SWITCHBOARD AT 711 and AT1800
|Taken from GPO Technical Instructions XXIII (1930)
For Private Branch Exchanges in Automatic areas. Similar to PMBX CB873.
AT1800 Mark 234 was made by British Ericsson their Diagram No. N16376 dated 1933.
Both switchboards operate on 12 volts DC.
Diagrams AT711 and AT1800.
Switchboard AT 1800 (5+20) - Picture dated 1932
Switchboard AT 1800
During the setting up of an exchange connection, the line is held when necessary by one or other of the following:-
An arrangement similar to (3) is not in circuit whilst ringing back on an answering cord; in such circumstances the speaking key must be operated.
When a plug is inserted into an extension jack, the line signal is transferred to a local circuit and is controlled by a series relay; it thus acts as a positive supervisory signal giving an audible alarm.
When a connection between an exchange line and an extension has been made and the speaking key restored, there is no loop across the line at the P.B.X. switchboard and the switches or signals at the public exchange are controlled by the switch hook on the extension telephone. "Through clearing" conditions are thus provided and it is not possible for an extension to call in the P.B.X. operator without releasing the connection at the public exchange on an originating automatic call, or actuating the supervisory signal at the public exchange on any call incoming or outgoing which has been completed via a manual board.
These switchboards are arranged for conversion to the condition described in Para. 145, and should a subscriber demand "calling-in facilities" after a switchboard of this type has been fitted it may be converted in situ.
The dial is in the operator's telephone circuit and, to dial a number the speaking key of the cord circuit connected to an exchange line must be thrown. In this condition the calling extension is in parallel with the operator's telephone and, in order that the impulses may be sent out to the exchange, the calling extension loop must be disconnected by throwing the ring back key whilst dialling. The key is therefore labelled DIAL and RING-BACK.
If power ringing is provided, a "Power Ring-Back" key must be used in the manner described in Para. 81.
Switchboard AT 711
Hand-restored drop indicators are provided as calling signals on the exchange lines and eyeball indicators are fitted on the extensions. Double cord circuits are provided, each with a ringing and speaking key and a dial, and ring-back key.
When a plug is in an extension jack, the line signal is controlled by a series relay and operates as a positive supervisory signal giving an audible alarm.
During conversation there is no holding coil across the circuit but, when the extension receiver is replaced, the line is looped by the extension signal as described above. It is, therefore, possible for an extension to attract the attention of the B.E. operator without releasing the connection or giving a clearing signal at the main exchange. During the setting up of an exchange connection, the line is held when necessary by one of the following:-
An arrangement similar to (4) is not in circuit while ringing back on an answering cord, in such circumstances the speaking key must be operated.
The dial is in the operator's telephone circuit and to dial a number, the speaking key of the cord circuit connected to the exchange line must be thrown. In this condition the calling extension is in parallel with the operator's telephone and in order that the impulses mar be sent out to the exchange, the calling extension loop must be disconnected by throwing the ring-back key while dialling. The key should therefore be labelled "DIAL AND RING-BACK".
Holding Circuit and Cord
Modifications required to Switchboard A.T. 711 for working to an
When a breast-plate transmitter and head-gear receiver are required, they should be wired to Figure 2, C.B. 1208 (N. 960).
The jacks are drilled for the reception of labels on which the circuit designations should be marked. The labels for the extension jacks are supplied engraved, those for the-exchange line jacks are supplied blank.
Any necessary service marking should be applied to the indicators by coded labels.
On local connections the maximum extension line resistance is 50 ohms.
Last revised: November 13, 2020