C MARKETING - INSTALLATION
Issue 1, Dec 1970
This Instruction describes Switchboards, AT 3796, which are floor-type
double-cord switchboards for use in either C.B. (manual) or Automatic areas. The
switchboards are available in three sizes 10+30 (65), 10+50 (65) and 10+60 (180).
On the 10+30 and 10+50 the strips of jacks associated with the exchange lines is mounted
at the bottom of the jack field, the corresponding indicators being fitted above the jack
field, each indicator vertically above the corresponding jack.
In the 10+60 switchboard the exchange line jacks are fitted above the extension jacks and
immediately below the corresponding exchange line indicators.
The wiring of the switchboards is complete for the maximum
capacity except for exchange lines where only 10 are wired and extension circuits on the
10+60 size, where only 120 circuits are provided. Wood spacing-strips occupy the position
of indicators and jacks not provided.
Mark 3 - 1953
Mark 3 - 1957
|Wiring diagrams - all circuits
|Wiring diagrams -
|Cord Circuit - divided feed
|Inter PBX pw's
||N 721 - N 729
|Inter PBX extn's
||N 775 Sig. Grp. A(i)
|Inter PBX extn's
||N 778 Sig. Grp. A(ii)
|Inter PBX extn's
||N 779 Sig. Grp. H
|Cord-circuit modified for secrecy against operator
|Modification for high-impedance monitoring
|S.T.D. metering, Schemes A, B and D1
|Schemes C2 and D2
|Modification to provide transmission to the operator
during P.B.X. power supply failure
Transmission and signalling limits.
The maximum permissible resistance of extension lines and of exchange plus extension lines
is given in EI Transmission, Teles B 3536; for inter-P.B.X. extensions see EI
Transmission, Teles B 3590.
In future, Switchboard, AT 3196 will not be modified for divided-feed working.
When signalling limits are exceeded, Switchboard N 1070 must be used, if only a
few extensions are beyond limits Unit, Auxiliary Apparatus No. 18 or 13A U.A.A. CBS 536 may be used.
Design and equipment
The switchboards are designed to stand on the floor, and holes are provided in both sides
for leading in cables. In later versions of these switchboards an additional cable entry
is provided at the bottom of the switchboard. There is no projecting apparatus on either
side of the switchboard and the skirtings are detachable: it is, therefore, possible to
install two switchboards in a continuous line.
The exchange lines terminate on hand-restored drop-indicators on the P.M.B.X., and the
extension lines on dolls-eye indicators. Double cord-circuits are provided - each equipped
with a RING and SPEAK key, DIAL and RING-BACK key, and two supervisory indicators. The
supervisory indicators, together with the keys, are fitted in the keyshelf.
A Key-stop No. 1 is fitted to the DIAL and RINGBACK key on switchboards to be used in
manual areas, to prevent movement of this key to the DIAL position.
Keys are provided in the face of the switchboard to disconnect the battery feed from the
first ten cord-circuits. Any extension can, therefore, be left connected to any exchange
line for night service via any of these cord-circuits.
The standard operating instrument consists of a combined transmitter and receiver headset
(Headset No. 1), connected to a four way plug.
Supervisory signals are of the negative type, disks being displayed while a call is in
progress and automatically restored at the termination of the call. Double clears are
given on exchange connections; on local calls and on inter-P.B.X. circuits individual
supervision is provided.
Ringing Facilities for ringing, either by hand generator or from power
ringing lead, are included in the equipment of the switchboard. Dgm. N 1093.
A bell and key are included in the equipment of the switchboard. Dgm. N 1093.
The cord test jack, JKD, is the last position of the jack No. 510 fitted above the
exchange line jacks. The circuit is shown in Dgm. N 1093, Fig. 5.
Later supplies of switchboards are provided with a dial and dial mounting. In manual
exchange areas the dial and dial mounting should be recovered and a Plug, Dummy, No. 1
(a) Incoming exchange call
To answer an incoming exchange call, insert the plug of an
answering cord into the jack of the calling line. R elay S in the cord circuit operates to
the earth on the sleeve of the exchange line jack. Contacts of relay S extend a holding
loop to the exchange line. The answering supervisory indicator operates to exchange
battery and earth via the holding loop. Restore the exchange line indicator by hand,
operate the SPEAK key and ascertain the demand. If connection with an extension is
required, insert the plug of the calling cord into the jack of the wanted extension and
momentarily operate the RING key. When the called extension answers the calling cord
supervisory indicator operates. Then restore the SPEAK key. Relay C operates via the
supervisory indicator contacts to the earth on the sleeve of the exchange line jack and
disconnects the cord circuit hold loop. The call is now established.
(b) Outgoing exchange call
To make an outgoing exchange call, insert a 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 the dialling tone
operate the DIAL key and dial the required number.
The cord-circuit operation is similar to that described in (a).
(c) Extension-to-extension call
On extension-to-extension calls relay S does not
operate. Its unoperated contacts provide parallel-feed battery and earth to the
Power supply data
Information for the calculation of power requirements is given in Q 6040.
Here the switchboard is provided with an all mains power unit the operator's circuit
should be modified as shown in Dgm. N 1099 (on later supplies of switchboards this
modification will have already been carried out). The modification applies only to the
10+50 and 10+30 sizes of switchboard. During power failure, relay MF in the switchboard
releases and provides for the operator's transmitter to be powered from an exchange line. Under power failure conditions 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, make outgoing exchange line calls, or leave exchange lines
permanently connected to selected extensions. Communication between extensions is not
Taken from EI Telephones PBX's B 1030.
Made by British Ericsson, their model No.