The facilities of much larger P.A.B.X.'s are given by this small single-unit system for two exchange and seven extension lines. it provides for three simultaneous conversations - two external and one internal. Enquiry calls can be made by extensions engaged on exchange calls without using the local link, and exchange calls can be transferred.

Being entirely extension-operated and employing simple components of known reliability, the 2+7 PABX offers the advantages of dependable 24-hour service and low running costs. It is completely self contained in a cabinet which can be table or wall mounted.
Component finishes and p.v.c. insulation on connecting wires are suitable for tropical conditions.

Maximum extension-line loop resistance, including the telephone, is 500 ohms. Exchange-line loop resistance is up to 1000 ohms depending upon the limits imposed by the public exchange equipment. Extensions are numbered 1 to 7.

Facilities

1. Extension - to - Extension Calls a redialled direct.
2. Outgoing Exchange Calls are made by pressing the button on the extension telephone.
3. Incoming Exchange Calls are signalled on a special bell or bells and answered by any extension.
4. Enquiry Calls, with exchange line held, can be made by extension to extension.
5. Exchange Call Transfer by any extension to extension.
6. Intrusion, for enquiry or exchange call transfer, on local connections.
7. Exchange-Line Switching to predetermined extensions under mains-failure conditions.
8. The Internal Link is not used when setting up or transferring exchange calls or making enquiry calls, nor is it held if a handset is not replaced.

Equipment
The apparatus, including power supply and ringing and tone generators, is contained in a brown and cream enamelled sheet-metal cabinet having two removable hinged covers which open out- wards from the centre. The components are on two frameworks which back on to each other; the one on the right-hand side is hinged on the front edge to allow access to wiring and terminating strips behind. Cables enter through the base of the cabinet via an aperture in the rear face of a shallow plinth.

The switching mechanisms are heavy-duty uniselectors, common-yoke line relays and conventional BPO relays for other circuits. Under normal conditions the components should require little more than cleaning and lubrication. Ringing current is obtained from a converter fed from the 200V tappings of the power-supply transformer. The output of the transformer is also used for the derivation of dial and ring tones.

Busy and intrusion tones are obtained from a transistor-type oscillator.

Power Supply
The power-unit transformer input is stepped for a.c. mains voltages between 100-125 and 200-25OV, 50/60Hz. Smoothed outputs of 40/55V, 2A d.c. and 4.23V a.c. are provided.

Telephones
BPO Type 746 telephones (catalogue section 5, sheet 7281 ) or Etelphones (catalogue section 5, sheet 7145) are recommended for extension telephones. However, any telephones suitable for 2-wire and earth or 3-wire working may be used, provided it is equipped with a non-locking press button and incorporates a dial with a 2:1 break/make ratio and pulsing
speed of 10 p/s.

Dimensions
Height: 28.3in (721 mm)
Width: 12in (31l mm)
Depth: 14in (356 mm)

Weights (approx.)
PABX unit: 120lb (54.4kg) net

When ordering, please specify PABX code number N24146A with extension lines as required (7 or 10).

Taken from Plessey Publication (1970)

Last revised: October 07, 2019