Cord-type PMBX
Indicator Signalling
with Automatic, CB, or Magneto exchange lines

n712.jpg (10784 bytes)This switchboard may be used either as a private exchange with magneto extension lines only, or as a private branch exchange connected to an automatic, C.B. or magneto public exchange. Twenty automatic, C.B. or magneto exchange lines can be accommodated; magneto exchange lines may be either additional lines or converted extension lines. Extension capacity is 100 with 17 cord circuits, but stock units are available with 50 or 70, extension lines, 10 or 12 cord circuits and wired but not equipped for exchange lines.

For installations from 100 up to 200 extensions, two units may be bolted together to form a two position, non-multiple switchboard. When up to the maximum of 400 extensions is required, four units, with provision for multiple working, can be similarly installed. Where two or more positions are installed en suite, each is equipped with longer connecting cords and double pulley weights.

For magneto working, one speak/ring key per cord circuit together with a common ring back key are fitted. When auto exchange lines are to be connected, additional equipment includes one dial key per line and a dial.


  1. Single supervision when either party rings off.
  2. Ring back.
  3. Audible alarm.
  4. Visual ringing indication.
  5. Provision for automatic, C.B. or magneto exchange lines.

A welded pressed-steel frame with integral plinth forms the main structure and component mounting, and is enclosed by laminated-wood panels. These are surfaced with hard-wearing, buff-coloured plastic, edged and secured with metal trims finished in glossy elephant grey stove enamel. In contrast, the face panel is coloured black and the key shelf buff linette. When two or more units are installed en suite adjacent intermediate side panels are removed and the units bolted together. Similarly, the front panel below the key shelf and the rear door can both be re- moved for convenience of access.

The face panel has indicators arranged in two groups on either side of the jack strips, and includes provision for equipment for other facilities as required. In addition, the jack panel has capacity for up to 100 multiple jacks for use when switchboards are multipled together. The key shelf is arranged with supervisory indicators and switching keys for cord circuits and miscellaneous facilities. Internal equipment is positioned and mounted for maximum accessibility. Incoming cables enter via covered apertures in the open constructed plinth, and terminate on soldered terminal fields positioned at the bottom of the unit.

For convenience of handling (e.g. during installation) the hinged key shelf can be raised and its apron lowered to reduce the depth by 8in (216mm). The weight of stock units varies depending upon the extension-line capacity.

Switching keys and signalling indicators are of miniature type. The wedge-shaped ivory-coloured key handles are tipped in contrasting colour, rendering operated keys instantly discernible. Indicators have twin shutters which are restored by pressing the plastic window and, when operated, display a black numeral on a back-ground of the same colour as the associated jack label.

Operating equipment, supplied as standard, includes a lightweight plug-in handset with capsule receiver and a manually operated ringing-current generator. Alternative equipment, available in lieu of or in addition to, is a lightweight plug-in headset and a transistor ringing unit powered from a separate 6V source. On switchboards with auto exchange-line connection, a dial adjusted to transmit 10 pps with 2:1 break/make pulse ratio and with standard numbering, i.e. 1 to 0, is supplied as standard. Alternatively, a dial with any required adjustment and numbering can be provided if required.
Internal components are to British Post Office specification and are afforded tropical finish, i.e. with coils suitably impregnated and metal parts specially finished. Conductors are insulated with p.v.c. giving protection against moisture, fire, attack by insects and fungoid growth. Connecting cords are additionally protected by p.v.c. sleeving and nylon braiding.

A 3 or 4.5V transmitter battery and a 3V night-alarm battery are required and may be housed within the switchboard.

Line Limits
For exchange-line working, using magneto Etelphones (catalogue sheet 7145) as extension instruments the maximum permissible loop resistance between the public exchange and extension is 1000 ohms.

Height: 48in (1220mm)
Width: 30in (772mm)
Depth: 32in (813mm)

Weights (approx.)
1 Switchboard, between 1881b (85.3kg) net and 205lb (93kg) net
1 Switchboard, export packed,
3451b (156.5kg)

Plessey Publication No. 7134 (1970)

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Last revised: October 12, 2019