PMBX No. 2/3

Descriptive Leaflet

3 Internal
April 1971

Private Manual Branch Exchange No. 2/3
(P.M.B.X. No. 2/3)


 The Switchboard, PMBX No. 2/3A is a lamp-signalling cordless switchboard with five connecting circuits and has a capacity for 3 exchange lines and 12 extensions. The switchboard has been designed to stand on a table or similar support.

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 (manual) and automatic exchange areas, and also, with Unit, Auxiliary Apparatus, CBS536 fitted at the public exchange in CBS No. 1, 2 and 3 areas.

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

The colour of the switchboard is two-tone grey and a matching Telephone No. 706 is normally used as the operator's telephone. The switchboard is 17 in wide by 8 in high by 13 in deep and it weighs 33 lb. It has a metal chassis and an acrylonitrile-butadiene-styrene copolymer (ABS) cover.

1. The lamp cover strip is removed by moving 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.

2. 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. 37A) is removed by applying pressure on the lower edge and sliding the strip upwards towards the lamp cover strip. This releases the fixing clips, which slot into the face panel in each half of the designation strip mounting. 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.

3. Subscribers' 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. 36A) is removed by pivoting the locking projection on the under side, and is replaced by a narrower strip (Strip, Designation, No. 33) which covers the upper part of the meter front. 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 Dgms N1005 and N1006. Metering connections within the switchboard are made on the auxiliary terminal strip fitted on the key panel adjacent to the exchange line keys.

4. The cover is held in position by an angled 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.

5. 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.

6. 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 slip at each end, is provided to protect the relays.

Case removed - keyboard and relay mounting folded back to expose wiring

7. A terminal block (Block, Terminal, No. 46A) with 122 terminals is mounted on the switchboard base immediately below the relay mounting plate. The conductors of a Connector No. 1068A are terminated on 68 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 block, together with the tags on a Strip, Connection, No. 153/10B 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 slots in the flange at the rear of the base. When viewed from the rear the right-hand slot is for the operator's telephone cord and the second slot is for the main 68-way cord. A dummy grommet is also provided on the right-hand side of the rear base flange. This is removed and a Jack No. 84C fitted when a Headset No. 1 is required in addition to the handset. Wiring is provided in the switchboard for the connection of this jack. When a Headset No. 1 is fitted the Telephone No. 706... is replaced by a Telephone No. 710, for details see Dgm N1103.

The picture above shows an Autodialler associated with the Switchboard.
This is not a standard item and would have been installed at extra cost.

Circuit connections
Four-wire extension circuits may be connected on all extension positions. Private circuits, inter-switchboard circuits, inter-switchboard extensions, and 2-wire extensions must, however, be connected on extension positions 7-12.

A Unit, Auxiliary Apparatus, No. 96A (or 96) should be provided on all 2-wire extensions and terminated in accordance with Dgm N1127.

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 E0402.

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 relay AC operates to an incoming ring. The pilot relay has a low-resistance winding which is connected in parallel with the operating coil when the relay operates in order to reduce the amount of series resistance added by the relay. If the calling lamp of an exchange line becomes disconnected an audible alarm is given (provided the ON key is operated) as the holding circuit for relay AC 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 signalling lamp.

A clear is given when the extension handset is replaced on both extension-to-extension and extension-to-exchange calls. On the 4-wire extensions control is by an additional gravity switch spring-set in the extension telephone. In the case of 2-wire extensions the control is by a supervisory relay in the Unit, Auxiliary Apparatus, No. 96A (or 96) (see Dgm N1103 explanatory figures).

Press-button recall
This is a standard facility and a switch is provided in accordance with Dgm 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 or 96) 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 910 ohm 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 relay CT is re-operated in the switchboard. Contacts of this relay disconnect the extension circuit from the exchange line circuit and reconnect the calling relay AC 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 relay AC; 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 calling relays are also disconnected, i.e. relay CT is released to prevent the exchange line calling lamps glowing. A 200k ohm resistor in series with a 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 the NS terminal which can be used as a switching condition to provide non-standard night service arrangements. Contacts of the NS relay (which is released) provide night service switching arrangements for any Units, Auxiliary Apparatus which may be connected to extensions 7 to 12. The operator's 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 7-12 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. been this facility is required the operator's telephone is changed from a Telephone No. 706 to a Telephone No. 710 When fitted, the headset utilises the regulator and induction coil contained in the telephone. Additional spring-sets in the telephone 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 can be used by the operator for both incoming and outgoing calls. The remaining exchange lines are connected through to selected extensions to give normal exchange access.

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 25 Hz 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 Dgm N1103 and for battery float system installations from Dgm N2332.

Dgm SA7164 is the schematic diagram for the switchboard and the detailed circuit description is given in Diagram Notes SA7164. Dgm N1103 gives details of the operator's telephone circuit (with and without headset), the apparatus and the circuit elements together with installation wiring and cabling instructions.

On all internal extensions three wires plus an earth wire are provided to each extension 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 (or 96) should be provided and connected as shown on Dgm N1127.

The switchboard has a parallel feed transmission bridge for extension-to-extension calls using a 300 + 300 ohm feeding coil with a 0.1 PF capacitor across the output to improve the sidetone balance. Non-removable relay shields are fitted to the transmission bridge feed relays; these reduce the crosstalk level between connecting circuits to below 75 dB. For extension to exchange calls the transmission feed is obtained from the exchange.

The switchboard should be installed in accordance with Dgm N1103. The 80-wire cable to the Box, Connection No. 5B should be terminated on the Connector No. 101/1A in accordance with Dgm N1103 and in the manner shown on Drg SD113.

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

The extension instruments should be terminated in accordance with Dgm N806.

STD meters, if required, should be provided in accordance with E5001 and Dgms N1005 and N1006.

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 Q0060 or 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. 5B

User Guide

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: January 18, 2022