This private manual branch exchange switchboard with capacity  for two exchange lines, six extensions and three connecting  circuits has been designed in   accordance with modern needs.  Compared with earlier types of   switchboard, it offers the attractive combination of  improved facilities   and compact design, both features having been obtained largely  through the use of miniature keys  and 4-wire extension lines. The switchboard operates from a 50V Apparatus supply and its physical design  features are similar to the 3 + 1 2 and 4 + 18 types in the cordless  PMBX range.

Up to half the extension line  circuits can be converted to 2-wire working by the use of wall mounted auxiliary relay units of which there are five types for:

1. Long extension lines.
2. Short and medium distance  private wires, tie lines and PBX  junctions.
3. Long-distance private wires, tie lines and PBX junctions.
4. Medium - distance lines to PABX.
5. Long-distance lines to PABX.

Construction and Finish
The switchboard is in two-tone grey with matching Etelphone as the operator's instrument. The lift-off switchboard cover is a one-piece moulding made of impact-resisting light-grey plastic, whilst the plastic-faced metal equipment panel and the metal base are elephant grey. The front and the rear equipment mountings hinged on the base can be opened book fashion for maintenance.

The switchboard is fully tropicalized and uses p.v.c. insulated wire throughout.

Apparatus
Keys are miniature type N9300 with ivory-coloured wedge handles tipped with inserts of contrasting colours to assist accurate operating. The lamps are behind a lens strip held by clips at the ends. When this strip is removed, the lamp- jack assembly can be pulled forward after releasing its fixing screws. Relays are Types N45000 (BPO 600) and N30000 (BPO 3000) protected by strip covers.

An internal terminal field permits direct termination of incoming lines. Alternatively, for plug-in switchboard connection, a 6ft (1.83m) flexible cord with matching plug can be supplied, together with an associated line jack.

At installations where no source of power ringing exists, a wall mounted ringing converter, operating from 200-25OV, 50Hz a.c. mains, can be supplied. Alternatively, a wall-mounted transistor ringing unit can be supplied.

Facilities

1. Lamp signalling on all lines.
2. Individual clearing signals on extension-to-extension calls.
3. Secretarial hold, enabling the operator to hold the exchange and consult an extension without being overheard.
4. Press button operator recall on extension and exchange calls.
5. Night service.
6. Power-unit operation with mains failure safeguards.
7. Extension-to-exchange line limit of 1000 ohms, or 900 ohms if 4-wire/2-wire auxiliary unit is used.
8. Connection of private circuits and inter-switchboard circuits without modification to the permanent wiring of the switchboard.

Power Details
The required 50V d.c. supply is usually derived from the a.c. mains via a battery eliminator. Full load current drain: 1A without auxiliary unit, 2A with auxiliary unit.

Extension Telephones
These are Plesseyphones (catalogue sheet 7159) or Etelphones (catalogue sheet 7145), arranged for use with the switchboard 4-wire lines. For auto working, dials are adjusted to transmit 10 pps with a 2:1 break/make ratio, and have standard numbering (i.e. 1 to 0).

Switchboard Dimensions
Height: 6.5 in (155mm)
Width: 9.5 (24lmm)
Depth: 11 in (279mm)

Weights (approx.)
12lb (5.4kg) net.

Plessey Publication No. 7128 (1971 )

Last revised: October 12, 2019