3 Internal
E 4050
Issue 1, Dec 1970


This Instruction describes Switchboard CB 873 which is a floor-type double-cord with an ultimate capacity of 25 lines.
The Switchboard is suitable for use in either C.B. (manual) or automatic. The wiring of the switchboard is complete for 5 exchange lines, 20 extension circuits and 8 cord circuits, with provision for a maximum of 10 cord circuits.
Old version (3 + 10 shown) Later version

Height 4ft 3ins
Width 1ft 7ins
Depth 1ft 8ins
Weight 164 lbs

Circuit Diagrams
Wiring diagram-all circuits D 128/...
Cord circuit (explanatory) N 930
Cord circuit, divided-feed N 1095
Exchange line, extension line, operator's circuit, dial circuit, ringing circuit, hold jack and cord test, night alarm (explanatory) N930
Long extension or private circuit
N 715 Sig. Grp. B
N 718 Sig. Grp. G
N 710 Sig. Grp. A (i)
N 713 Sig. Grp. A (ii)
Inter-P.B.X. private circuit
N 716 Sig. Grp. D
N 717 Sig. Grp. E
N 719 Sig. Grp. H
N 770 Sig. Grp. A (i)
Inter-P.B.X. extension
N 773 Sig. Grp. A (ii)
N 774 Sig. Grp. H
Earth dialling to P.A.B.X.s N 880
Conference facilities:-
Amplified, key-switched SA 7093
Unamplified, key-switched SA 7092
Unamplified, cord connected SA 7090
Press-button recall N 851
S.T.D. metering, Schemes A, B and DI N 1021
S.T.D. metering, Scheme Cl N 1022
S.T.D. metering, Schemes C2 and D2 N 1023
Observation facilities SA 2402
Monitoring facilities SA 7102
Cord circuit modified for secrecy against operator N 992

Transmission and signalling limits
Extension line limits can be increased by the use of auxiliary apparatus units. Where it is uneconomical to provide auxiliary apparatus units the cord circuits may be modified for divided-feed on extension-to-extension calls in accordance with Works Spec. S(W) 2064 and Dgm. N 1095.

The switchboard is designed to stand on the floor against a wall, to which it is fixed by screws. The front of the switchboard is hinged on the left-hand side, to give access to the interior. The night-switching keys and the alarm cut-off key project from the face of the switchboard and are mounted above the extension line indicators.

A Hook, Receiver. A.F. is provided on the left side of the switchboard face for the operator's instrument. A hole is provided in the backboard to enable wiring to be led into the switchboard.

The exchange lines terminate on hand-restored drop indicators, and the extension lines on self restoring dolls-eye indicators. Double cord circuits are provided, each equipped with a RING and SPEAK key and a RING-BACK, DIAL CALL key.
Supervisory signals are of the positive clearing type, the extension calling indicator also giving supervisory signals when an extension is connected to the exchange or another extension. The extension indicator operates when the extension clears, and actuates a bell to provide an audible alarm. Individual supervision is given on extension-to-extension calls. On exchange-to-extension connections the clearing signal is received only on the extension indicator.

Operator's telephone
The switchboard can be provided with either a Telephone No. 164 or Headset No. 1 connected to a four-way plug. At existing installations fitted with a handset only, the operator's circuit may, if required, be modified to accommodate either type of instrument. Works Spec S 628 refers.

Later supplies of switchboard are issued with a dial and dial mounting.

Audible alarm
A bell is included in the equipment of the switchboard so that an audible alarm is given by the operation of any indicator. The alarm can be disconnected by means of a key mounted adjacent to the NIGHT SERVICE keys. In earlier supplies the alarm was also wired through the operator's telephone switchhooks, now replaced by a Hook, Receiver, A.F.

Ringing current can be provided by the hand generator which is included in the switchboard or by a ringing lead or ringing converter. If the hand generator is not the sole ringing source a HAND GENERATOR KEY must be fitted in the last dial key position and wired in accordance with Dgm. N 930. When a ringing lead is provided a Resistor, Bulb, No. 2 should be fitted inside the switchboard and wired in series with the ringing supply (see TI Q0005).

Hold jack and cord test
A strip of jacks, mounted between the exchange line and the extension line jacks, is equipped for holding exchange calls and for cord test purposes.

Night service
Keys are provided in the face of the switchboard to disconnect the battery feed and the operator's telephone from the first five cord circuits (on earlier supplies, the first eight cord circuits). Hence any exchange line can be connected to any extension for night service via any of these cord circuits. Contacts of the night service keys also disconnect the battery feeds to all indicators.

(a) Incoming exchange call. To answer an incoming exchange call, insert the plug of an answering cord into the jack of the calling line. Relay S in the cord circuit operates to earth on the exchange line jack. Contacts of relay S disconnect battery and earth from the cord circuit. Restore the indicator by hand and operate the SPEAK key; operation of the SPEAK key places the operator's circuit across the line; this loop trips the exchange ringing. Ascertain the caller's requirements. If connection with an extension is required, insert the plug of a calling cord into the jack of the wanted extension line and operate the non-locking RING key. Restore the SPEAK key when connection has been established with the extension. It should be noted that an answering supervisory is not received as long as the RING key is operated.

(b) Outgoing exchange call. To make an outgoing exchange call, insert the calling plug into a disengaged exchange line jack, operate the SPEAK key, and

(i) where the public exchange is of the manual type, wait for the public exchange operator to answer,

(ii) where the public exchange is of the automatic type, on receipt of dial tone operate the DIAL CALL key, and dial the required number; if power ringing is used the HAND GENERATOR key must be operated before the DIAL CALL key to prevent the application of ringing to the calling extension which may be connected to the answering cord.

Each extension circuit contains a supervisory relay A, which, via auxiliary springs on the extension jack, operates the extension indicator. The extension indicator operates when the extension calls and restores when an answering cord is inserted in the extension jack. When the operator calls an extension, the extension indicator remains operated until the extension answers. During extension-to-extension calls relay S in the cord circuit is normal. Contacts of relay S extend battery and earth, via retard coil LA, to the cord circuit and thus to the extension telephones which are, in effect, in parallel.

Mains failure
At installations without a standby power supply the NIGHT SERVICE key in that operator's circuit must be short-circuited. In the event of mains failure the NIGHT SERVICE keys should be operated. Operation of these keys disconnects the retard coil LA, which would otherwise shunt the operator's circuit and impair its transmission performance. The operator may then receive and extend incoming exchange line calls, or leave exchange lines permanently connected to selected extensions. Communication between extensions is not possible.

Issue No.1 of this E.I. was in 1939 and the earlier model was in Technical Instructions XXIII dated 1924, when the Switchboard was known as the Switchboard B.E., C.B. 3 + 10/25 & 3 + 25/25.

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Last revised: November 27, 2010