SWITCHBOARD AT 711 and AT1800

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 AT 711 and AT 1800.

Switchboard AT 1800 (5+20) - Picture dated 1932

Switchboard AT 1800

Operating Features
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.

During the setting up of an exchange connection, the line is held when necessary by one or other of the following:-

1. The operator's telephone.

2. The extension telephone.

3. A resistance spool whilst the extension is being rung on a calling cord.

4. By the insertion of a plug into a separate holding jack, if the circuit is left without an extension connected.

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:-

1. The operator's telephone.

2. The extension telephone.

3. The extension signal.

4. A resistance spool while the extension is being rung on a calling cord.

5. By the insertion of a plug into a separate holding jack if the circuit is left without an extension connected.

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 Test
When no special hold jack is fitted on Switchboards, A.T. 711, the last extension jack should be used for this purpose and labelled HOLD, no alteration to the circuit being necessary.  If it is desired to connect an extension to the jack, a separate jack in single mounting should be fitted in the left-hand wood stile and wired to Fig. 1 of Diagram C.B.1209 (N. 961).  New supplies of switchboards will be provided with such jacks in position.

Modifications required to Switchboard A.T. 711 for working to an Automatic Exchange
Particulars of the modifications necessary when the switchboard is required to work to an automatic exchange are given in T.I. XXXIV Part 3.

Night Service
The operation of the night-switching key cuts the battery feeds from the tip and ring of the cord circuits ; the Earth from the sleeves of the exchange line jacks, and also disconnects clearing signals from some or all of the extension equipments.  Any extension line available for night service, must be connected to one of these equipments and may then be connected to any exchange line by using any pair of cords.  When such connections are set up, there is no current drawn from the B.E. bus-bars.  A dial is necessary on each extension requiring direct night service to the main exchange, and, if auxiliary equipment is fitted on such an extension, a break-jack for use on night connections should be inserted in the line.  If available, a spare extension jack, should be used for the purpose and suitably labelled.

Operators Telephone
A fixed transmitter and a Bell receiver are included in the equipment of the switchboard.  Current for speaking is drawn over the exchange line or from the cord circuit, according to whether the connection is via the exchange or purely local.

When a breast-plate transmitter and head-gear receiver are required, they should be wired to Figure 2, C.B. 1208 (N. 960).

Night Alarm
A bell is included in the equipment of the switchboard. The removal of the receiver from the switchhook disconnects the night alarm circuit.

Ringing Circuit
If power ringing is provided, a double throw change-over key and resistance lamp, if required should be fitted locally and wired to the circuit shown on Diagram C.B. 894 figure 3.

Labelling
This should conform to the general scheme outlined in Part 1.

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.

Signalling Limits
To ensure satisfactory operation of the P.B.X. relays and signals on main exchange connections the exchange line resistance plus extension resistance must be such that, with the 20 ohms + 30 ohms relay in the un-operated position, the current through the extension is not less than .25 rnA.  The permissible value of the resistances depends upon their relative magnitudes and the type of main exchange equipment.

On local connections the maximum extension line resistance is 50 ohms.

Last revised: November 11, 2021