Issue 1, Dec 1970
PRIVATE MANUAL BRANCH EXCHANGES
Switchboard CB 873
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.
The 1928 Rate Book
instruction advised that an earlier Switchboard was available that was
equipped with 3 exchange lines, 10 extension circuits and 5 cord
circuits, with provision for a maximum of 5 exchange lines, 20 extensions and 10 cord circuits.
Diagram CB 874 was initially dated 1914 and described two switchboards with
the maximum capacity of 25 but supplied to the field as 3+10 and 5+20.
|3+10 (Picture dated 1919)
||5+20 (Picture dated 1924)
||5+20 (Picture dated 1938)
|5+20 Mark 5 (Picture dated 1953)
||5+20 Mark 8 (Picture dated 1962)
||5+20 Mark 8 (Picture dated 1963)
Dimensions and weights
Height - 4ft 3ins.
Width - 1ft 7ins.
Depth - 1ft 8ins.
Weight (3+10 (25) - 135 lbs.
Weight (5+20 (25) - 164 lbs.
|Switchboard Ccts - explanatory (3+10)
|Switchboard Ccts - Wiring details (3+10)
|Wiring diagram-all circuits (Mark 7 & 8)
|Wiring diagram-all circuits (Mark 5)
|Cord circuit (explanatory) (5+10)
|Cord circuit, divided-feed (5+10)
|Exchange line, extension line, operator's circuit, dial circuit,
ringing circuit, hold jack and cord test, night alarm (explanatory)
|Long extension or private circuit
||N 986 Sig. Grp. B
N 715 Sig. Grp. B
N 718 Sig. Grp. G
|Inter-P.B.X. private circuit
||N 710 Sig. Grp. A (i)
N 713 Sig. Grp. A (ii)
N 716 Sig. Grp. D
N 717 Sig. Grp. E
N 719 Sig. Grp. H
||N 770 Sig. Grp. A (i)
N 773 Sig. Grp. A (ii)
N 774 Sig. Grp. H
|Earth dialling to P.A.B.X.s
Unamplified, cord connected
|S.T.D. metering, Schemes A, B and D1
|S.T.D. metering, Scheme C1
|S.T.D. metering, Schemes C2 and D2
|Cord circuit modified for secrecy against operator
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
Diagram 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 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
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.
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 Diagram 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
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.
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.
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.
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.
This Switchboard requires a power unit rated at 50 volts DC. The 1939
instruction advises that the Switchboard would be powered from one set of
fifteen "Cells Leclanche No. WK1".
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
Issue No.1 of this Engineering Instruction was published in 1939.
Before this date switchboards were in Technical
Instructions XXIII dated 1924 and 1930. In the 1924 publication this Switchboard was known as the Switchboard B.E., C.B., 3 + 10/25 & 3 + 25/25.
The 1928 Rate Book advises that there was a Mark 234 and a Mark 235.
In general there was little difference, but the Mark 234 used 5 black and 5
red cords, whilst the Mark 235 used 10 red cords and different key
Facing woodwork is Honduras Mahogany, Alba Mahogany or
African Mahogany (Lagus or Benin). No exposed woodwork is plywood.
Keyboard coving is 1/16" red fibre (semi-matt).
Made by British Ericsson - Model No. N203B - their Diagram Number N73077
UK Military part - YA6257.
Drawings - 7429 and 76730.
Specification - S32.
Telephone No. 164.
Mounting, Dial No. 8.
Dial, Automatic No. 14.
Box Connexion No. 2A.