TELEPHONE No's N1674 & N1675

2 + 10 House Exchange

n1674b.jpg (10727 bytes)This system provides ten stations with intercommunication facilities and access to two external circuits, which may be automatic or C.B. public exchange lines or extension lines of PBX systems. It employs press button intercommunication and exchange-line access, and ideally suited to organisations with an anticipated high calling rate between stations.

Direct press button intercommunication.
Up to five simultaneous conversations.
Direct press button access to exchange lines.
Lamp indication of exchange-line conditions.
Secrecy on exchange- line calls.
Enquiry calls (exchange line held).
Call transfer.
Operator recall on lines to PBXS.
Exchange bell on/off.
Conference calls from any station to all or selected stations.
Continued exchange-line service if power fails.
Night-service working.
Barred or restricted exchange-line access.

Station Multiple
Station telephones are connected in order of priority by a maximum of 400yd (365m) of cable. The first station on the multiple has priority over all other stations in making and answering exchange-line calls; when in use, it disconnects all other stations from the line. Similarly, the second station disconnects stations 3 to 10 and so on.  Cabling between stations is direct except where a station is inconveniently situated. A spur cable is then used to minimise the length of the main cable; this does not affect the order of priority.

Station Telephones
These instruments have been approved by the Council of Industrial design and are available in black, grey and ivory. For convenience of installation and maintenance each telephone cord plug connects to an associated desk/junction box. Both the instrument cover and handset are moulded from Acrylonitrile Butadiene styrene (ABS), a material noted for its high gloss, light fastness, and impact strength. The telephone interior, which is built on a pressed-steel base, includes a d.c. buzzer and an a.c. ringer, together with printed-wiring panels, carrying:-

  1. transmission components, including plug-in line regulator.
  2. intercom-signalling contact springs. These engage with the printed wiring on depression of an intercom button and require no tensioning as necessary with conventional springsets.

Press buttons
These total 15 on the press button panel; ten for intercom calls, two for exchange lines (labelled 1 and 2), two for exchange line release (labelled R) and one for conference (labelled C). Provision is made above the panel for four inline buttons for bell on/ off and other ancillary operations such as applying restricted exchange line access to a station user.

Designation Labels
Slide-in reversible paper labels numbered 1 to 5 and 6 to 10 and protected by plastic windows are provided for the intercom keys. Space is left for writing the names of persons or departments on the labels.

Two pairs of lamps ('exchange' and 'seize') serve for exchange line visual signalling and supervision. The seize lamp gives a steady signal at the originating station on seizure of the exchange line, whereas the exchange lamp gives the following three signal
indications at all stations:-

  1. A pulsating signal at ringing periodicity until an exchange call is answered.
  2. A steady glow during an established exchange call.
  3. A 'wink' when a station holds an exchange line while making a call to another line. This signal becomes steady when the user returns to the held line.

Desk/Junction Box
This houses screw terminals for the main cable and a 56-way connector for acceptance of the plug ended telephone cord. Where a station is connected via a spur, the spur cable is connected to the main cable at an auxiliary junction box similar to the desk/junction box.

Exchange-line Relay Unit
This unit accommodates relays for exchange-line switching and lamp signalling, together with a transistor device for regulating the current supply for the signalling lamps.   Components mount on a raised platform, hinged to the base to facilitate access to the relay wiring. Cable entry is via rubber grommets in the base.

Installation and Cabling
Systems to be connected to exchanges under the jurisdiction of the British Post Office (or similar administrations overseas) are supplied and installed by the administration. Systems not in this category can be customer installed.

A mains-operated power unit is normally provided, giving a smoothed regulated 50V supply of up to 2.7A.
Note: the call lamps are fed via a transistor current-regulating device which ensures long lamp life.

Equipment Code Numbers

Equipment   Code No's GPO No.
Telephone   N1674B (automatic) Tele Intercom No. 4/1L
Telephone   N1675B (C.B.) Tele Intercom No. 4/1CB
Desk/junction   N3317A
Exchange-line relay unit   N24213C
Power unit   N22404A

Plessey Publication No. 7132 (1971)

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Last revised: October 03, 2019