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


TELECOMMUNICATIONS INSTRUCTIONS
C MARKETING - INSTALLATION
3 Internal
E400l

PMBX No 2/2..
 

GENERAL
 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.


 

PHYSICAL DESIGN
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.

FACILITIES
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 Diagram 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 Diagram N1102 explanatory figures).

3.2 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 (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 50V 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.

POWER AND RINGING SUPPLIES
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 Diagram N1102 and for battery float system installations from Diagram N2332.

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

INSTALLATION
The switchboard should be installed in accordance with Diagram 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 Drawing 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)

Click here for an article in the Ericsson Bulletin


The Post Office Electrical Engineers Journal
Volume 54 - Part 1

A New Small Cordless P.M.B.X. Switchboard
by C. M. HALLIDAY and E. J. LIDBETTER

The first of a new range of private manual branch exchange switchboards of the cordless type has a capacity for two exchange lines and six extensions. It is a lamp-signalling switchboard of compact design incorporating new features and the power supply is derived from a separate mains-driven unit supplying 50 volts d.c. and 25 c/s a.c.

INTRODUCTION
The requirements for small manually-operated  telephone switchboards for use in subscribersí  premises have, for many years, been met by a range
of cordless switchboards, all of which are of a similar basic design. Private manual branch exchanges (P.M.B.X.s) of this type have a maximum capacity of three exchange lines and nine extensions; lever keys are used to interconnect the circuits, and the signalling facilities are provided by indicators.

A new range of cordless switchboards is now being designed to supersede the existing series, and the first of these new designs, the P.M.B.X. No. 2/2A, has a maximum capacity of two exchange lines and six extensions. The type of switchboard that it supersedes has a maximum capacity of two exchange lines and four extensions. The new switchboard has been given a modern appearance, approved by the Council of Industrial Design, so that it will harmonize with other new designs of subscribersí apparatus. Lamp signalling is used and the connecting keys are of a new miniature type.

The number of extensions that can be connected to the new switchboard has been increased to six compared with four on the existing design, as it has been found more economical to have a higher extension to exchange line ratio for a switchboard of this size. It has been confirmed nevertheless that the traffic-carrying capacity of three connecting circuits will suffice even with the increased number of extensions, and therefore no change has been made in the number of such circuits.

The adoption of the 4-wire extension principle has enabled all the requisite facilities to be provided in the most economical manner. Briefly, this principle consists of running an extra wire plus an earth lead to each extension telephone for supervisory purposes. A 4-wire/2-wire conversion relay-set has been designed for use on external extensions that need to be connected via the public-exchange local-line network. All the extension line circuits on the switchboard can be used for 4-wire extensions, while extension circuits No. 4-6 can, by simple rearrangement of cords and straps within the switchboard, be used for 2-wire extensions, private circuits, or inter-switchboard extensions. In addition to the conversion relay-sets necessary for 2-wire extensions, auxiliary units are also required for private circuits and inter-switchboard extensions.

The power supply for the switchboard is derived from an external a.c. mains-driven power unit supplying 50 volts d.c. and 25 c/s a.c. for ringing purposes.

Fig 1

FACILITIES PROVIDED
The main facilities provided by the new switchboard are as follows:-

(a) Lamp signalling for exchange lines and extensions.

(b) A transmission and signalling limit of 500 ohms for an extension circuit and 1,000 ohms for an exchange-to-extension connexion when connected to an exchange with equipment for 1,000-ohm lines.

(c) Press-button recall on extension-to-extension and extension-to-exchange calls under all conditions.

(d) Individual clearing on extension-to-extension calls.

(e) Holding of an exchange call by the P.M.B.X. operator while speaking to an extension on the same connecting circuit, the conversation being inaudible on the exchange line.

(f) Connexion of private wires and inter-switchboard circuits without modification to the permanent wiring of the switchboard.

PHYSICAL DESIGN
The new switchboard, which is shown in Fig. 1, will normally be provided in two-tone grey with a matching Telephone No. 706 as the operatorís instrument. The switchboard has a metal chassis and a plastic cover, and its dimensions are approximately 8 in. wide by 5 in. high by 10 in. deep. The design of a compact item has been made easier by the adoption of the 4-wire extension principle and by the introduction of lamp signalling to replace the indicators used on the present type of switchboard. In addition, a new miniature key (the 1,000-type key) has been used instead of the standard lever key. The springs of this new key, which are comb-operated, are similar to relay springs and have twin contacts. The key handles are small and wedge-shaped (see Fig. 2).

The keys, lamp jack and the circuit-designation label-strip are mounted on the panel in such a manner that all screw heads are concealed. When the lamp-jack lens-strip is removed by releasing the phosphor-bronze clips at each end of the cover, the lamp-jack screw-heads are exposed. By releasing these screws the lamp-jack may be drawn forward to permit lamps to be changed without the use of a lamp extractor or the removal of the switchboard cover.

The thermoplastic cover of the switchboard is vacuum-formed by deep drawing p.v.c. sheet and is polished after forming. The leading edge of the cover slots under the front of the face panel and adjustable plates on the sides of the panel form dust seals. The cover is removed by releasing the captive screws at the rear on the base.

 

Fig. 2

The chassis consists of three sections; the front and rear panels, hinging on the base plate, form a triangle when the chassis is closed, as shown in Fig. 2. By releasing the screws at the apex the hinged panels can be opened outwards to permit inspection of the wiring and components, as shown in Fig. 3. To conserve space in the switchboard, small wire-wound resistors and tubular capacitors have been used and are mounted on insulated tags fixed direct into the rear panel of the chassis. Plastic dust covers are provided to shield the 600-type and 3,000-type relays.

A connexion block with 70 terminals is mounted on the base. Some of the terminals are used for the connexion of the operatorís telephone, subscriberís private meter and a 38-conductor flexible cord for the exchange lines, extensions and miscellaneous terminations. The remainder of the terminals are provided to permit rearrangements of straps and cords when 2-wire extensions, private circuits or inter-switchboard extensions are required, thus avoiding the modifications to the permanent wiring that are necessary in some applications on the present type of switchboard. The free end of the 38-way cord is terminated on a multi-way plug which fits into a wall-mounted jack cabled to the connexion box. If subscribersí private metering is required, a Meter No. 19, as used on direct exchange lines, is associated with the switchboard.


ELECTRICAL DESIGN
A new parallel-feed transmission bridge has been adopted for economic reasons. For 50-volt working the transmission bridge consists of a 300 + 300-ohm coil with an 0.1uF capacitor across the output to give improved side-tone balance. This combination is suitable for all extension-to-extension calls up to a signalling limit of 500 ohms, with either 300-type or 700-type telephones. Non-removable relay shields fitted to the transmission-bridge relays increase the crosstalk attenuation between circuits to at least 75 db.

A new lamp (Lamp No. 2, 45 volts) has been developed for use on the switchboard. It has a reasonably flat lumens/resistance response over the range 0-500 ohms line-plus-telephone loop resistance, and the use of this lamp has obviated the need for a line-signalling relay.

No hand generator is fitted in the switchboard; the ringing supply is derived from a frequency-division mains-operated unit producing a 25 c/s a.c. output.

The switchboard circuits have been designed to operate from a nominal 50-volt d.c. supply but are capable of operating from a supply in the range 45-55 volts, and this has permitted economies to be made in the design of the power unit. Arrangements have been made to ensure that exchange calls already in progress are maintained if a mains failure occurs. It is recognized and accepted, however, that extension-to-extension and inter-switchboard calls will fail under such conditions, but it is considered that little inconvenience will be caused thereby, since failures of the mains supply in this country are generally few and of short duration. Under mains-failure conditions night-service arrangements are adopted. At installations where a break in service cannot be tolerated, however, the switch-board will be operated from a floated-battery system.

FIG. 2 - CHASSIS OF SWITCHBOARD CLOSED


FIG. 3 - CHASSIS OF SWITCHBOARD OPENED

CIRCUIT DESCRIPTION AND OPERATION
The 4-wire extension principle is used on all internal extensions and can be applied to external extensions within the curtilage of the subscriberís premises. Where external extensions are connected via the local-line network it will generally be desirable and more economical to use a 4-wire/2-wire conversion relay-set. For all extensions without a conversion unit the exchange-to-extension transmission and signalling limit is the same as that of a direct exchange line, since a relay is not required for supervisory purposes. On external extensions for which conversion units are required this limit is reduced to 900 ohms. The difference is due to the resistance of the supervisory relays in the conversion unit.

If the switchboard is used to replace an existing installation, the internal extensions will have to be wired for 4-wire-working and it will be necessary to change the extension telephones to include the additional auxiliary switch-hook spring-set.

A simplified circuit diagram of the new switchboard is shown in Fig. 4.

Extension-to-Extension Calls
When a call is originated at an extension by lifting the telephone handset, earth via KX1, the telephone loop, KX2 and the calling lamp operates the pilot relay, P, connected to -50 volts, and the extension calling lamp glows when contact P2 operates and short-circuits the high-resistance coil of the P relay. Contact PI causes an audible alarm to be given if the alarm on key, KA, is operated. If more than one extension is calling the switchboard, overhearing between the extensions is suppressed by capacitor Cl. The call is answered by operating the operatorís telephone key, KO, associated with the chosen connecting circuit and the appropriate connect-extension key, KX, and by lifting the handset of the operatorís telephone. The transmission-bridge relay, L, feeds transmitter current to both telephones.

If connexion to another extension is required, the operator checks that the extension is disengaged by observation of the keys, and calls the extension by the operation of the appropriate ringing key, which connects 25 c/s ringing current to the extension. When the called extension answers, the calling lamp glows and the KX key associated with that extension is operated to complete the connexion. The operator then restores key KO and replaces the operating telephone handset. Either extension can recall the switchboard by depressing the recall button on the extension telephone. An earth (on
the fourth wire from the connexion box) is then extended to the C wire (the third wire) and the calling lamp glows while the recall button is pressed. When the call is completed and the extension handset is replaced, an earth is extended via the auxiliary switch-hook spring-set, SWA, to the C wire to give a clearing signal on the calling lamp via key contact KX4. Individual clearing signals are given when either extension replaces the handset.

Extension-to-Exchange Calls
If the calling extension requests connexion to an exchange line, the connect-exchange key, KE, of a free exchange line is operated. This disconnects the local 50-volt supply from the connecting circuit at KE1 and KE2 and extends the extension to the exchange line. As there are no supervisory relays in the connecting circuit (the third wire being used for supervisory purposes) no additional relays are required to prevent the calling lamp flashing to pulsing if the exchange line is connected to an automatic exchange. The exchange-line connecting-keys are, however, arranged as on the previous type of switchboard, to prevent two exchange lines being connected together. When the exchange call is completed the clearing signal is given on the extension calling lamp only, when the extension telephone handset is replaced.



Note: There are three connecting circuits
FIG. 4 - SIMPLIFIED CIRCUIT OF NEW SWITCHBOARD

Incoming Call on Exchange Lines
Incoming ringing current on the exchange line will operate relay AC over one coil via capacitor C2 and relay contact MF1 normally operated. Relay contact AC1 lights the calling lamp and AC2 completes a holding circuit for relay AC (to provide a locked calling signal on the exchange line) and also operates the pilot relay, P.

The circuit is arranged to ensure that, provided the alarm on key, KA, is operated, an audible alarm is given at the P.M.B.X. even if the exchange-line calling lamp becomes disconnected. The operation of relay AC is delayed by the short-circuit maintained across the hold coil by contact AC2. This avoids false operation due to line surges when switching takes place. Specific safeguards have also been incorporated to avoid the possibility of lost calls due to misoperation of the switchboard keys. A calling signal is not extinguished until the operatorís handset has been lifted and both the KE and ICO keys have been operated. If an exchange-line key only is operated with all the remaining keys in the same connecting circuit normal, the L relay of the connecting circuit is operated via KX3, K03, KE5 to earth at K04, and the calling signal is maintained by contacts LI and L2 (contacts L3 and L4 are similarly connected in the second exchange-line circuit). When key KO is operated, relay L releases, but the ringing current is extended to ring the bell in the operatorís telephone and this is continued (unless the calling party clears) until the call is answered by lifting the operatorís handset. The circuit thus ensures that a signalling device is always connected across the exchange line until a call is answered, and the need to connect a calling device incorporating a capacitor permanently across the line is thus avoided. A call can be extended to an extension as already described, and the clearing signal is given on the extension calling lamp when the handset of the extension telephone is replaced.

Exchange-Line-Hold Facility
A hold key, KH, is provided in each exchange-line circuit and these keys have the dual function of applying a resistive loop to hold the exchange equipment and of reconnecting a 50-volt supply via a bridging coil to the connecting circuit, which is disconnected from the exchange line so that the operator may use the same connecting circuit to speak to an extension without the conversation being audible on the exchange line. This facility is normally used on incoming exchange-line calls. A single call held lamp common to both exchange lines is provided to give a visual indication to the operator when a hold condition is applied.

Mains Failure
As the switchboard has been designed to work from a mains-operated 50-volt d.c. supply unit, the circuits have been arranged so that exchange-line service is maintained under mains-failure conditions. A mains-failure relay, MF, has been included and is operated by the 50-volt supply. When a mains failure occurs relay MF releases and contacts MF1, 2 and 3 connect the bell of the operatorís telephone to the first exchange line so as to give an audible indication of an incoming call. The operator can answer the exchange call under mains-failure conditions by operation of the appropriate KE and KO connecting keys and by removing the handset from the operatorís telephone. Outgoing exchange calls can also be originated by the operator in the normal manner. The operating instructions for this type of switchboard will advise subscribers to connect the remaining exchange line to a selected extension, as for night-service working, should a mains failure occur, and the selected extension will then have direct access to the second exchange line.

Press-Button Recall
By fitting a press-button key with a make contact in parallel with the auxiliary switch-hook spring-set of each extension telephone, press-button recall is effected by extending the earth on the fourth wire to the C wire (the third wire), and recall can be provided on all extension and exchange calls.

Night Service
Night-service arrangements are provided by the night service key, KNS, which disconnects the pilot relay, the extension calling lamps and also the AC relay locking circuit. Selected extensions can then be connected to the exchange lines by operating the KE and KX keys to a specific connecting circuit.

Private Circuits , Inter-switchboard and External Extensions
Using the additional terminals which have been provided for extensions No. 4-6, private circuits, interswitchboard and external extensions can be connected without modification to the permanent wiring of the P.M.B.X. When an exchange-line key and an extension connecting key are operated together in the same connecting circuit, earth is applied to the extension P terminal from K04 via KE5 and ICX5. This earth can be used to operate a relay in an auxiliary unit. The contacts of this relay disconnect the line, thus providing a prohibition facility to prevent the interconnexion of exchange fines and private circuits.

An auxiliary switch-hook spring-set, SWB, controls relays in the 4-wire/2-wire conversion relay-set when this is provided. The circuit of the relay-set is such that, when the operator intervenes on an established exchange call, the exchange fine is held by a loop and current for the extension is provided from the local transmission feed in the relay-set. The SWB spring-set ensures that calls are not inadvertently released by the operator entering the circuit by operating key KO before lifting the handset of the operatorís telephone.

CONCLUSION
It is confidently expected that the new cordless P.M.B.X., with its use of modern materials, compact design and an increased range of facilities, will be more attractive to the subscriber and to the Post Office than the existing type of cordless switchboard which it supersedes.

ACKNOWLEDGEMENT
The authors wish to thank Ericsson Telephones, Ltd. who, as the liaison manufacturers, worked in close co-operation with the Post Office on this development.


 

Additional Pictures

Rear view with case removed
 

Side view with case removed


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. 5A


 
 
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Last revised: June 06, 2020

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