3 Internal
E 4060
Issue 1, Dec 1970

Switchboard AT 3796

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.

Mark 3 - 1953 Mark 3 - 1957 Mark 3 - 1963

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.

Size Exchange Lines Extensions Cord Circuits
Capacity Equipment Capacity Equipment Capacity Equipment
10+30 15 10 30 30 15 11
10+50 15 10 50 50 15 15
10+60 20 10 160 60 18 15

These are shown in the Table below:-
Size Weight Dimensions
ft. In. Ft. in. ft. in.
10+30 280 4 7.25 2 1 2 2.5
10+50 309 4 7.25 2 1 2 2.5
10+60 336 5 1.25 2 2 2 6.5

These are shown in the Table below:-

Circuit Diagrams

Wiring diagrams - all circuits - Mark 1 and Mark 2 LD 41/0
Wiring diagrams - all circuits - Mark 2 and Mark 3 LD 41/1
Wiring diagrams - Explanatory N 1093
Cord Circuit - divided feed N 1096
Inter PBX private wires N 721 - N 729
Inter PBX extensions N 775 Signalling Group A(i)
Inter PBX extensions N 778 Signalling Group A(ii)
Inter PBX extensions N 779 Signalling Group H
Audible clearing SA 2050
Conference facilities:-
Amplified, cord-connected SA 7091
key-switched SA 7093
Unamplified, cord-connected SA 7090
key-switched SA 7092
Cord-circuit modified for secrecy against operator N 993
Modification for high-impedance monitoring SA 7101
Press-button recall N 852
S.T.D. metering, Schemes A, B and D1 N 1021
Scheme C1 N 1022
Schemes C2 and D2 N 1023
Modification to provide transmission to the operator
during P.B.X. power supply failure
N 1099

Transmission and signalling limits.

The maximum permissible resistance of extension lines and of exchange plus extension lines is given in EI Transmission, Telephones B3536; for inter-P.B.X. extensions see EI Transmission, Telephones B3590.

In future, Switchboard, AT3196 will not be modified for divided-feed working.  When signalling limits are exceeded, Switchboard N1070 must be used, if only a few extensions are beyond limits Unit, Auxiliary Apparatus No. 18 or 13A U.A.A., CBS536 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.

Night service
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.

Operator's telephone
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. Diagram N1093.

Night alarms
A bell and key are included in the equipment of the switchboard. Diagram N1093.

Cord test
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 Diagram N1093, 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 inserted.

(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 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 un-operated contacts provide parallel-feed battery and earth to the extensions.

Power supply data
Information for the calculation of power requirements is given in Q6040.

Mains failure
Here the switchboard is provided with an all mains power unit the operator's circuit should be modified as shown in Diagram 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 possible.

Taken from EI Telephones PBX's B 1030.

Made by British Ericsson, their model No. N308A their Diagram Number N72537 (Dated 1948).

UK Military model - YA 3778.

The Mark 1 was introduced circa 1935.

Drawings - 9475, 63481, 60813_2_2, 60813_2_3, 60813_2_4, 60814_1_1, 60814_1_2, 60814_1_3 and 60814_1_4.

Specification - S99 and S428 (Mark 2).

Keyboard - The covering is 1/16" red fibre and the end capping is 3/32" red fibre.

User Guide

Additional Pictures

At suite of two 10+50 switchboards at Messrs Eyre and Spottiswoode, Printers
Note the breast plate transmitters
Picture dated 1935


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