. The Switch 31B is now being fitted to all new and repaired LSTs.
LST 4D on PMBX No 4
If an LSD 4D containing Tone Ringer No. 9A is fitted on PMBX 4 a Resistor
Coil No. 35A (3.9 kg) should be connected in series with the tone ringer as shown in N822.
N4475 is to be amended on the next issue to show this.
Radio Interference where problems of radio interference occur, it is recommended that the
Area Radio Interference Group should be consulted before any case is referred to Regional
Model number against functionality
|LST No. 4A-1 Mk 1
||ORIGINAL FORM, HAS DIAL, 5 WAY LINE
CORD AND SPRING-SET TYPE GRAVITY SWITCH
|LST No. 4A-1 Mk 2
||UPDATED VERSION WITH MICRO-SWITCH
TYPE OF GRAVITY SWITCH
|LST No. 4A-1 Mk 1A
||Mk 1 VERSION UPDATED BY P.O.
FACTORIES, IDENTIFIED BY 2 DOME HEADED STUDS ON FRONT PANEL
|LST No. 4A-1 Mk 3
||WITHOUT THE 2 DOME HEADED STUDS AND
WITH INCREASED TONE RINGER OUTPUT
|LST No. 4A-2
||AS 4A-1 BUT WITH NO HANDSET OR SWITCHOOK
ON REAR PANEL
|LST No. 4B
||EXPERIMENTAL MODEL - NEVER ISSUED
|LST No. 4C-1
||DC CODE C VERSION OF LST 4D
|LST No. 4D
||SIMILAR TO LST 4A-1 Mk 2 BUT WITH VARIABLE TONE RINGER, EXTERNAL LOUDSPEAKER AND
MICROPHONE SOCKETS ON BACK PANEL
|LST No. 4E
||SC VERSION OF LST 4D
|LST SA 4255
||DC CODE C VERSION OF LST 4E
|LST SA 4265
||MF VERSION OF LST 4E