3 Internal
Issue 2, Feb 1978


Loudspeaking Telephones No. 5 and 5B are superseded by Loudspeaking Telephone No. 7A (LST No. 7A). See TI C3 B2006.

Stores Required:
Control Unit No. 8B with Connector 1014B
Telephone No. 708 with Connector 1014C
Amplifier and Switching Unit No. 1B
Transmitter No. 27B

Transmitter No 27B
This is an extension microphone which is switched into circuit in place of the normal microphone fitted in the control unit. It is usually mounted close to the users mouth. The other items are normally placed on a shelf or unit provided by the customer.

Field of Use Direct exchange lines and shared service.

Installation Details were given in Works Specification S(W) 2139.

Stores Required:
Unit Signalling SA 4226 - 1
Transmitter No. 27B
Telephone No. 708 with Connector 1018A (or Telephone No. 716 if more than one press button is required)
Control Unit No. 8A
Amplifier No. 138B

Field of Use Direct exchange lines and shared service.

Installation Diagram N4476 refers.

Adjustments required:-
Test the LST No. 1 in accordance with TI C3 B2001 with the exception that no attempt need be made at this stage to establish the optimum setting of the receive amplifier volume control.

The volume control in the Amplifier No 138B is not used and must be rotated fully clockwise, i.e. to its maximum position. Two independent controls have been provided instead in the Signalling Unit. Use the control marked "Receive volume, manual" (RV2) to adjust the howl point for normal operation, and the one marked "Receive volume, POSM" (RV1) for POSM operation. On POSM operation the set must be just stable at all times during the setting up of a call. Once the call has been established and the distant correspondent is on the line, the POSM user is enabled to increase the volume from the basic setting.

Switch on the LST manually, dial a single digit to clear dialling tone and then adjust the "Volume control, manual" to a point below the threshold of howl and test but as described in TI C3 B2001. Switch off.

Request the subscriber to engage the "telephone" outlet on the POSM apparatus. Cheek that Relay LA operates, dial tone is received over the loudspeaker, the extension microphone is live, the control-unit microphone is dead, and the lamp in the control-unit glows. Dial a single digit manually using the control unit to clear dial tone, and then adjust the "Volume control, POSM" to a point below the threshold of howl. Bear in mind that the subscriber may subject the relative positions of loudspeaker and microphone to small variations, and the howl point should be set to allow for this. For shared service request the subscriber to engage the "telephone" outlet and then to "call exchange" and note that Relay CE, operating after Relay LA, causes dial tone to be returned. Relay CE must of course be released immediately upon receipt of dial tone. as it presents to the exchange the same condition as that from a normal telephone with the "call" button pressed, i.e. an earth is extended on either the A or B wire. Proceed to adjust "Volume control, POSM" as above, then request the subscriber to clear down.

Check the subscriber's operation of the volume control on an established call. Relays VW, VX, and VY should operate in the sequence shown in Diagram SA/SAW 4226 to increase the volume progressively. If howling at the higher settings causes distress, temporarily reduce the present volume control to establish that the whole range of 8 steps is operating correctly.

Dial Circuit Finally, contact the Test Desk by dialling on the control unit, and carry out a dial test on the dialler circuit. Ask the subscriber to select each digit in turn, and ascertain that the digits sent are within normal tolerances. If necessary, adjust the pulsing speed and ratio of relay PU by adjusting RV3 and strapping tags TC14, 16, 18 and 20 as indicated on Dgm SA/SAW 4226.

Introduced in 1969.

Diagram SA 4212 - LST 5 Mk2
Diagram SA 4213 - LST 5A Mk1
Diagram SA 4227 - LST 5B

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Last revised: February 25, 2023