Issue 1, Jan 1978
LOUDSPEAKING TELEPHONE No. 7A
Loudspeaking Telephone No. 7A (LST 7A) supersedes the LST 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 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:-
(i) Amplifier 138B. This line powered amplifier was also used with the LST 5B.
(ii) Telephone 708. An additional spring set, Part 1/DSP/1256, must
be fitted for this application.
(iii) Control Unit 33A. This unit is based on the Control Unit 8A (used with LST 5B) but the dial has been replaced by a 10 button keypad.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 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 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 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.)
(iv) Signalling Unit 37A-1. This unit accepts control signals from the Control Unit 33A or
privately supplied selection mechanism. These signals cause the LST 7A to seize the line,
alter the volume setting (minimum, medium and maximum) and send call set up signals to the
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 7A via the external
socket mounted on the Signalling Unit 37A. A retaining clip is provided on the Signalling
Unit to secure the connection.
(v) Transmitter 28A. This supersedes Transmitter 27B and is connected to the goose neck
(when supplied) using the special clamp provided.
If a Transmitter 28A or 27B is provided ensure that strap 4-5 is in place in the
Signalling Unit 37A.
(vi) 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 N 4478 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
138B for maximum receive gain consistent with avoiding instability. With the privately
provided equipment disconnected from the Signalling Unit 37A-1 a few tests calls should be
made using the Control Unit 33A to set these up. The following procedure should be
- 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.
- Use the keypad in the normal fashion to set
up a call.
- Check that the volume key functions to give minimum, medium and maximum volume.
- To transfer to the handset of the Telephone 708, lift the handset then restore the ON/OFF
key to the OFF position.
- To revert to loudspeaking place the ON/OFF key to the ON position
AND THEN replace the handset.
- 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.