PMBX No. 2/4

Descriptive Leaflet

3 Internal

Private Manual Branch Exchange No. 2/4
(P.M.B.X. No. 2/4)


 The Switchboard PMBX, No. 2/4A is a lamp-signalling cordless switchboard with seven connecting circuits and has a capacity for four exchange lines and eighteen extensions. The switchboard has been designed to stand on a table or desk.

Each extension telephone is normally connected to the switchboard by four wires (A, B, C and an earth). External extensions may however, where necessary, be provided on a 2-wire basis by fitting a Unit, Auxiliary Apparatus, No. 96A at the PMBX end of each such extension. Similarly inter-PBX circuits can be connected via a suitable Unit, Auxiliary Apparatus, e.g. No. 97, 98 or 99. The switchboard can be used in CB and automatic exchange areas, and also, with Unit, Auxiliary Apparatus, CBS 536 fitted at the public exchange, in CBS No. 1, 2 and 3 areas.

Power and ringing supplies are normally obtained from mains-operated units but may be derived from secondary cells when standby power is required.

The switchboard is coloured two-tone grey and a matching Telephone No. 706... is normally used as the operator's telephone. The switchboard is 22 in wide by 9 in high by 15 in deep and it weighs 56 lb. It has a metal chassis and an acrylonitrile-butadiene-styrene copolymer (ABS) cover.

The lamp cover strips are removed by sliding the phosphor-bronze clips at each end inwards towards the centre of the switchboard and pulling forward. On removal of the lamp cover strip the two lamp-jack fixing screws are exposed. By unscrewing these screws the lamp jack is drawn forward. The lamps can then be removed without the use of a lamp extractor or removal of the switchboard cover. The individual lamp prisms are a push fit and can be readily removed by pressure on the rear. A non-metallic instrument should be used for this purpose to prevent scratching the prism. A new lamp prism can be inserted from the front by the application of slight pressure.

There are two designation strips fitted on the face panel, one for the extensions and the other for the exchange lines. The extension designation strip (Strip, Designation, No. 40A) is removed by applying pressure on the lower edge and sliding the strip upwards towards the lamp cover strip. This disengages the fixing clips attached to each half of the designation strip mounting from slots in the face panel. The paper label, which is printed on both sides, can then be removed for marking as required. With the extension designation strip removed access is given to the label for the exchange lines, which can be removed by sliding it out of the right-hand side of its mounting.

Subscriber's private meters of the cyclometer trip type (Meters No. 21B, Grey) can be provided on the face panel in the space normally occupied by the exchange line designation strip. When these are required the existing designation strip (Strip, Designation No. 41A) is removed by pivoting the locking projection on the under side, and is replaced by a narrow strip, (Strip, Designation, No. 35A), which covers the upper part of the meter front. In addition a Strip, Designation, No. 34A is fitted above the FREE LINK signalling lamps. When replacing designation strips care should be taken not to damage the panel surface. For details of metering schemes see E5001 and for the application of these schemes to the switchboard see Diagrams N1008 and N1009. Metering connections within the switchboard are made on the two auxiliary terminal strips fitted on the key panel adjacent to the exchange line keys.

The cover is held in position by an angle bracket which pulls the cover to the rear by means of the screws in the flange at the rear of the baseplate. To remove the cover the screws (which are captive) are unscrewed, and the latching bracket springs clear of the projections on the inside of the cover. The leading edge of the cover fits under the front of the face panel and adjustable plates, which are positioned on each side of the panel form dust seals.

The chassis consists of three sections, the front and rear panels hinging on the baseplate, forming a triangle when the chassis is closed' By releasing the screws at the apex, the hinged panels can be opened outward to give access to the wiring and components.

The relays in the switchboard are standard PO 600 and 3000 type and are mounted on the rear panel. A metal cover, retained by a spring clip at each end, is provided to protect the relays.

Two terminal blocks (Blocks, Terminal Nos. 47A and 47B) with 55 terminals each are mounted on the switchboard base immediately below the relay mounting plate. The conductors of a Connector No. 1100A are terminated on 100 of these terminals. The free end of the connector is plug-ended and jacks into a wall- mounted socket to which the permanent cabling is terminated. Terminals are also provided for the connection of the operator's telephone. The remaining terminals on the terminal blocks, together with the tags on the Strip, Connection No. 153/7B which is also provided on the base of the switchboard below the face panel, permit the termination of 2-wire extensions, inter-PBX extensions or private circuits using associated Units, Auxiliary Apparatus, without modification to the switchboard permanent wiring.

Cordage enters the switchboard via the two entry holes in the flange at the rear of the base. When viewed from the rear, the right-hand hole is for the operator's telephone cord and the second hole is for the main 100-way cord. A dummy grommet is provided on the left-hand side of the baseplate when viewed from the front. The dummy grommet is removed and a Jack No. 84C fitted when a Headset No. 1 is required in addition to the operator's telephone. Wiring is provided in the switchboard for the connection of this jack. When a Headset No. 1 is fitted the operator's telephone is changed to a Telephone No. 710; for details see Diagram N1104.

Circuit connections Four-wire extension circuits may be connected on all extension positions. Private circuits, inter-switchboard private circuits, inter-switchboard extensions, and 2-wire extensions must, however, be connected on extension positions 10-18.

A Unit, Auxiliary Apparatus, No. 96A should be provided on all 2-wire extensions and terminated in accordance with Diagram N1128.

Private circuits etc require the provision of a Unit, Auxiliary Apparatus No. 97, 98 or 99. Details of the particular N diagrams applicable to any signalling group are given in F0402.

Lamp calling on exchange lines and extensions
A Lamp No. 2-45V which has a reasonably flat lumen/resistance response is used to eliminate the need for a line-calling relay on extensions. The lamp is connected in series with the battery feed via the pilot relay and glows when the extension loop is completed by removal of the telephone handset at the extension. The pilot relay is shunted by an electrolytic capacitor to prevent overhearing between calling extensions. The exchange lines use the same type of lamp and a locked signal is given when the signalling relay operates to an incoming ring. In order to reduce the amount of series resistance added by the pilot relay it has a low-resistance winding which is connected in parallel with the operating coil when the relay operates. If the calling lamp of an exchange line becomes disconnected an audible alarm is given (provided the ALARM ON key is operated) as the holding circuit for the signalling relay is via the pilot relay.

Clearing and recall
The use of the 4-wire extension principle separates the functions of transmission and supervision. As a result it has been possible to arrange that individual clearing and recall signals are given by each extension lamp.

A clear is given when the extension handset is replaced on both extension-to-extension and extension-to-exchange calls. On 4-wire extensions control is by an additional gravity switch spring-set in the extension telephone. On 2-wire extensions the control is by a supervisory relay in the Unit, Auxiliary Apparatus, No. 96A (see Diagram N1104 explanatory figures).

Press-button recall
This is a standard facility and a switch is provided in accordance with Diagram N806 on each extension telephone. Recall is provided on 4-wire extensions by connecting the make contact of the switch in parallel with the additional gravity switch spring-set provided in the extension telephone. On 2-wire extensions where a Unit, Auxiliary Apparatus, No. 96A is provided the press-button is arranged to earth the telephone loop and operate a differentially connected relay in the unit; the relay contacts operating complete the extension signalling lamp circuit. The facility is effective on both extension-to-extension and exchange-to-extension calls.

Holding of exchange calls
Automatic holding of exchange calls is provided on all connecting circuits. If, on an exchange call, the operator restores the operator's key with no extension connected to the circuit, a previously operated hold relay connects a 910ohm resistor across the exchange line to hold the exchange equipment. The hold condition is removed by the re-operation of the operator's key or the restoration of the exchange line key. If, however, the exchange line has been extended to an extension circuit, the holding circuit of the relay is dependent upon the earth extended over the C-wire from the extension telephone auxiliary gravity switch, and the hold condition is maintained on the exchange line while the extension handset is on its rest. The hold condition is removed when the extension answers. If the extension does not answer the hold condition is removed either by the re-operation of the operator's key or the restoration of the exchange line key. Visual indication of a call being held is given by the glowing of the CALL HELD lamp which is common to all connecting circuits.

Exchange and extension line limits
For transmission and signalling resistance limits see E0110.

Follow-on-call trap
When an exchange-to-extension connection is cleared down by replacing the extension handset the 'call-trap' relay is operated in the switchboard. Contacts of this relay disconnect the extension circuit from the exchange line circuit and reconnect the signalling relay to the exchange line. Therefore, if a follow-on-call is received on an exchange line which has not had its exchange line key restored after a previous call, it will be trapped on the signalling relay; the exchange line calling lamp will glow but the extension telephone bell will not ring.

Night service
The NIGHT SERVICE key should be operated and selected extensions connected to the exchange lines whenever the switchboard is left unattended overnight or for long periods. Under night service conditions the pilot relay is disconnected from the extension calling lamps to prevent these from glowing. The exchange line signalling relays are also disconnected to prevent the exchange line calling lamps glowing. A 200k ohm resistor in series with a 2pF capacitor is connected across each exchange line to ensure that the lines do not test as disconnected should the NIGHT SERVICE key be operated without the exchange lines being extended to extensions. Operation of the NIGHT SERVICE key connects an earth to terminal NS of the switchboard which can be used as a switching condition to provide non-standard night service arrangements. Contacts of the night service relay (which is released) provide night service switching arrangements for any Units, Auxiliary Apparatus which may be connected to extensions 10-18. The operator's telephone bell circuit is also disconnected from the first exchange line when the NIGHT SERVICE key is operated.

Prohibition of exchange service on private circuits
These circuits terminate on extensions 10-18 and a prohibition relay is connected to the P terminal of an extension on which a Unit, Auxiliary Apparatus is fitted and strapped to provide exchange prohibition. The contacts of this relay are arranged to disconnect the private circuit from an exchange line when both the exchange and extension keys are operated.

Audible alarm
This is switched on when the ALARM ON key is operated. An additional bell or buzzer can be provided, if required, by making use of the NA terminal. Any type of bell or buzzer suitable for 50v working may be used.

Lightweight headsets
A Headset No. 1 may be provided in addition to the operator's telephone. When this facility is required the operator's telephone is changed from a Telephone No. 706 ... to a Telephone No. 710... and additional spring-sets provided in the telephone to disconnect the headset when the handset is removed from the rest. The Telephone No. 710... is used because its gravity switch has the greater mechanical advantage needed to operate the additional change-over springs required.

Operation under mains failure if the power supply to the switchboard fails at installations supplied by a mains-operated power-unit, exchange connections in progress are maintained but extension-to-extension calls fail. An audible indication of an incoming call on the first exchange line is given by ringing the bell in the operator's telephone. Under power failure conditions the first exchange line is normally used by the operator for both incoming and outgoing calls. The remaining exchange lines may be connected to selected extensions to give normal exchange access.

Free-link signalling
A free-link signal is provided to assist in the selection of a free connecting circuit. Two lamps are provided for each row of keys, one for the link connected by the upward movement and the other for that connected by the downward movement. In the idle condition all lamps are extinguished. On receipt of an incoming call the pilot relay operates and the first lamp glows (connecting link A).

When this circuit is taken into use by the operation of the appropriate exchange line or extension line key, to answer an incoming call, the transmission bridge relay operates. The contacts of this relay disconnect the lamp associated with connecting link A, and connect the lamp of connecting link B which glows to indicate that this is the next free connecting link.

The sequence is repeated as further links are taken into use. When all seven connecting links are engaged the eighth (OVERCALL) lamp glows. Should any further calls then be received, they can be answered by the operator on the overcall circuit.

Should a connecting link be taken into use out of turn this does not affect the order or the display of the signals, i.e. the lamp glowing will always indicate the first free connecting link starting with circuit A. If a circuit becomes free the associated lamp glows if it is the first free connecting link in the sequence.

This enables the operator to answer a call when all seven connecting links are engaged.

  1. To answer an extension call the SPEAK ON OVERCALL key is operated. An additional transmission feed relay now provides current to the operator's telephone and, in operating, its contacts disconnect the ringing supply from the extension ringing key and extend the operator's telephone thereto. The operator can now speak to the calling extension by holding operated the appropriate ringing key.

  2. To answer an exchange call the SPEAK ON OVERCALL key and appropriate exchange line OVERCALL key are operated. Contacts of the latter key disconnect the additional transmission feed relay and connect the exchange line to the operator's telephone. The transmission feed for the operator's telephone is now supplied by the public exchange and a contact of the exchange line OVERCALL key connects a bridging coil to provide a holding condition on the exchange line. This permits the operator to leave the circuit and offer the call to an engaged extension and then if required restore the previously set-up connection and extend the exchange call to the extension.

The switchboard has been designed to operate from a nominal 50v dc supply but is capable of working in the range 45-55v. This supply is normally obtained from a mains-operated power unit but a battery float system is available for use at installations where the limited facilities available under mains-failure conditions are insufficient.

A 25Hz ringing supply is provided by a mains driven ringing converter at installations using power units and by a DC operated ringing converter at installations provided with stand-by batteries.

Details of power and ringing equipment for mains operated power unit installations may be obtained from Diagram N1104 and for battery float system installations from Diagram N2332.

Diagram SA7165 is the schematic diagram for the switchboard and a detailed circuit description is given in Diagram Notes SA7165. Diagram N1104 gives details of the apparatus, circuit elements, installation wiring and cabling instructions and of the operator's telephone circuit (with and without headset).

The switchboard should be installed in accordance with Diagram N1104.

On all internal extensions three wires plus an earth wire are required to each telephone; four wires should also be provided for external extensions subject to the restrictions as detailed for Extension Plans 1A in C0010. For all 2-wire extensions a Unit, Auxiliary Apparatus, No. 96A should be provided and connected as shown on Diagram N1127.

A 50 pair cable to a Box, Connection, No. 5C should be terminated on a Connector No. 101/2A in accordance with Diagram N1104 and in the manner shown on Drawing SD113.

The cabling between the switchboard and the Box, Connection, No. 5C must not exceed 100ft to avoid an excessive voltage drop under full load conditions.

The extension instruments should be connected in accordance with Diagram N806.

STD meters, if required, should be provided in accordance with E5001 and Diagrams N1008 and N1009.

For a mains-operated installation the power unit should be installed in accordance with Q0020.

For a battery float installation the rectifier and battery should be installed in accordance with Q0071.

The above picture shows from top to bottom, a Unit, Auxiliary Apparatus, No. 96A (one each external
extension or inter-switchboard circuit), a Power Unit and a Box, Connection, No. 5C

User Guide

Drawings - 91838/0, 91838/1, 91891, 91892, 91893 and 91923.

Label No. 431 - Exchange lines.
Label No. 430 - Extensions.

Made by Ericsson

Click here for an article from the Ericsson Bulletin


BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: May 05, 2023