Cordless PMBX
Lamp Signalling with Automatic or C.B. exchange lines

n116.jpg (15012 bytes)This private manual branch exchange switchboard with capacity for three exchange lines, twelve extensions and five 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 supply and its physical design features are similar to the 2 + 6 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. The lift-off 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 as shown in the illustration overleaf.

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

Keys are miniature type N9300 with ivory-coloured wedge handles tipped with inserts of contrasting colours to assist accurate operation. The lamps are behind a lens strip me 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 200-250V, 50Hz a.c. mains, can be supplied. Alternatively, an in-built or wall mounted transistor ringing unit can be supplied.

The separate two-tone grey table type Etelphone provided for the switchboard operator can be arranged for use in conjunction with an operator's lightweight headset, if required.


  1. Lamp signalling on all lines.
  2. Individual clearing signals on extension-to-extension calls.
  3. Automatic holding of exchange calls.
  4. Follow-on call trap on incoming exchange calls.
  5. Pressbutton operator recall on extension and exchange calls.
  6. Night service.
  7. Power unit operation with mains failure safeguards.
  8. Extension-to-exchange line limit of 935 ohms, or 850 ohms  if 4-wire/2-wire auxiliary unit is used.
  9. 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).

Height: 8in (203mm)
Width: 17in(435mm)
Depth: 13.5in (346mm)

Weights (approx.)
36lb (16.3kg) net.

Plessey Publication No. 7129 (1970)

Click here for an article from the Ericsson Bulletin

Click here for the GPO Switchboard No. 2/3

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Last revised: October 12, 2019