3 Internal
Issue 2, Oct 1973


This instruction gives details of the installation of Loudspeaking Telephone No. 2.  Reference should be made to B2000 for a general description of loudspeaking telephones, and for information on the acoustic problems associated with loudspeaking telephone use.

The Loudspeaking Telephone No. 2 requires a d.c. supply of either 50, 26 or 6V nominal.  The total current consumption is 250 ma at 50 or 26V, and is 80 ma at 6v.  The d.c. supply may be provided by a power lead from the exchange battery, by a separate dry battery, or by a Power Unit No 53A.  The cheapest method of providing the power supply should be chosen for each installation.

If dry cells are used, five Cells, Dry, R40 (in a Box, Battery, No. 6) should be provided, and this should be noted on the exchange record card.  At any subsequent maintenance visit to the installation, the dry cells should be replaced automatically if they have been in use more than nine months, and the date of renewal entered on the exchange record card.

Before using a power lead from an exchange battery, the daily voltage excursion between no load and full load should be ascertained, and it should be verified that this variation does not exceed the limits laid down.

At installations where there is not an existing exchange power lead, and where it is undesirable to use dry cells, then a Power Unit No. 53A may be used.

Agree with the subscriber a suitable position for the cabinet.  It should be kept away from radiators and, if possible, from direct sunlight to avoid the possibility of overheating the equipment.

Determine a suitable position under a desk or on an adjacent wall to fix the external junction box which is connected to the cabinet by an 8ft. length of multi-way cordage.  Extend the exchange line leads and the d.c. supply leads to this position but do not connect.

Diagram SA 4156 shows the circuit arrangement of the Loudspeaking Telephone No. 2 together with the power-feeding connections. Installation should proceed as follows:-

(a) The amplifying equipment is designed to work over a range of 5-7,8 volts.  Three links, two straps and a voltage control potentiometer are provided to achieve this voltage range with different input voltage supplies.  The three permissible nominal voltages are clearly marked on the links as 50, 26 and 6v.  The links are set initially for a nominal supply of 50V and require alteration for a nominal supply of 26 or 6v.  Check that for a nominal supply of 50, 26 or 6V the measured voltage of the supply does not exceed 62, 32 or 7.8v respectively.  Set the links, straps and voltage control potentiometer for the appropriate nominal voltage as detailed in the notes on Diagram SA 4156, Sheet 2, and connect the power supply to the set.  It is important to ensure that the voltage control potentiometer is set fully clockwise before the power supply is connected, as stipulated in the diagram notes.

(b) After connection of the d.c. supply to the. set, the voltage control potentiometer requires adjustment if a nominal 50 or 26V supply is used (the voltage control is rendered inoperative when the links are set to the 6v position).  Fig 1 shows the amplifier voltage (VA) which should be obtained by appropriate adjustment of the voltage control potentiometer.  This graph applies only where the power supply is known to fluctuate between the limits 38-59V or 18-29V. The voltage control adjustment is made as follows:-

Switch the set on and establish a connection without signal input from the line.  In an automatic exchange area this can be best arranged by dialling another number and holding the connection in the 'called subscriber held' condition, thus ensuring quiet line conditions.  Measure the supply voltage (VS) across terminals J20 and J29 of the junction box. Read off from Fig 1 the amplifier voltage (VA) corresponding to the voltage Vs just measured.  Turn the voltage control anti-clockwise until this value (VA) is measured across terminals J15 and J29 of the junction box.

(c) A Power Unit No. 53A may be used when it is undesirable to use dry cells because of difficulty in periodical renewal or accommodation.  A Power Unit No. 53A will provide sufficient power for only one Loudspeaking Telephone No. 2.  This power-unit gives a 12v d.c. output, and two additional resistors are required to reduce it to 6v. The two resistors should be requisitioned as Resistor Coil 35-15 ohm and Resistor Coil 35A-39 ohm.  Terminals J1, J4 and J8 are spare terminals in the junction box which can be used for mounting the resistors.  The Loudspeaking Telephone No. 2 is then set up for the 6v condition as previously described.  Great care should be taken to ensure that the polarity of the 12v d.c. supply is correct before it is connected to the junction box terminals.

(a) Make test calls , both local and junction, and check that the transmission performance in both directions is satisfactory, using either the loudspeaking equipment or the handset.  These test calls should first be made to handsets at the distant end, and then, if possible, further tests should be made to another Loudspeaking Telephone No. 2 to test the performance when both instruments are switched to the loudspeaking condition.

(b) Check the clarity of reception on each volume control position.  There should not be undue clipping of the first syllables of received speech, particularly on the first words in sentences.  The presence of room noise at one or both ends can affect the amount of noticeable clipping, but with average room noise arising from other conversations within the room, closing of doors and moderate traffic noise, it should still be possible for each end to interrupt conversation from the other end.  If there is excessive speech clipping, or if the voice switching is otherwise unsatisfactory, then the receive amplifier may be readjusted as described in par 6, if the requisite testing apparatus is available.

(c) Check that the indicator lamp glows when the ON key is operated.

The receive amplifier gain control (RV2, Diagram SA 4156) may be adjusted to suit local conditions, or the subscriber's requirements.  Adjustment should be made with the INCREASE VOLUME key on the front panel in the normal position. RV2 is mounted on the chassis of the loudspeaking telephone.  Other than this the receive amplifier does not normally require adjustment.  If however, the receive amplifier is to be readjusted because of excessive clipping of received speech, arrange the test circuit as shown in Fig 3 and then proceed as follows:-

(a) Connect a d.c. supply to the loudspeaking telephone as described in para. 4.

(b) Disconnect the microphone lead from the microphone potentiometer.  This potentiometer consists of four carbon resistors mounted on an insulating board mounted vertically on the top of the chassis adjacent to the calling buzzer.

(c) Set potentiometer RV2 fully anti-clockwise, i.e. for maximum receive volume.

(d) Set the Attenuator No 5 for maximum attenuation and set the Oscillator No. 29A to give an output of 0dB at 800 c/s.

(e) Adjust the Attenuator No 5 to give a voltage of 8.2v d.c. across capacitor C12 (Diagram SA 4156) when measured by a Meter, Multi-range, No 12 or other instrument of equivalent sensitivity (20000 ohms per volt).  Drawing CD 1652 shows the position of C12 on the amplifier label.

(f) Adjust RV1 (Diagram SA 4156) to give a level reading of -5 dB on the Tester RP790.

(g) On completion of the tests, RV2 may be readjusted to suit local conditions.

In common with all loudspeaking telephones the performance of Loudspeaking Telephone No. 2 is dependent upon the local acoustic conditions and upon the line attenuation.  Under extreme conditions it will be necessary to use the associated handset to make a successful call. For further information on this aspect reference should be made to B2000.

Formerly EI Telephones, Stations, F3003

Originally called Telephone No. 1000L (GEC Paxmaster 660592) and introduced in October 1959.  Description was changed to LST No. 2 in 2/60.

This Telephone was produced by GEC.

LST No. 2B is similar to LST No. 2A except that the Handset No. 1 has been replaced by a Handset No. 3 and the rear amplifier panel has an improved mounting arrangement.

Diagram SA 4156_1 - LST 2
Diagram SA 4156_2 - LST 2
Diagram SA 4161 - LST 2A
Diagram SA 4175 - LST 2B

Specification - S608.

Wiring Instructions
(taken from diagram SA4156)

Please refer to the picture above, which shows the external junction box:-

The following terminals are linked in the external terminal block:-

  1. 23 and 24
  2. 25 and 28
  3. 26 and 27

Line connections:-

  1. A leg to terminal 27
  2. B leg to terminal 28

For a nominal 50v supply:-

  1. Set voltage control fully clockwise (the knob to the left of the capacitor).
  2. Set all three links to the 50v position.
  3. In the loudspeaking telephone ensure that there are no straps or links between terminals 26, 27 and 28.
  4. Connect the 50v + to terminal 29 on the external terminal block.
  5. Connect the 50v - to terminal 32 on the external terminal block.
  6. Adjust the voltage control as per the Telecommunication Instructions above.

For a nominal 6v supply:-

  1. Voltage control is inoperative.
  2. Set all three links to the 6v position.
  3. In the loudspeaking telephone ensure that terminals 26, 27 and 28 are linked together.
  4. Connect the 6v + to terminal 29 on the external terminal block.
  5. Connect the 6v - to terminal 31 on the external terminal block.



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