PMBX No's 2/2 and 2/2b

3 Internal

PMBX No 2/2..

 The Switchboard, PMBX, No. 2/2A is a lamp-signalling cordless switchboard with three connecting circuits and has a capacity for two exchange lines and six extensions. The switchboard has been designed to stand on a table or similar support. The later version, Switchboard, PMBX No. 2/2B differs only in internal detail, e.g., electronic buzzer in lieu of the earlier mechanical type.


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 (or 96) 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, CB 506 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 manufactured to Specification S629 and Drawing No 91508.


1. 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 8 in wide by 5 in high by 10 in deep and it weighs 12 lb. It has a metal chassis and an acrylonitrile-butadiene-styrene copolymer (ABS) cover.

2. 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.

3. The designation strip is removed by applying firm outward pressure to the lug at each end. This releases the fixing clips, which slot into the front panel on each half of the designation strip mounting. The paper label, which is printed on both sides, can then be removed for marking as required. When replacing the designation strip care must be taken not to damage the panel surface. The two parts of the designation strip are not interchangeable.

4. The cover is held in position by two angle brackets which pull the cover to the rear by means of the two screws mounted on the flange at the rear of the baseplate. To remove the cover the screws, which are captive, are unscrewed. 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. The plastic dust covers for the relays are removed by releasing the screws situated on the side supports of the rear panel. On early models the 600 type relays are positioned on the baseplate and their dust cover is removed by releasing the single screw fixing in the centre.

7. A terminal block with 70 terminals is mounted on the base. The conductors of a Connector No. 1038A are terminated on 38 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 and STD meters. The remaining terminals 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 three grommet entry slots on the flanges at the rear of the base. When viewed from the rear the right-hand slot carries the cord for the operator's telephone, the centre the main 38-way cord and the left-hand slot the STD meter cordage. As STD meters will not always be required, a dummy grommet is supplied for the left-hand slot.

8. A Headset No. 1 may be fitted in addition to the operator's telephone. When this is required the operator's telephone is changed to a Telephone No. 710 the cover is replaced by a Cover, Switchboard, No. 1A, Grey and a Jack No. 84C is fitted.

1. 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 4-6.

A Unit, Auxiliary Apparatus, No. 96A (or 96) should be provided on all 2-wire extensions and terminated in accordance with Dgm N1126.

Private circuits etc require the provision of a Unit, Auxiliary Apparatus, No. 97, 98 or 99.

2. 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. A resistor which is connected in series with the exchange line lamp has the same value as the low- resistance winding of the pilot relay. This resistor is provided to reduce the variation in brilliance between exchange and extension lamps. 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 relay AC is via the pilot relay. 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.

3. 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.

3.1 Clearing
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. 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 N1102 explanatory figures).

3.2 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.

4. Holding of exchange calls
A separate HOLD key is provided below the connecting circuit keys of each exchange line. Operation of the HOLD key disconnects the exchange line from the connecting circuit, applies a 430 ohm loop to hold the exchange line connection and reconnects a transmission feeding battery. This permits the operator to speak to an extension on the the connecting circuit with complete secrecy from the subscriber held on the exchange line.

5. Exchange and extension limits
For transmission and signalling resistance limits see TI E0110.

6. Night service
The NIGHT SERVICE key should be operated whenever the switchboard is left unattended overnight or for long periods. This prevents locked lamp signals on the exchange lines should an incoming call be received during this period. With the NIGHT SERVICE key operated the call is indicated on the switchboard only for the duration of the incoming ringing signal. Any selected extension can be extended on night service including those fitted with a Unit, Auxiliary Apparatus, No. 96A (or 96). If a non-standard night service switching arrangement is required an earth extended to terminal NS can be used for operating any switching relays which may be necessary.

7. Prohibition of exchange service on private circuits
A prohibition relay is connected to the P terminal of extensions 4 to 6 when a Unit, Auxiliary Apparatus is fitted thereto and strapped to provide exchange prohibition. The contacts of this relay are arranged to disconnect the private circuit if an attempt is made to connect it to an exchange line.

8. 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 5OV working may be used.

9. Lightweight headsets
A Headset No 1 may be provided in addition to the operator's telephone by minor modification to the switchboard. When this facility is required the operator's telephone is changed from a Telephone No. 706 to a Telephone No. 710. Additional spring-sets in the telephone disconnect the headset when the handset is removed from the rest. (The significance of the use of the Telephone No. 710 is purely mechanical as the gravity switch has a greater mechanical advantage to operate the additional change-over springs required).

10. 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 second exchange line can be connected to a selected extension 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 N1102 and for battery float system installations from Dgm N2332.

Dgm SA7163 is the schematic diagram for the switchboard and a detailed circuit description is given in Diagram Notes SA7163. Dgm N1102 gives details of the apparatus, circuit elements, installations wiring and cabling instructions and of the operator's telephone circuit (with and without headset).

The switchboard should be installed in accordance with Dgm N1102.  On all internal extensions three wires plus an earth wire are required for each telephone.

  1. A 41-wire cable to a Box, Connection, No. 5A should be terminated on a Connector No. 100A in accordance with N1102 and in the manner shown on Drg SD113.

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

  3. The extension instruments should be terminated in accordance with Diagram N806.

  4. STD meters, if required, should be provided in accordance with E5001 and Diagrams N1003 and N1004.

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

  6. For a battery float installation the rectifier and battery should be installed in accordance with Q0060.

  7. A lightweight headset, if required, should be fitted in accordance with Works Spec S(W) 2064.

(This instruction succeeded EI Telephones, PBXs, B1051)

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Last revised: December 18, 2010