3 Internal
E 4070
Dec 1970, Issue 1

Switchboard N 1070

Switchboards N 1070 have been introduced for use, in place of Switchboards AT 3796, where the extension or exchange line resistances are in excess of the signalling limits for Switchboards AT 3796.  They supersede for new work Switchboards AT 3796 modified to Diagram N 1096.  These latter switchboards, while they increase the extension to extension signalling resistance limits, do not give any increase in the exchange to PBX, or exchange to extension, signalling-resistance limits which, for the AT 3796 switchboards, are lower than for other types of PBX.

The N 1070 - 10+60(180) is shown pictured to the right.

Physical Design
The outward appearance, face-equipment layout, etc. of the new switchboard are identical with that of Switchboards AT 3796.

Equipment, Capacity, Dimensions and Weight.
The Switchboard N 1070 is issued in the same three sizes as Switchboards AT 3796, viz. 10 + 30(65), 10 + 50(65), 10 + 60(180), each size being approximately 30lb heavier than the weight quoted in TI E4060 for the equivalent size of AT 3796 switchboard.

The facilities offered by the Switchboard N 1070 are similar to those described in TI E4060 for Switchboards AT 3796 with the exception that only the first five cord circuits are wired for night service working.

Cord circuit - N1070
Wiring diagram (all circuits) - LD 133

Circuit Details
The cord circuits give "divided feed" working on extension to extension and extension to private circuit calls, but revert to through-signalling conditions on exchange line connections.  Basically, the cord circuit for the N 1070 type switchboard is similar to that for the AT 3796 type and, for signalling purposes, the limiting factor is the current requirement for the satisfactory operation of the supervisory indicators In the cord circuit, i.e. 30ma.  The extended limits on exchange calls have been obtained as follows:-

(a) Exchange to PBX calls
The local transmission bridge is cut out on the operation of relay AS to earth on the exchange line jack sleeve, and a loop. is offered to the exchange.  The resistance of this loop is the resistance of the bridging coil (L2) and supervisory indicator only and is thus 160 ohms less than the loop offered by the Switchboard AT 3796.

(b) Exchange to extension calls
The supervisory indicator on the extension side of the cord circuit is operated to the PBX battery when the extension answers and, after operating, holds to the public exchange battery.  The contacts of relay CS prevent the extension circuit being shunted by the bridging coil (L2) before the extension answers, at which point the circuit is switched through and the bridging coil is disconnected.  By this means, satisfactory operating current for the supervisory indicator is obtained with a longer extension circuit than is possible with the Switchboard AT 3796 modified to Diagram N 1096.

Apparatus required
Box Connexion No. 3A.
Headset No. 1.
Mounting Dial No. 9.
Dial, Automatic No. 14.
Case No. 9.
Glasses No. 21A (in the case of a blind operator).



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Last revised: November 03, 2021