3 Internal
Issue 1, Jan 1978


Loudspeaking Telephone No. 7A (LST No. 7A) supersedes the LST No. 5B and is primarily intended for use by severely physically handicapped customers who have one of a number of patient operated environmental control mechanisms. LST No. 7A contains a line powered loudspeaking amplifier and an exchange signalling unit which is normally mains powered. In the event of mains failure re-chargeable standby batteries provide power for emergency use.

LST 7A comprises:-

  1. Amplifier No. 138B. This line powered amplifier was also used with the LST No. 5B.
  2. Telephone No, 708. An additional spring set, Part 1/DSP/1256, must be fitted for this application.
  3. Control Unit No. 33A. This unit is based on the Control Unit No. 8A (used with LST No. 5B) but the dial has been replaced by a 10 button keypad. The unit also contains a microphone key, a volume key and an ON/OFF key. The principle use of the Control Unit is to functionally test the LST No. 7A when the selection mechanism is disconnected. The 10 button keypad sends digit selection control signals to the Signalling Unit. The ON/OFF and volume keys also generate control signals.

    Future Control Units No. 33A will come provided with Cords Connecting No. 16/1A but prototype units have cords where the colour scheme may not match that shown on installation diagrams. The wiring of the cord at the Control Unit end is not changed for plan arrangements and so the wiring as issued must not be altered.

    (NOTE: The Control Unit No. 33A is not a Keyphone and cannot be used on its own to set up telephone calls. It must be associated with the rest of the LST 7A.)
  4. Signalling Unit No. 37A-1. This unit accepts control signals from the Control Unit No. 33A or privately supplied selection mechanism. These signals cause the LST No. 7A to seize the line, alter the volume setting (minimum, medium and maximum) and send call set up signals to the exchange.

    This unit is normally mains powered but two Batteries Secondary No. 23 are fitted to provide standby power under mains failure conditions. The unit will not function correctly if the batteries are omitted.

    The privately supplied selection mechanism is connected to the LST No. 7A via the external socket mounted on the Signalling Unit No. 37A. A retaining clip is provided on the Signalling Unit to secure the connection.
  5. Transmitter No. 28A. This supersedes Transmitter 27B and is connected to the goose neck (when supplied) using the special clamp provided.

    If a Transmitter No. 28A or 27B is provided ensure that strap 4-5 is in place in the Signalling Unit 37A.
  6. Batteries Secondary No. 23. Two are required to be fitted to the Signalling Unit using the plugs and sockets provided. (The batteries MUST be connected before the mains voltage is applied.)

The patient operated environmental control mechanisms to which this Instruction refers are not PO provided.

All the items are listed individually in the Vocabulary of Engineering Stores. The Control-Unit, Signalling Unit and Transmitter are 'E' controlled items and should be listed together on one requisition and submitted as detailed in the Vocabulary. This requisition should also show the customer's name, the A/N number and the full delivery address for the stores.

Using the Diagram N4478 connect the equipment together. When connected for shared service the signalling unit automatically connects an earth to seize the exchange line at the commencement of a call. Following the details given in TI C3 B2001 adjust the Amplifier No. 138B for maximum receive gain consistent with avoiding instability. With the privately provided equipment disconnected from the Signalling Unit No. 37A-1 a few tests calls should be made using the Control Unit 33A to set these up. The following procedure should be adopted:-

  1. Ensure that the mains supply is connected. With the ON/OFF key of the Control Unit in the ON position wait till dial tone is returned. 
  2. Use the keypad in the normal fashion to set up a call.
  3. Check that the volume key functions to give minimum, medium and maximum volume.
  4. To transfer to the handset of the Telephone No. 708, lift the handset then restore the ON/OFF key to the OFF position.
  5. To revert to loudspeaking place the ON/OFF key to the ON position AND THEN replace the handset.
  6. Calls may be initiated using the handset by simply lifting the handset and keying out the required number.

Introduced in 1978 and superseded the LST No. 5B.


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Last revised: March 11, 2022