Introduced in 1960.

Loudspeaking Telephone No. 1 (LST No. 1) is transistor operated. Current for the amplifiers is drawn from the telephone line and therefore a separate power supply is not required.

It comprises three items:-

(a) Control-unit No. 8A with Connector...

(b) Telephone No. 708 with Connector...

(c) Amplifier No. 138A or B

The three units are interconnected by means of plug-ended cords.

The control unit contains the microphone, indicator lamp, dial, volume control key and on/off key.  Loudspeaking calls are controlled completely from this unit.  To make a call using the loudspeaker, the on/off key is operated and the required number is dialled.  When the call is answered, the received level of speech can be adjusted by means of the 3-position volume control key.  To terminate the call, the on/off key is restored and the volume control key is moved to the low volume position to prevent howl on re-use.

On early models with Control-unit No. 8A (Mk. 1), change-over to the handset whilst a loudspeaking call is in progress is effected simply by lifting the handset from the telephone; the call is switched over automatically and the on/off key is self restored.  To revert to the loudspeaking condition it is necessary to depress the on/off key on the control unit whilst the handset is returned to the telephone.  On current models with Control-unit No. 8A (Mk. 2), the self-restoring feature has been withdrawn because of functional and mechanical weaknesses of the relay-released key.  To change-over to the handset whilst a loudspeaking call is in progress the handset is lifted and then the on/off key is restored manually.  To revert to the loudspeaking condition the on/off key is re-operated and then the handset returned to the telephone.

Mk. 1 control units had an omni directional microphone.  Mk. 2 units have a microphone the sensitivity of which is directional. Maximum pick-up is from the front and rear of the unit and minimum pick-up is from the sides.  When this type of microphone is used there is less acoustic feedback from the loudspeaker provided the telephone unit (which contains the loudspeaker) is correctly positioned to one side of the control unit.

The Mk. 2 control unit has slotted openings in an escutcheon plate fitted to the rear of the unit to preserve the directional properties of the microphone.  The lamp, which would otherwise obstruct the frontal opening and hence interfere with the performance of the microphone, has been moved to a position below the dial on the front edge of the unit.  The microphone is resiliently mounted to reduce the amount of noise transmitted to line when the control unit is handled.

Because the directional-type microphone is less sensitive than the one used in Mk. 1 units, a pre-amplifier has been added to the Mk. 2 control unit and the output level adjusted 3 dB higher than that of Mk. 1.  Under ordinary acoustic conditions about 3 dB increase in receive gain will also be realised because of the improved acoustic stability.  The volume control key of the Mk. 2 unit has three locking positions instead of two locking positions and one non-locking position. Mk. 1 and Mk. 2 units are interchangeable.

The telephone instrument is a standard 700-type telephone modified by the removal of the dial in place of which is fitted a loudspeaker and its covering grille.  On early models an extra gravity switch was fitted for the automatic on/off key restoration facility. The switch is omitted from current models but the appropriate terminals are strapped through to maintain circuit compatibility.

Amplifier No. 138B supersedes Amplifier No. 138A.  The amplifier will usually be mounted in an inconspicuous position under the desk or on a wall.

Loudspeaking Telephone No. 1A is similar to the No. 1 except that Telephone No. 716 with Connector... is used instead of Telephone No. 708 with Connector... Telephone No. 716 has provision for the optional fitting of four push-buttons.  Amplifier No. 138B is used.

When working to a normal handset at the other end of the line the performance of this type of loudspeaking telephone is satisfactory except under conditions of high total line attenuation.  In such circumstances the high gain positions of the volume control key should be tried but, depending upon the local conditions, it may be found that oscillation occurs.  If this should happen then the handset must be used.

Field of use
The equipment is designed for use on direct exchange lines on 50V exchange systems (including shared service) and should also function satisfactorily on other exchange systems where the line current through the telephone is not less than 30 ma.

It may be used as an extension instrument on a P.B.X. and also on the following extension plans:-

Extension Plans 1 and 1A: Main or extension
Extension Plan 9: External and internal extensions on P.M.B.X. only, with or without recall
House Exchange System No. 3: Non-multiple extensions

3 Internal
Issue 2, July 1973


This Instruction gives details of the installation of Loudspeaking Telephones Nos. 1 and 1A.  Reference should be made to B2000 for information on the acoustic problems associated with their use.

Loudspeaking Telephone No. 1 incorporates a telephone instrument capable of being fitted with a single push button, whilst the instrument of Loudspeaking Telephone No. 1A may have up to four push buttons.  The type of loudspeaking telephone to be fitted in individual applications is indicated in the relevant N diagram.

(a) Loudspeaking Telephone No. 1 (Mk II) comprises the following items
Telephone No. 708, Grey, with Connector 1018A
Control-unit No. 8A, Grey, with Connector 1014B
Amplifier No. 138B

(b) Loudspeaking Telephone No. 1A (Mk I) comprises the following items

Telephone No. 716, Grey, with Connector 1018A
Control-unit No. 8A, Grey, with Connector 1014B
Amplifier No. 138B

The separate items are supplied complete with cords and plugs for inter-connection and additional items are not normally required.  If connectors longer than 72 in are required for control-units or telephones, the following are available:

Connector 1014B, 144 in
Connector 1014B, 216 in
Connector 1018A, 144 in
Connector 1018A. 216 in

Arrange with the subscriber the most suitable layout on his desk for the control unit and the telephone, bearing in mind the following considerations:

(a) To ensure the best possible quality and level of transmission, the control-unit should be placed at the front of the desk as near to the subscriber as possible so that the speaking distance from the microphone is a minimum.  This also places the controls and dial in the most convenient position for making a call.

(b) The subscriber should be instructed always to speak closely to the microphone, a distance of 15-18 in (380 mm-460 mm) being reasonable.

(c) The loudspeaker should be separated from the microphone by at least 2 ft (600 mm) to prevent "howling".  The Telephone No. 708 or 716 containing the loudspeaker should therefore be positioned at the required distance from the control-unit but within reasonable reach of the subscriber for the occasions when use of the handset is required.  The control-unit must be positioned to one side of the telephone instrument so that advantage is taken of the directional properties of the microphone.

(d) Choose a suitable position for mounting the Amplifier No. 138B where it is unlikely to be knocked and damaged.  An inconspicuous position on a wall or under the desk will be suitable but it must be mounted within reach of the connecting cords from the control-unit and the telephone.  The amplifier should be kept away from heating appliances and direct sunlight to avoid overheating.

Dgm N 4472 details the connections required for direct exchange lines, shared service and switchboard extension lines.  For other applications, diagrams in the series N 4451-4499 refer.  Procedure as follows:

(a) Remove the cover of the Amplifier No. 138B.  Three terminal blocks, one 4-way, one 5-way and one 7-way, numbered 1-16, are provided in the amplifier.  The straps on these blocks are set initially for a DEL installation as shown in Dgm N 4472.

(b) Connect the cords from the control-unit and telephone to the appropriate plug and socket in the amplifier and anchor them by their grommets in the two outer slots provided.  The separate grommet packed with the amplifier is for use with the line cord and should be located in the centre slot.

(c) Before connecting the exchange line to the line terminals on the terminal block, proceed as follows:-

(i) Measure the line current with a Resistor Coil No. 35A - 300 ohm in place of the loudspeaking telephone.  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 the extension connected through to the main exchange.

The current measured should not exceed 90 ma or 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.

(ii) On shared service, measure the earth current under calling conditions using the 300 ohm resistor in place of the instrument.  If the current exceeds 90 ma a Resistor, Coil, No. 35A, 200 ohms, should be included in the earth circuit in accordance with Dgm N 4472.

(iii) Measure the line loop resistance. The gain of the 'send' amplifier should be adjusted to suit the line loop conditions, by means of a strap provided on terminals 12, 13 or 14 in the amplifier.  The strap is initially set between terminals 12 and 13 to give the correct output for lines having a loop resistance less than 500 ohms.  For lines exceeding 500 ohms loop resistance, change the strap from terminals 12 and 13 to terminals 13 and 14.

(d) Connect up the installation in accordance with the relevant N diagram.

(a) Place the control-unit and telephone in their correct positions on the desk and operate the ON key.  The supervisory lamp should glow, and in automatic areas dial tone should be heard from the loudspeaker.  Remove the dial tone by dialling a suitable digit.  Set the 3-position volume control key to the low volume position, and adjust the continuously variable volume control in the amplifier (marked REC VOL) to a setting just below the 'howl' point.  In CB areas this adjustment should be made under conditions of calling the exchange, i.e. the operator withholding the plug from the line.

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 locking screw.

(b) Make several test calls, both local and junction, and check that the transmission and loudspeaking reception are satisfactory when speaking closely, i.e. approximately 18 in (460 mm) from the microphone.

(c) On a loudspeaking call, lift the handset and restore the ON key. Check that:-

(i) the indicator lamp is darkened, and

(ii) the call is switched from the loudspeaking equipment to the handset.

(d) While the call is switched to the handset, test for correct restoration to the loudspeaking equipment, by depressing the ON key and restoring the handset to the switch-hook cradle.  Check that:-

(i) the call is switched back to the loudspeaking equipment, and

(ii) the indicator lamp glows.

(e) Perform all the usual tests to a standard telephone, such as dial and bell tests.

(f) Anchor the two cords securely inside the amplifier and replace the cover.

NOTE:- The high volume position of the volume control is provided to give extra receive gain on calls of high line attenuation.  On normal calls it is expected that howling will occur if this position is used.

Formerly EI Telephones, Stations, F3002

Diagram SA 4130, SA 4177 and SA 4174 - LST No 1 & 1A
Diagram SA 4179 - LST 1A Mk 1

Installation Instructions (PDF)

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Last revised: February 10, 2022