TELEPHONE No. 706 & 8706

Original Telephone 706 - with steel dial and straight grebe handset cord	Early Telephone 706 with PVC cords and plastic dial
Telephone 706CB	Later Telephone 706 with no number ring and all number dial

Reference

Click here to here a Telephone No. 706 bell ringing

Many models made by British Ericsson - model N1340

How to wire your Telephone No. 706 to make it work on Plug and Socket

Diagram for telephone circuit - N806 and SA/SAW 10077.

Diagram for Auxiliary Units - N808.

Diagram for Additional Buttons - N809C.

Diagram for earlier style Addition Buttons - which was a holder that accepted a Perspex style insert, called a Label No. 358 - N809B.

Specification - S586.

Drawings
Telephone - 91196, 91272/0/2 (Mark 2), 91272/1/1 (Mark 1), 92272/1/2 (Mark 2), 91272/2/1 (Mark 1), 91272/2/2 (Mark 2), 91272/3/1 (Mark 1), 91272/3/2 (Mark 2A) 91272/4/1 (Mark 1), 91272/4/2 (Mark 2A) & 93010/2.
Circuit Board - 91273/0/1, 91273/1/2 and 91273/2.

First issued in 1959, this telephone superseded the Telephone No. 310F and Telephone No. 700.  Early model were produced with a stainless steel dial and a grebe handset cord.

Both C.B. and Automatic models were available.

Numerous manufacturers were involved with this phone and two variants were produced, those being the Mark I (hard wired) and the Mark II (PCB).  The GPO used both variants.

Telephone No. 8706F was wired for the new plan plug and socket system and fitted with a plug ended Cord Connection No. 4/502 3000mm.  This model was introduced in 1982, using recovered Telephone No. 706's, to augment supplies of the Telephone No. 8746.

Introduced in 1959 this telephone also had a wall type variant called the Telephone No. 1/706.  This model was made obsolescent in 1961 and replaced by the Telephone No. 711 (originally black only) in 1962.

The telephone was available in the standard seven colours and was to specification S586.

TELECOMMUNICATIONS INSTRUCTION
C MARKETING INSTALLATION
3 Internal
B1001
Issue 3 May 1977

TELEPHONE No. 706
Description and Facilities

SCOPE OF INSTRUCTION
This Instruction describes the Telephone No. 706 ... and the various add-on units that can be fitted to extend its range of facilities.

GENERAL
The Telephone No. 706... is suitable for use on exchange lines up to a maximum of 10dB at 1600Hz and is for use at installations requiring either no press-button or only one press-button.  The telephone can be modified for a variety of uses by fitting adapters internally.

DESIGN
The telephone is available in black, blue, two-tone green, two-tone grey, ivory, red and yellow. Click here for the colour range.

There are two types of the Telephone No. 706: the conventionally wired (Mark I) and the printed wiring (Marks II and IIA) versions. The Mark I has a plastic base on which all the components are mounted and interconnection made by conventional wiring.  Marks II and IIA have a metal base on which is mounted a printed wiring board with most of the components soldered directly on to it.  All electrical components other than the gravity-switch spring-sets and bell mechanisms are identical in each version.

Note that in a later supplies, a dust cover was provided over the back of the dial (part No. 1/DCO/631).

Mk1 - Conventional Wiring

Mk2 - Printed Circuit Board

HANDSET
This is coded Handset No. 3 and consists of the body (Part 2/DBO/48), Earpiece No. 26A, Mouthpiece No. 21A, Receiver-inset No. 4T and Transmitter-inset No. 16.  The body handle is hollow to allow the passage of the receiver connections.  The Receiver-inset No. 4T and Transmitter-inset No. 16 are held against the earpiece and mouthpiece respectively by spring rings (Parts 1/DRI/50) which are positioned and keyed by the three-ribs in the respective cavities.

The receiver terminals must be positioned between the two webs in the receiver cavity to prevent the receiver-inset turning when the earpiece is screwed on.  The Transmitter-inset No. 16 is prevented from turning by locating the three projections on the transmitter case in the three grooves in the transmitter cavity.

Earlier versions of the handset were fitted with a Transmitter-inset No. 13C.  This was located by three tags and the cord conductors are connected to by a nut termination on the transmitter spring -ring and by a Phosphor-Bronze split pin (1/DPL/397).  These Transmitters should be replaced by Transmitter-inset No. 16 with part 1/DRI/50.

The instrument cord enters the handset by a keyed entry hole in the moulded insert which is cemented into the end of the handset.  The cord has a square section grommet secured by a brass ring crimped in two places.  To insert the cord in the handset the four conductor 'tails' should be threaded through the entry hole and the crimped ring passed through the keyway to the full extent allowed by the grommet.  The grommet should then be twisted to position the square part of the grommet in the square lead-in to the entry hole.  The torque thus imposed will twist the crimps in the brass ring away from the keyways and the cord is locked in the handset.

To remove a cord from the handset, turn the brass ring until it lines up with an arrow on the casing.  The arrow is part of the moulding and indicates the keyway.  The pull the cord away from the handset.

COVER
The telephone cover (Part 2/DCO/609, colour) is an injection moulding.  The cover is secured to the telephone assembly by two lugs on the front skirt which engage two recesses in the leading edge of the base, and also by two fixing screws (Part 1/DSC/100).  These screws pass through bushes (Part 1/DBU/282) with escutcheon plates (Parts 1 and 2/DPL/746 left and right respectively) or through a handle (Part 1/DHA/23), which seat in recesses in the cover moulding between each pair of horns.  The screws engage tapped holes in the tops of the gravity-switch support brackets to provide cover location in the horizontal plane and the two bushes or the handle locate the cover in the vertical plane.  The escutcheon plates are secured to the cover by lugs which are slightly bent.  To remove the escutcheon plates from the cover, these lugs must first be straightened by means of an Adjuster, Detent, No. 1 to allow easy passage through the cover hole.

An aperture for the press-button is provided between the front horns but when a press-button is not required it is blocked by a dummy-button (Part 2/DBU/258, Colour), held in place by a spring clip (Part 1/DSP/506).  When a press-button is fitted in the telephone the displaced button and clip should be stored within the telephone for subsequent refitting.  It will usually be possible to secure the dummy and its fixing clip to the gravity-switch frame with a piece of insulating tape.

The dial surround ring is a Label No. 479A.  It should be fitted by hooking the lower two lugs over the bottom of the hole in the cover in such a position that the upper two lugs will pass through the cut outs at top of the hole.  The ring should then be rotated until the cut out for the dial finger stop in the cover and the ring is aligned.

Early supplies were fitted with Labels No. 355 or 473A secured by a clamping ring (Part 1/DSP/504).

The cover has a moulded aperture in the rear edge for the cord grommets.

Two types of cover were produced:-
Part 1/DCO/609 - Polymethyl Methacrylate moulding material (superseded by the 2/DCO/609).
Part 2/DCO/609 - Acrylonitrile Butadiene Styrene Copolymer moulding material.

BASE
Conventionally Wired Telephone
The base is moulded in toughened polystyrene and is virtually unbreakable.  The raised portion round the edge is designed to mate with the cover and in the corners there are countersunk holes for the telephone feet.  Two raised tiers of terminals are moulded into the rear of the base and the terminals are formed by standard 6BA nuts and screws, the hexagonal nuts being pressed into holes in the moulding.

In front of the terminals two platforms are raised in the moulding on which are riveted the two gravity-switch supports and between these two positions a jack position is moulded for mounting the regulator.  Five spring connections are fitted into this jack, and a keyway is provided to prevent incorrect insertion of the regulator.

A small platform is raised near the left-hand gravity-switch bracket to retain the dial cord terminations when the telephone is required for CB or LB working.  There are four holes in this platform, one of which is fitted with a nut and screw terminal.  The two leads to the dial pulsing springs are short-circuited under this screw and the other three dial lead spade terminals are inserted in the other three holes.  The bell mechanism is mounted on a relief in the moulding and is secured by two screws.

Two clip positions are moulded into the base adjacent to the bell mounting, one for securing the induction coil and the other for the capacitor.  Two radially slotted domes are raised in the front of the base with screw holes at their zeniths for mounting the bell gongs.  There are five 'knockouts' in the base, three of which are for mounting the Plan-set N 625, and the other two for wiring purposes.  At the cord entry the edge of the base is thinned to form part of the cord grommet clamping arrangement which is completed by a metal clamp (Part 1/DPL/733) held in place by insertion in two slots in the base moulding.

Printed Wiring Telephone
The Mark II telephone base is made of mild steel with a black crackle enamel finish. It is basically the same shape as the moulded version already described but has different mounting facilities.  Six pillars are fixed on the base, four for mounting the printed wiring board and two for the bell gongs.  A series of holes is punched in the base between the bell gong positions and 'knock-out' holes are provided similar to those in the moulded base.  Two holes are raised at the rear of the base for securing a plastic support for the terminations on the printed wiring board.  The cord grommet locking device is a T-shaped metal extension riveted to the rear of the base.

The Mark IIA base is similar to the Mark II except that additional supports are provided for the printed wiring board and the 'knockouts' are already removed, the large oval hole being fitted with a plastic plug.

GRAVITY-SWITCH MECHANISM
In both types of telephone the gravity-switch mechanism is mounted on a bracket which is drilled and formed for securing the dial mounting, the Switch No. 5A-3 and the various adapters associated with the Telephone No. 706.  The plungers are two black nylon stubs moulded on the tips of the two lever arms. These arms are pivoted on projections of the mounting bracket and rock on one edge of the pivots.  The two lever arms are joined by a horizontal member, the whole being one metal stamping.  This bar actuates the lifting comb of the gravity-switch springs when the plungers are depressed.  The extreme ends of the lever arms carrying the nylon plunger tips pass through guides in the mounting bracket and the cover-fixing screw holes are situated adjacent to these guides to ensure adequate plunger clearance of the holes in the escutcheon plates in the cover.  The gravity-switch springs, protected by a transparent dust cover, are of nickel-silver with palladium contacts and, in the conventionally wired version, the assembly is secured to the gravity-switch mounting bracket at an angle of approximately 30 degrees from the horizontal.  One fixing screw in the spring-set bracket passes through an elongated hole in the gravity switch bracket providing positional adjustment for the spring-set comb with respect to the operating bar.

In the printed wiring version these springs are mounted vertically and the tags are soldered directly into the printed wiring pattern.  In both instances the plunger lift is provided by the gravity-switch pressures until the spring-set is normal and the remainder of the travel is provided by a horizontal spring exerting pressure at its extremities on the gravity-switch lever arms.  This horizontal spring is positioned so that the pressure at the extremities is exerted downwards and also towards the rear of the telephone to ensure that the gravity-switch arms rock on the front edge of the pivots.

A latch is fitted on one of the gravity-switch brackets for locking the gravity-switch down when the covet is removed for maintenance purposes.  It is so designed that when the telephone handset is replaced after maintenance the latch is automatically released.

BELL
For the Mark I and Mark II versions the bell used is the Bell No. 59A-1, Unmounted.  This bell is supplied with the two coils connected in series and two 'flying' leads with ring tags.

The Mark IIA is fitted with a Bell No. 59B-1, Unmounted.  This is similar to the Bell No. 59A-1, Unmounted, except that the coil tags are positioned to permit two of them to be soldered directly into the printed wiring pattern, which provides the series connection of the coils and additional fixing for the bell assembly.  The Bell No. 59B-1 is not available for maintenance replacement in the field, Bell No. 59A-1 should be used instead.

The two gongs used are one each of Bell-gongs Nos. 24A and No. 24B, the former having the higher pitch.  The gongs are fitted with the largest radius, denoted by the code stamping, pointing towards the rear of the telephone.

AUTOMATIC SENSITIVITY REGULATOR
Regulator No. 1 consists of a small printed wiring board on which are mounted a Resistor, Bulb No. 15, a Rectifier-Element No. 209 and two carbon resistors of 15 and 47 ohms connected as shown on Diagram N801.  These components are soldered directly into the printed wiring pattern which is connected to the telephone circuit by five plug points formed on one end of the board.  Three more plug points, which are strapped together, are provided at the other end of the board so that reversal of the regulator board in its jack will remove it electrically from the telephone circuit.  All the printed wiring plug terminations are coated with a thin layer of solder to provide good contact with the jack springs when the regulator is plugged in.

Regulator No. 1 front and rear views

CAPACITOR AND INDUCTION COIL
These are secured by spring clips which fit into slots in the base moulding of the conventionally wired telephone or are soldered direct on to the printed wiring board. The capacitor in both versions of the telephone is the Capacitor No. 7719 which consists of a 1.8 uf and a 0.9 uf capacitor in the same can. It is mounted horizontally in the conventionally wired telephone and vertically in the printed wiring telephone.

The Coil, Induction, No. 31 has a closed magnetic circuit core made up from laminations forming one interleaved joint and one butt joint. The windings are wound on a moulded bobbin and brought out to five terminal pins protruding from one end.

DIAL AND DIAL MOUNTING
Early supplies of the Telephone No. 706 were fitted with the Dial, Automatic, SS No. 12LA or Dial, Automatic No. 21CA colour but subsequently the Dial, Automatic, No. 21FA, clear has been fitted and is described in M0017.  It is secured to the telephone by a circular metal clip clamped tightly around the periphery of the dial body.  This circular clip is secured at the top to the gravity-switch bracket by a screw, and at the bottom by two lugs which engage two slots in a bracket located across the tops of the bell gongs.  The dial may subsequently be removed, complete with mounting, by unscrewing the fixing screw on the gravity-switch bracket and hinging the dial outwards to disengage the lugs at the bottom of the dial mounting.  On current supplies of the telephone a dust cover (Part 1/DCO/631) is provided to fit over the rear of the dial.

TELEPHONE WITHOUT DIAL
The Telephone No. 706CB is no longer stocked.  The original Telephone No. 706CB was fitted with a Dial, Automatic, Dummy No. 6.  Where a dial-less instrument is needed, the Dial, Automatic No. 21... is recovered and a Dial, Automatic, Dummy No. 7, (Colour) is fitted in its place.  The dial leads of the conventionally wired telephone are secured in the positions described previously.  In the printed wiring telephone the dial leads are secured in a plastic strip (Part 1/DST/558) which fits on the dial mounting-bracket fixing screw.  The pink and orange leads should be clamped under the screw terminal in the plastic strip, and the remaining three leads should be retained by pushing the spade tags into the slots provided. On an original CB hard wired variant the orange and pink wires are clamped under a screw, on the base moulding, near the Induction Coil and the remaining three wires pushed into three holes adjacent to the screw.

CORDS
The telephone is supplied fitted with two cords.  The line cord is Cord, Instrument, No. 4/121AT colour, 3000mm and the handset cord is Cord, Instrument, No. 4/118AX 200mm.  The AT and AX cords are made to metric dimensions, and supersede the AA and AK types previously fitted.

TERMINAL BLOCK
Originally a three way line cord was fitted to the telephone and a Block, Terminal No. 30, colour, was provided.  The telephone is now supplied with a Block, Terminal, No. 52A, colour.  This terminal block has eight terminals, strapped in four pairs, with one removable link (Part 1/DLI/19) strapping terminals 3 and 4.  The block has the same dimensions as the Block, Terminal No. 30 and uses the same cover.

FEET
The telephone feet, Parts 2/DBU/259, are manufactured from non-staining chloroprene; early issues of the Telephone No. 706 were fitted with dome shaped feet Part 1/DBU/259.  A projection at the top of the foot is provided to force fit in the holes in the telephone base.  To insert a foot in the base, slightly moisten the projection, engage it with the hole at one point of its periphery, and a firm twist will then cause the rest of the lip to engage with the hole.  The later feet can be identified as they have a depression in the centre.

CARRYING HANDLE
Some issues of the Telephone No. 706 are provided with a carrying handle (Part 1/DHA/23).  The handle replaces the cover fixing escutcheon plates and screw retaining bushes, and clips into the apertures normally occupied by the escutcheon plates.  Handles may be fitted to existing telephones when requested by the subscriber, except when the telephone is attached to a Plan Set N625.

ADD-ON UNITS
The telephone can be modified for a variety of uses by the addition of simple auxiliary units and parts.  Diagram N808 shows the wiring of the auxiliary units; Diagram N809 shows the various press-buttons.

Switch No. 5A-3 - Press Buttons
The switch consists of a micro-switch with a change-over contact action, with three flexible wire leads fitted with spade-type tags, mounted on a bracket. It is used on installations requiring extension, main or exchange signalling facilities or a bell cut-off facility.

A range of press-buttons, which have an integral plunger, is available for fitting to Switches No. 5A-3 and are listed in Table 1.  The Parts 3 and 4/DBU/264 have a projecting step to provide the locking facility.  When the button is pressed down on its right-hand side the projection engages with the mounting bracket and locks the plunger in the down position.  To release the switch, the press-button is pressed on the left-hand side, allowing the projection to disengage and restore to normal under the action of the spring in the micro-switch.

The buttons are available in the seven colours of the telephone range.  Where the moulded legends of Parts 3/DBU/263 and 3/DBU/264 are not suitable, the blank buttons may be engraved with the required legend.  An engraving schedule should be submitted with the requisition for the labels giving details of the legend required.

To fit the button to the switch, remove the two screws fixing the switch to its mounting bracket, insert the button in the channel formed in the mounting bracket and reassemble.

The Switch No. 5A-3 is fitted in the telephone as follows:-
Withdraw the captive screw to its limit, engage the slotted lug in the switch bracket with the similar slot in the forward horizontal member of the gravity-switch bracket.  When correctly engaged this allows the captive screw to engage in the vertical slot in the gravity-switch bracket adjacent to the top dial fixing point.  Tighten the captive screw to secure the switch in position.  The pictures below shows a Switch No. 5A-3 with a Part 3/DBU/263 fitted to the Telephone No. 706.

Switch No. 5A-3 - Front view	Switch No. 5A-3 - Rear view

TABLE 1

Thermistor No. 1A-1
This item prevents bell-tinkling on shared service installations.  The two spade tags are secured under the appropriate terminal screws and the Thermistor is supported on the ends of the two connection leads in a convenient safe place inside the telephone.  The Thermistor is covered by a rubber sleeve and shown in the picture above.

A Thermistor No. 1A-1 together with a Switch No. 5A-3 superseded the Adapter, Shared Service, No. 3.

Adaptors Shared Service No. 3

Auxiliary Gravity-Switch Spring-Sets
These are required on some extension plan arrangements and on telephones associated with switchboards in the 2/.. series. ("C wire signalling").

Part1/DSP/1233 (one change-over unit), Part 1/DSP/1252 (one break unit) and Part 1/DSP/1256 (one make unit) are available to satisfy these requirements.  As long as demand for the three individual spring-sets is large, it is more economical to have each of the spring-sets available than to use the Part 1/DSP/1233 for all requirements.

These spring-sets are similar to the normal gravity-switch spring-set in the Mark I version of the telephone except that the mounting bracket is formed with a reverse angle to permit mounting on the right-hand vertical member of the gravity-switch bracket.

The spring-sets are supplied complete with fixing screws and spade-tag wire connection leads.  To fit the spring-sets to the telephone, pass the fixing screws through the holes in the right-hand vertical member of the gravity-switch bracket and screw into the tapped holes in the spring-set mounting bracket.  The top fixing hole is elongated to allow the depth of engagement of the spring-set with the gravity-switch lever to be adjusted.  A Part 1/DSP/1233 is shown fitted in the picture below.

Adapters, Local Battery
When a Telephone No. 706 is used in a local battery installation, a local battery adapter must be fitted.  The adapter consists of a gravity-switch operated spring-set and an inductor mounted on a bracket, and wire connection leads with spade-tags.  The inductor which has a low d.c. resistance and a high impedance to audio frequencies, completes the local battery circuit to the transmitter but does not shunt the speech currents in the telephone circuit.

Two local battery adapters are available, the Adapter, Local Battery, No. 5, which has a spring-set with one make contact unit, and the Adapter, Local Battery No. 6, which has a spring-set with one make and one change-over contact unit.  The Adapter, Local Battery, No. 6 is used on local battery installations that require an auxiliary gravity-switch spring-set to be fitted.

The adapters are fitted in a similar way to the auxiliary gravity-switch spring-sets.

The use of Local Battery Private circuits is to be depreciated, particularly where these pass through a Public Exchange where apparatus could be fitted to enable CB working to be adopted.

Adapter, Local Battery, No. 6

Adapter, Plan-Set No. 1
The adapter is for use in the main station of an Extension Plan 105 or 107 in conjunction with a Plan-set N 625.  The assembly of a Telephone No. 706, Adapter Plan-set No. 1 and a Plan-set N 625 is known as a 'Planphone A'.  The adapter consists of an auxiliary gravity-switch spring-set (one make unit, one change-over unit) and a 1.8 uf capacitor, mounted on a bracket, complete with wire connections with spade-tags.  The adapter is fitted in the same position as the auxiliary gravity-switch spring-sets described above.  See picture below.

Buzzer No. 32C-1
This is a 10v dc buzzer complete with mounting bracket, spark quench and wire connection leads with spade-tags.  The buzzer is fitted to the Telephone No. 706 when used on some plan extensions.  The buzzer mounting bracket is secured to the telephone by the fixing screw of the right-hand bell gong.  The Buzzer No. 32C... supersedes Buzzer No. 32A...  The picture below shows a buzzer mounted in a telephone.

Strip, Connection, 6-Way (Part 2/DST/836)
This is a six-way terminal strip for use in the telephone when additional terminals are required.  The terminal strip is a moulded block fitted with six tags and six 6BA cheese-head terminal screws.  Secure the terminal strip to a Part 1/DBR/366 (mounting bracket) with the two screws and nuts provided with the terminal strip then fit the mounting bracket to a vertical member of the gravity-switch bracket.  Two six-way terminal strips may be fitted, one to each vertical member of the gravity-switch bracket.  See picture below.

Strip, Connection, 18-Way
When more than twelve additional terminals are required a Strip, Connection, No. 155A, should be used.  The frame has two fixed metal studs on one side which locate in two holes in one vertical member of the gravity-switch bracket and on the other side there is a spring loaded metal stud which engages with a hole in the other vertical member.

Additional Capacitor Capacitor No. 7712-2 and Clip No. 90
These are available for use on certain installations where an additional 2 pf capacitor is required.  The capacitor is supplied complete with two Connection leads with spade-tags and is secured to the gravity-switch bracket by the Clip No. 90.

Acoustic Shock Suppressor
A Rectifier, Element, No. 205 may be fitted to the telephone to prevent acoustic shock.  It is complete with wire leads and should be connected in parallel with the Receiver-inset No. 4T directly across the receiver terminals, or across terminals 1 and 2 in the telephone itself.  Whilst not fitted at the introduction of the Telephone No. 706 they became a standard fitment later.

Handset Weight
When certain add-on units and auxiliary-gravity switch spring-sets are fitted to the telephone, the telephone handset may have insufficient weight to operate the gravity-switch fully.  A handset weight (Part 1/DWE/7) is available to overcome this difficulty and a description of it, together with fitting instructions is contained in B1005.

Additional Receiver
The Receiver, Watch, No. 8T and Hook, Receiver, AG are available when an additional receiver is required.  A description and fitting instructions for the items are given in M0070.

Handsets
A Handset No. 4, 5, 6, 7 or 14A may be used in place of the Handset No. 3 supplied with the telephone.

Lamp Signalling Units
Lamp Signalling Units Nos. 1 and 2 are available for use in conjunction with the Handset No. 7.  These units are mounted on a frame similar to the frame of the Strip, Connection, No. 155A and are fitted to the telephone by the same method.  A description of the Handset No. 7 and use of the lamp signalling units is contained in B2141.

Dial Lock
A Lock No. 29 can be fitted to the case to prevent outgoing dialled calls.  999 can still be dialled.

REPLACEMENT PARTS
Table 2 lists the replacement parts which are available.

TABLE 2

PART TO BE REQUISITIONED AS:-

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Brief History of the Telephone No. 706

The Tele 706 was designed jointly by Ericsson's Telephones Ltd, The GPO Engineering Department and the Council of Industrial Design.  It featured new ideas already used in several telephones introduced during the previous decade.

Unlike the 200 and 300 types the 706 was at last a truly modern looking telephone.  It had smooth curves and ergonomic design, was lightweight, colourful, and versatile.  The same telephone could be used for many applications by changing the removable dial surround and also had a blanking plate covering a hole which could be removed to allow different switches to be fitted.  Inside it contained the latest circuit technology including a plug-in current regulator and option of printed circuit board wiring (see below).

Three telephones which possibly influenced the final design of the 706 are:-

• (1953) Ericsson's/GEC `1000' type - smooth curves and various colours.

• (1957) Siemens 'Centenary Neophone' - lightweight plastic, removable dial surround, various colours, the first UK telephone with printed circuit wiring.

• (1949) #500 Rotary Desk Set - designed for Bell Telephone Co in USA by Henry Dreyfuss (This is the 'standard' American desk telephone).
  

A quote from Henry Dreyfuss regarding the #500 Telephone goes 'The form had to be classic so that it would not be out of place with shorter lived objects in homes and offices' - he estimated that it would have a twenty year life span.  This telephone is known to anyone who has read a magazine or watched a television programme during the last 30 years and must surely have influenced the design of many of its predecessors.

The Telephone No. 706 was available in 7 colours ... Two-tone Green, Two-tone Grey, Topaz Yellow, Concorde Blue, Lacquer Red, Black, Ivory.  Other variations exist - One-tone Green, Red/lvory etc.  A clear plastic version was used for demonstration purposes.  The Council of Industrial Design originally suggested 'Colonial Blue' for the blue version but Ericsson's asked for it to be changed because "it was inappropriate in a changing world".  The two tone grey version reflected "trends in matching office equipment".  The Post Office also experimented with other colours and advertised this publicly.

The case and handset were made of lightweight thermoplastic acrylic.  Initially there was a choice between modified Polystrene and an acrylic resin called Diakon which was manufactured by ICI.  Diakon was chosen as it seemed the best option at the time.  But due to breakages, the plastic was changed to ABS around 1962.

Internally the '706' contained improved (over the 300 type) circuitry including a plug-in electronic regulator. Click here for more information on the cases

The Telephone No. 706 was shipped without a carrying handle due to a GPO ban on lifting telephones.  This ban was lifted in 1964 and a carrying handle was expected on the Telephone No. 736.  But the case for a handle rolled on and internally there was discussions which normally revolved around how much these handles would cost.  In 1967 the GPO placed an order for 800,000 glass loaded nylon carrying handles.  The choice of plastic for lifting such a heavy item was wrong and the handle generally snapped in two.

Two designs were submitted for the internal wiring of the Telephone No. 706... Conventional wiring and printed wiring.  In 1959 printed wiring techniques were in their infancy and thus the BPO allowed manufacturers to choose either method of production. The two methods had the same terminal layout and used the same components.  Siemens Edison Swan Ltd designed the printed wiring version, whereas Ericsson's Telephones Ltd preferred the conventional method. (Siemens had already produced the first telephone with printed wiring inside - the Centenary Neophone).

Instruments with conventional wiring have a plastic base with raised domes for the bell gong mountings, whereas printed wiring instruments have a pressed steel base which is flat.

Early 706's came with grebe cords, steel dials and domed feet.  The cords soon became PVC, as stocks were made available, the dials where replaced with the No. 21 (plastic dial which was replaced in the late 60's by a clear finger plate) and the feet tended to slide so were replaced by a flat foot with a recessed centre.  There were many arguments over the clear finger plate and tests were carried out on all sorts of finger plates, one being the spoked dial pictured below.  The spoked dial was not a success.

The following manufacturers were initially involved in the production of the 706:-
The Automatic Telephone & Electric Co, Ericsson's Telephones Ltd, The General Electric Co Ltd, The Phoenix Telephone & Electric Works Ltd, The Plessey Co Ltd, Siemens Edison Swan Ltd/ AEI, Standard Telephones & Cables Ltd and The Telephone Manufacturing Co Ltd. Tele's 706 have also been seen with HAS (Association Automation, London) & CWL (Pye Cambridge works) manufacturers codes.

GEC called the 706 'The New Gecophone' whilst Ericsson's called it 'The Etelphone' - it was also known as 'The Mod T' by Post Office sales staff (Modern Telephone).  This telephone was given the title '800' type by AEI and some Mark 2 GPO drawings are annotated with an "S" and "800".  A version with a 2000 Ohm ringer and North American number ring was made for the Canadian market.

On earlier models the resistance lamp on the regulator board had just a clear glass finish - unfortunately some users were perturbed by the glowing coming from within their instruments if they were used in the dark (especially red models!) and later releases have the resistance lamp painted black.  The picture below shows the location of the regulator - the regulator is in circuit.  Note:- that the regulator board can be inverted, which defeats the regulator, but still allows the telephone to work.

In the early 1980's, with the modern press button telephones coming on stream, BT had a large number of dial telephones in stock and being returned from the field.  A lot of these phones were sold on to Africa and other countries would would buy them but some were made into intercom sets.  The intercom set included two Telephone No. 706's with a roll of connecting cable.  Each phone had a battery box in the case and a buzzer.  The telephones in this intercom set where called the Telephone No. 706IC.

An additional earpiece could be fitted, hanging on a small arm at the rear of the phone, that allowed second person to listen in.

Misc Notes :-
To remove the case undo the two screws next to the switch hooks.

To remove a dial fingerplate on the coloured plastic dial:-
Insert a thin plated screwdriver, between the fingerplate and the number plate, on the right-hand side on the dial. Slide the screwdriver towards the centre of the dial and it will locate in a slot. Rotate the screwdriver and the label cover should pop off. (on no account strike the dial in an attempt to loosen the cover).

To remove a dial fingerplate on the clear plastic dial:-
The correct method is to use an 'Extractor No 29' which is basically a rubber sucker!  An alternative is to use a piece of selotape.  Stick it onto the dial centre face and pull away from the dial.

Type Codes:-
Some instruments are marked (for example) 706F, 706L, 706R, etc.

The suffix letters F, L & R have the following meanings:-
F - All Figure dial or Dial Surround
L - Figures & Letters on dial Surround
R - Recall button fitted

Plug and Socket
87xx Series Telephones

Some telephones in the 7xx series were issued as low impedance versions with a PSTN type lead. The type number for these instruments was the same as the original instrument prefixed by an '8'. Generally only Tele 8706R was issued and this was a factory repaired 706 re-issued with a PBX recall switch due to a shortage of Tele 746.

Line Cord Connections
Line cord connections to a standard Tele 706/746 are as follows:-
Red - T8, White - Tl8, Blue - T5, Green - T15
straps T5-T6, T8-T9, Tl6-Tl7-Tl8-Tl9
Note : this is for the original 4 way lead

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An early production Telephone No. 706 in Concord Blue, with a Dial No. 12.

An extract from
THE POST OFFICE ELECTRICAL ENGINEERS' JOURNAL
Volume 53, Part 1 - April 1960

Trial of a New Type of Wall Telephone
TELEPHONE No. 1/706

THE Post Office is obtaining limited quantities of a new wall telephone which will be used in selected areas to test customers' reaction to a wall telephone available in a wide colour range. This telephone, coded Telephone No. 1/706, is a modified version of the table telephone, Telephone No. 706, which has already been described in an earlier article.  To make it suitable for mounting on a wall, the table telephone has its dial turned through 180 degrees and the cradle extended to retain the handset. Means are also provided for fixing the telephone to the wall.

To enable the dial to be reversed, additional cut-outs have to be made in the telephone cover and the outer number ring clip to accommodate the new position of the dial finger stop. Also, the mounting ring within the telephone that clamps around the dial has to be slightly modified to give clearance for the new position of the dial governor bearing screw. These changes, which neither affect the use of the parts in the table telephone nor increase their cost, are already incorporated in a proportion of table telephones.

The modification to the cradle consists of replacing the two small metal plates that surround the plungers in the table telephone by a chromium-plated cradle extension held by the two cover fixing screws, which pass down through the normal cradle close to the plungers. The plungers in the telephone are actuated by the side of the handset by a wedging action.

The telephone is fixed on the wall by means of two subsidiary parts. The first of these is a narrow bracket that is mounted horizontally on the wall, using a screw through the central fixing hole. The ends of this bracket are formed into hooks; the telephone is hung on these hooks using the holes in the base into which the two front rubber feet are fitted for the table telephone. The second part is a small oval plate with two holes in it. The plate is fastened to the wall telephone with a screw, which is passed through a "knock-out" hole in the telephone base and into the threaded hole in the plate. The other hole is used to fix the plate to the wall and is large enough to accommodate possible inaccuracies in the wall fixing. This mounting arrangement has the advantage that it eliminates the need to hold up the weight of the telephone while the sometimes difficult wall fixings are made. Also, the telephone can subsequently be very easily removed, and changed if necessary, without interference with the wall fixings. This method of fixing the telephone to the wall by two screws only, in a vertical line, has been adopted because in some forms of house construction only vertical timbers in the walls, spaced some distance apart, give the firm screw fixings needed to support the weight of a telephone.

The wiring of the wall telephone is identical with that of the table telephone, except that it has no line cord or terminal block; it may, therefore, be substituted for a single-button table telephone in any extension plan arrangement. As the changes involved are slight it would be possible to convert some table telephones into wall telephones locally if kits of the necessary extra parts were provided. However, because of the very limited use envisaged for this telephone and the limited availability of table telephones suitable for the conversion, this will not be done, and completely assembled wall telephones will be supplied. The development of the wall telephone described above was carried out by Ericsson Telephones, Ltd as supplied as their model N1065.

Available in 1961.

Superseded by Telephone No. 711.

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Taken from
THE POST OFFICE ELECTRICAL ENGINEERS JOURNAL
Volume 52, Part 1 - APRIL 1959

The New 700-Type Table Telephone-Telephone No. 706
by H. J. C. SPENCER and F. A. WILSON

The "700-type" transmission circuit was originally included in a prototype telephone the physical design of which was based on that of the "300-type" telephone. This article describes an entirely new table telephone incorporating the circuit. Of attractive appearance, the new instrument is being made in a wide range of colours using advanced constructional techniques, including printed wiring. It incorporates an automatic sensitivity regulator, shown to be necessary by trials with the prototype.

INTRODUCTION
The 700-type transmission circuit was first introduced by the Post Office in Telephone No. 700 and was described in an earlier article. Using the rocking-armature receiver this very efficient circuit promised considerable economies in the provision of local lines. The physical design of Telephone No. 700 was not new, however, being based on the 300-type telephone to minimise the cost of retooling by the telephone industry. This resulted in the rather dated case of the earlier telephone being retained, which clashed with the curves of the new handset. Subsequently it was decided that the use of Telephones No. 700 by the Post Office would be restricted to trials of transmission properties and that a completely new design of instrument, shaped to satisfy modern tastes but preserving the improved transmission performance, should be designed. This article describes the new telephone, which has been designated Telephone No. 706.

The trials carried out with Telephone No. 700 fully confirmed the high laboratory assessments of its transmission performance but revealed that subscribers with short lines would consider it too loud. After considering the various methods of overcoming this difficulty (they were briefly discussed in the earlier article) it was decided that the solution most suited to Post Office operating conditions was the inclusion in the telephone circuit of a sensitivity regulator controlled by the line current. The transmission circuit of the new telephone has therefore been modified to include a regulator developed for this purpose.

The British telephone manufacturers were invited by the Post Office to submit proposals for a new physical design. The Postmaster General decided that the most promising of these was that submitted by Ericsson Telephones, Ltd. The original design was modified to incorporate changes suggested by the Council of Industrial Design, and in its final form was approved by the Council.  See picture.

Choice of Moulding Material
Of fundamental importance in the design of the new telephone was the choice of material for the case and handset mouldings. In the past, the bulk of Post Office telephone mouldings have been made in thermosetting phenolic material. This is cheap and has a hard scratch-resistant and stain-resistant surface with good finish. A drawback is that it is naturally a dark colour and the range of alternative colours obtainable by the inclusion of pigments is very restricted. In recent years the use of thermoplastics has much increased and many new ones have become available in quantity. Generally these materials are stronger and less brittle than thermosetting materials and they are available in a wide range of colours. Against these advantages must be set the fact that they are more easily scratched, that some are susceptible to surface staining or chemical attack by such things as ink, furniture polish and cosmetics, and that they are more expensive. Thermoplastics are usually injection moulded, a much more rapid and therefore cheaper process than the compression moulding used for thermosetting materials, and because of their extra strength they can also be used in thinner sections. These factors help to reduce the difference in price.

After considering a number of alternative materials the Post office decided that polymethyl methacrylate, manufactured in this country by Imperial Chemical Industries, Ltd., under the trade name of Diakon, offered the best compromise between the sometimes conflicting requirements of cost, strength and finish. It is of interest that polymethyl methacrylate has been used by the Post Office in small quantities for coloured telephones since 1937 and has been found an excellent material. It has a translucency which gives it a very attractive appearance, and it has a hard surface, for a thermoplastic, which is not susceptible to staining. The decision to use it for all telephones was influenced by a substantial price reduction made possible by a change to a "dry mixing" moulding technique. This yields its economy by eliminating two manufacturing processes previously necessary to colour the base material and by reducing working stocks of materials.

Colour Range
The choice of a thermoplastic material for all telephones removes much of the economic necessity for supplying the majority of telephones in black and the new telephone will be supplied to subscribers primarily as a coloured instrument. It will be available in a range of six colours:-
Two-tone grey (light French grey with elephant grey)
Two-tone green (aircraft grey-green with forest green)
Light ivory
Concord blue
Lacquer red
Topaz yellow
It will also be manufactured in black.

In choosing these colours the Postmaster General was again advised by the Council of Industrial Design, while consideration was also given to the production and maintenance problems of colour matching. Slight differences in the chosen colours are harder to detect by eye than differences in pure colours such as the red and green previously used for Post Office telephones. The dyes and pigments used have been selected for their light-fastness qualities, and both natural and artificial fading tests that have been proceeding for some years show that all the colours have a light-fastness better than Standard No. 7 (BS 1006).

Weight Reduction
The use of thermoplastic material, helped by the absence of a chassis, has resulted in a considerable reduction in the weight of the telephone. Compared with 5.25lb for the 300-type telephone, the new telephone, conventionally wired, weighs only 3.1lb. The reduction in weight is most noticeable in the handset; the rocking-armature receiver weighs only 1.2oz and the total weight of the handset is 9 oz, compared with nearly 1lb for previous handsets. While the weight of the older handsets has been accepted hitherto without question or complaint, use of the new one leaves little doubt that lightness will be a feature much appreciated by subscribers.

Alternative Interior Designs
Among the designs which had been submitted to the Post Office was one which incorporated printed wiring for connecting the components instead of conventional wiring. The printed-wiring technique, although still in its infancy, shows promise of manufacturing economy and lends itself particularly to automatic assembly methods. The new telephone has therefore been designed so that it can be manufactured with the components connected by either conventional or printed wiring, the two versions having the same circuit and terminal layout and using the same components. Initially both types will be manufactured, but ultimately, should one be significantly cheaper and be satisfactory in all other respects, it alone may be made for Post Office use. To minimise the cost of the tooling changes that this will involve for some manufacturers, the alternative designs use as many common parts as possible, the differences being largely confined to the base plates.

EXTERNAL DESIGN
The chosen shape is essentially simple, obtaining its pleasing effect from its good proportions rather than by styling tricks or exaggerations. The telephone is considerably lower and a little longer and narrower than the 300-type telephone. To accentuate the low look the cover does not quite conceal the black base, which appears in shadow beneath it. The dial, which is fastened to the base and not to the cover, has been set at an angle of 30' to the horizontal, as against the 37' used previously, and has been surrounded by an enlarged number ring, it being felt that with the introduction of subscriber trunk dialling everything possible should be done to reduce the chances of mis-dialling. Above the dial is a rectangular hole, normally closed by a moulded dummy, in which a push-button may be fitted. The push-button used bears its own legend so that separate labelling on the telephone is not needed. To harmonise with the cover, and to provide a comfortable, secure grip, the handset has a rectangular handle section.

The Cover
The cover is an insert-free moulding with wall thickness varying from 0.11 to 0.14in. The cradle formed by the cover is shaped to provide a stable rest position to which the handset will naturally gravitate when slightly misplaced. Alternative positions are far enough from the normal to draw attention to incorrect replacement. To meet possible export requirements a carrying handle may be fitted to the cover.

The cover is fixed to the telephone by two screws which pass down through the cradle close to the plungers, and in addition it is held to the telephone at the front by moulded projections which hook into recesses in the base.

The external dial number-ring is a transparent moulding with characters reproduced on the back, which is then painted with the background colour. The ring is held in contact with the front of the cover, around the dial, by a spring ring. This allows it enough movement to accommodate slight variations in the position of the dial in the hole, while keeping it correctly orientated.

Handset No. 3

The Handset
The handset has been titled Handset No. 3. The design follows generally that of Handset No. 1 and in particular, the shape and relative positions of the mouthpiece and earpiece, which were the result of considerable research, have been exactly reproduced. It differs in having a rectangular handle section and in being designed for injection moulding. The problem in designing an injection-moulded handset is to reduce the thickness of material, particularly in the handle, sufficiently to make it easy to mould and to take advantage of the strength of the material. A core withdrawn through the cord entry, as in Handset No. 1, does not hollow the handle sufficiently and in the new handset a much larger core is used. The hole through which this is withdrawn is reduced to the cord-entry size by gluing within it a small auxiliary moulding (fig. 5). This method of manufacture has a number of advantages over the alternative method of making the handset from two half-shells stuck together along a longitudinal seam.

Fig. 5	Fig. 6

In Handset No. 1 the method of holding the cord required its full length to be threaded through the entry hole. This method is inconvenient with the coil type of extensible cord and an alternative method of cord fastening has been devised. In this a grommet is clamped on the cord by a metal ring, two lugs bring formed in the ring in the process (fig. 6). The cord is fixed to the handset rather as a bayonet-fitting lamp is inserted in its holder, and is released by twisting the metal ring inside the handset.

The receiver is held in contact with the earpiece by a spring ring, supported by, and captive on, ribs moulded inside the handset. Other ribs control the position of the receiver and prevent it turning as the earpiece is screwed on.

The transmitter end of the handset is designed to accept either a transmitter of the No. 13 type or a new one under development. Ribs prevent the transmitter turning and others support a spring ring, which will be required when the new transmitter is fitted. The transmitter used at present is the Transmitter-Inset No. 13C. This has a rear spring ring with three tabs to engage with the anti-rotation ribs in the handset, and has a terminal for the cord connection. A tight-fitting phosphor-bronze plug is used to provide a cord terminal on the rear electrode.

INTERNAL DESIGN
Although externally the conventionally-wired and the printed-wiring telephones look alike, their internal arrangements differ considerably and will be described separately. The gravity switch and dial mounting are identical in the two telephones and these features are described first.

The Gravity Switch
Most telephones use gravity switches that include plungers sliding in bores in the cases of the telephones. For reliability, particularly in dirty locations, such switches require appreciable force to overcome the friction of the plungers. The light weight of the handset of the new telephone halves the force available and because of this an entirely new switch has been developed. Important features of this are as follows:-

1. The "plungers," which are extensions of a pivoted bracket, do not touch the sides of the generous holes in the cover through which they protrude. The switch is therefore almost friction free and cannot stick due to dirt seizing the plungers.
    

2. The weight of the handset is opposed by the spring tensions of the contacts only; the whole weight can therefore be usefully employed.
    

3. The spring contacts are comb operated (fig. 7).

The switch is built upon a vertical bracket. This is riveted to the base of the conventionally-wired telephone and to the wiring board of the printed-wiring telephone. At the top of the vertical bracket are tapped holes for the cover fixing screws. It is the fixing of the cover to the telephone at the cradle, rather than at the base as in most telephones, that gives the accurate registration between the plungers and their holes necessary for friction-free operation. When the cover of the telephone has been removed for maintenance the switch may be locked in the handset-on position by a spring, riveted to the vertical bracket, which hooks over one plunger arm. A touch on the plunger releases the lock so that there is no risk of it being left inadvertently engaged.

Fig. 7	Fig. 8

The spring-sets are mounted upon the vertical bracket and the contact-operating combs are pushed directly by the transverse bar of the plunger bracket. They differ slightly with the type of telephone. The springs are of nickel silver with palladium contacts. Palladium, which replaces the silver of earlier telephones, has been proved to be the equal of platinum for low-current applications but costs much less. The moving springs are split for their full length, giving a true twin-contact action, and the comb operation ensures accurate twinning and proper sequence of operation. The contacts are protected from dust by clipped-on transparent covers.

Dial Mounting
Earlier telephones have had the dial mounted in the case. This is an inconvenient arrangement for many reasons and in the new telephone the dial has been mounted on the base. The dial is first held in a mounting in which it is clamped by a band around its body. The bayonet fitting used for so long is thus abandoned although the lugs required for it are still being provided on dials to give interchangeability. The dial in its mounting is supported at the front of the telephone by a simple link plate that bridges the tops of the bell gongs, and at the rear by the gravity-switch bracket.

Conventionally Wired Layout
The conventionally-wired telephone interior (fig. 8), is built upon a plastic base moulded in a tough resilient grade of polystyrene. Raised at the front of the base are two domes, perforated to allow egress of sound, on which the bell gongs are mounted. Terminal strips are provided in two ribs moulded across the rear of the base. Also formed in the base moulding are Supports on which are riveted the feet of the gravity-switch structure, ribs and holes for locating and fixing the induction coil and capacitor, and a jack for the plug-in automatic regulator. Although complicated in shape the base moulding is produced by a simple tool and it is free of moulded-in inserts, which would retard production; all threads are provided by nuts pressed into holes.

The bell mechanism is fixed directly to the base by two screws, and the induction coil and capacitor are held in moulded locations by simple spring clips. The carbon balance-circuit resistors are supported by their own terminations.

The telephone is wired with p.v.c. insulated 6.5lb/mile wire, except for the connections to the dial, which are made with stranded wire. When the dial is not fitted the ends of the wires are pushed into holes in the base moulding. The connections to the bell are separate from the cable form and are tag-ended to allow easy removal.

Printed-Wiring Layout
The printed-wiring telephone (fig. 9) is built on a pressed-steel base, riveted to which are pillars for mounting the bell gongs and the printed-wiring board. The board is supported about 0.25in. clear of the base on four of the pillars and to it is riveted the gravity-switch bracket fixing points of which coincide with the rear fixings of the board to the base, the board fixing screws actually passing through the feet of the bracket, so that all three parts are held rigidly together at these points. As a result the gravity switch is unaffected by possible warping of the board.  The board is manufactured from 3/32in. thick high-quality synthetic-resin-bonded paper clad on one side with copper 0.0014in. thick (fig. 10).

Fig. 9	Fig. 10

The terminals fitted to the rear of the board consist of U-shaped metal plates clipped to the edge and soldered to the wiring pattern. An "edge" for the second row is made by piercing slots across the board and as this weakens the board it is supported under its rear edge by a strip moulding which clips into holes in the base.

The bell mechanism is fastened to the board by the usual two screws but the capacitor and induction coil are not screwed down, being held to the wiring pattern by their soldered connections. The balance-circuit resistors are mounted directly on the board.

The bell is connected to terminals by tag-ended conventional wiring.

NEW COMPONENTS
The components used and proved in Telephone No. 700 could have been used in the new telephone and would have been quite suitable for the conventionally-wired version. Modifications have, however, been made to the physical forms of the capacitor and the induction coil to make them more suitable for the printed-wiring technique and, in the interests of standardisation, the new designs have been used in both versions of the telephone. A further component that has been altered is the bell, manufacture of which has been cheapened without loss of efficiency. The telephone has one entirely new component, the automatic regulator.

The Induction Coil
The new coil, Coil, Induction No. 31, is shorter and stouter than the No. 30 coil, which has been described previously. The change in proportions reduces the stresses on the terminal-pin connections when the coil is secured by them in the printed-wiring telephone. As grain-oriented material is still used for the core the new proportions have resulted in a slight loss in efficiency, the elongated core of the earlier coil being particularly suited to the material. As will be described later, however, the inclusion of a regulator in the telephone has enabled this loss to be compensated for in the circuit design.

The coil bobbin is a phenolic moulding with slots in one cheek through which the winding ends are led out to terminate directly upon five stout terminal-pins. The bobbin is wound throughout with one gauge of wire, which simplifies production. Because of its smooth finish, wrapping of the bobbin before winding is unnecessary and the low electrical stresses between windings have made paper interleaving unnecessary.

The laminations are assembled in the bobbin with one butt joint, equivalent to a small air-gap, and one interleaved joint. They are secured by running an air-drying varnish between them.  The balance-circuit resistors in the new telephone are separate carbon resistors, investigation having shown that if the telephone is designed for their use they are cheaper than non-inductive windings on the induction-coil bobbin. Separate resistors are particularly convenient for use in the printed-wiring telephone because of the flexibility they give to the pattern and the case with which they can be mounted.

The Capacitor
The proportions of the capacitor have been changed in the same way and for the same reason as for the induction coil. In addition, the capacitor has been given springy wiring tags which enable it to be plugged into holes in the wiring board and to remain there on its own until soldered. As ample space is available in the telephone the volume of the capacitor has been increased to make the internal design less critical.

The Bell
The changes to the bell do not affect the principle of operation or the performance and the new and old bells are interchangeable. Details of the changes are:

1. The cast and ground cobalt-steel magnet (material is too hard for normal machining) is replaced  by a moulded ceramic magnet. The new and old magnets are shown in Fig. 12.
    

2. The milled and drilled yoke is replaced by one manufactured from sheet by punching and bending operations.
    

3. The pivot for the armature has been simplified.

Fig. 12	Fig. 13

A section through the yoke, magnet and armature pivot is shown in Fig. 13. Apart from economy of manufacture the new design has the following advantages:

1. The ceramic magnet has greater retentivity.
    

2. Adjustment of the air gap is easier.
    

3. The armature pivot is more easily removed.
    

4. The hammer ball can be passed through the yoke so that the armature may be removed without demounting the bell.

To fit in the narrower telephone the diameter of the bell gongs has been reduced. Other constants of the gongs have been changed, however, so that the sound given by them is unchanged.

The Automatic Sensitivity Regulator
The need for, and electrical design of, the regulator are discussed later. Physically it consists of  a small printed-wiring board on which are mounted a multi-element rectifier, a resistance lamp and two carbon resistors. One end of the wiring board forms a 5-point plug for insertion in the jack in the telephone base; the other end is a dummy plug, insertion of which completes the circuit as an unregulated telephone. Slots are cut in the ends of the board which, in conjunction with a bar in the jack, prevents incorrect insertion. In the design of the plug and jack the emphasis was on reliability of connection rather than ease of insertion r suitability for frequent insertion. High pressures are used and the jack springs are concave in section, their edges biting into the solder coating of the copper pattern on the board, which forms the plug connections. Advantages of a plug-in unit for the regulator are as follows:-

1. It facilitates the locating of faults.
    
2. A faulty regulator may be changed, instead of a complete telephone.
    

3. Production testing is simplified. The regulator can be tested on its own, as are transmitters, etc., and a simple test to prove correct wiring is all that is then needed for the assembled telephone.

Feet
The feet are of the push-in type. A domed form has been used in place of an annular ring, to prevent pressure the marking of furniture, and they are made from a soft, non-staining artificial rubber. Their anti-sliding properties are superior to those of earlier designs.

Cords
The cords fitted to the new telephone are a marked advance on those previously used and the improvements have been achieved with a significant price reduction.

The cords have the following features:

1. Conductors
   The tinsel is laid up on high-tenacity synthetic yarn, e.g. Terylene, resulting in conductors having half the old diameter with the same tensile strength.  Life, as measured by reciprocating tests, has been  increased fourfold. The conductors are insulated with extruded p.v.c., which makes them waterproof, of a grade flexible enough to fully utilise the properties of the tinsel.
    

2. Sheathing
   The cords are made up in round section with a covering of either flexible p.v.c., of a grade chosen to minimise the risk of the plasticiser marring polished surfaces, or of braided synthetic yarn.
    

3. Terminations
   Clinched on spade tags have been used for ease of connection and economy. A double clinch tag has been preferred to the insulation-piercing r type as it gives a stronger, more reliable connection. The tag dimensions make it suitable for the variety of screw sizes met on new and old apparatus.
    

4. Grommets
   These are fixed to the ends of all old cords. They are preferred to strain cords, which are sometimes left untied. Moulded in flexible p.v.c. they prevent sharp bends where cords emerge, and so reduce wear.

Extensible handset cords will be fitted as standard to the telephones. Besides being convenient for subscribers they reduce the tendency of cords to kink and knot up, which increases wear.

The cords are manufactured in the form of a close coil, the retracting force coming from the resilience of the p.v.c. sheath. Cords of this type cost less than textile extensible cords and have a greater extension, up to six times the closed length. Life tests have shown they will withstand over half a million extensions.

Terminal Block
The new block (Block, Terminal, No. 30) is shaped to match the telephone. The mouldings are in impact polystyrene, the toughness of this material being preferred to the better finish of polymethyl methacrylate for this application. The mouldings are free of moulded inserts. Threads for the terminals and the cover-fixing screw are provided by pressing nuts into holes. There is an entry for a cord, with grommet, at one end of the block and an additional one can be provided by breaking out a thin section of the cover. Other "knockouts" provide cable entries. Six terminals are provided, normally linked in pairs to give a three-way block; the links may be removed to give six separate connections if needed.

FLEXIBILITY IN USE
The advantages of making a telephone suitable for a wide range of use have already been discussed in connection with Telephone No. 700. With the new telephone the need to restrict the number of variations stocked is even greater because of the wide colour range. Measures taken to achieve flexibility include:-

1. making provision for one push-button in every telephone,
    

2. incorporating an extra spring in the gravity-switch spring-set so that the basic telephone can be converted to shared-service working,
    

3. providing a number of holes for fixing auxiliary parts (these are punched during manufacture at the same time as other holes and add nothing to the cost,
    

4. incorporating a large number of connection links within the circuit, and
    

5. providing a number of spare terminals for connecting auxiliary components.

These features enable the basic telephone, stocked as Telephone No. 706L with a dial and Telephone No. 706CB with a dial-dummy, to meet all the requirements for a table telephone with or without a push-button. For some purposes auxiliary units have to be added to the telephone by the installer and some of these will be described.

Shared-Service Adaptor
The shared-service adaptor, when fitted in the telephone, makes it suitable for automatic separate- metering systems. It is an assembly of a micro-switch, a push-button and a Thermistor which are fixed within the telephone by a single screw.

Hitherto the circuit of shared-service telephones has included a rectifier. This is used to hold the A relay, in the selector seized at the exchange, during a short break in the loop when the telephone calling key is released. The micro-switch has a change-over time of less than 5 ms, which is too short to pulse the A relay so that it enables the rectifier to be omitted. Connection of the adaptor to the telephone circuit is made by flexible wires with spade ends. Fig. 16 shows the connections.
 

Fig. 16

The extra gravity-switch spring enables the new telephone to be used on C.B. common-metering systems without the extra capacitor that has to be added to Telephone No. 332.

Push-Button Units
These are assemblies similar to the shared-service adaptor but without the Thermistor. There are two, Switch No. 5A-1, which is non-locking, and Switch No. 5A-2, which locks down. Pressure on one end of the push button of Switch No. 5A-2 locks it down; pressure at the other end releases it. The non-locking push-button has no step. The snap-on tops of the push-buttons form labels. They are transparent mouldings with the characters and backing colour on their undersides.

Additional Spring-sets
Additional spring-sets can be fitted on the side of the itch bracket opposite to the normal spring-set. similar to those used in the conventionally wired telephone.

Local-Battery Unit
A C.B. telephone may be converted into a local-battery telephone by adding to its circuit a choke-coil, a battery and a gravity-switch contact. To alternating current the choke can be considered an open-circuit and the battery impedance is negligible so that for speech currents the circuit reduces to the transmitter alone. The choke is a low resistance to direct current, however, and provides a path for the local transmitter polarising current. An assembly comprising the extra parts converts the Telephone No. 706 into an efficient anti-side-tone local-battery instrument.

It allows the Post Office to offer local-battery subscribers the latest type of telephone and on subsequent conversion to automatic working it is only necessary to remove the local-battery unit and fit a dial to the telephone, instead of changing the complete instrument. The local-battery instrument is also very suitable for use on long private wires.

MAINTENANCE ADVANTAGES
To be economical to maintain, a telephone must be reliable so that attention is rarely required, and accessible, so that when faults do occur they can be rapidly found and cleared. On both counts Telephone No. 706 is an improvement on earlier telephones. The following factors should reduce the likelihood of faults occurring:

1. The moulding material is stronger than thermosetting materials and has been proved satisfactory by many years' service.
    

2. The "frictionless" gravity switch with its comb operated twin-contact spring-set and dust cover should be free of all sticking and contact faults.
    

3. The new cords have been proved by accelerated life tests and field trials to be superior to any used previously.

Factors which make the telephone easy to service are as follows:-

1. Only two screws have to be withdrawn to take off the cover and reveal the inside, compared with seven to give full access to a Telephone No. 332.
    

2. The cover is removed without turning the telephone upside down, so that dirt within does not get shaken up, causing further faults.
    

3. Because the dial is fixed on the base a completely working telephone is left when the cover is removed, and there is no risk of the dial cord being trapped in a vulnerable part of the dial when the telephone is re-assembled.
    

4. The bell can be replaced without the use of a soldering iron.
    

5. The gravity-switch lock allows the telephone to be worked on without losing incoming calls.
    

6. The plug-in regulator is easily replaced if faulty and helps in fault tracing.
    

7. Spade tags, and grommets in place of strain ties, speed cord changing.

The automatic regulator might be expected to increase fault liability and much attention has therefore been given to the reliability of its components. The reliability of resistors and rectifiers is well known, while the resistance lamp, not to be confused with a Barretter, has a robust filament which is much under-run and should therefore have a very long life. Confidence in the components has been established by accelerated life tests.

TRANSMISSION FEATURES
Assessment of Excess Sensitivity of Experimental Telephones and Need for Sensitivity Regulation

The earlier article suggested  that the level of transmission performance of  the Telephone No. 700 might be sufficiently high to become an embarrassment on short lines. To test the reactions of subscribers, trials were therefore immediately carried out concurrently with subjective assessments in Research Branch. Some 400 extension telephones with very short lines on the Telephone Managers' P.B.X.'s at Canterbury and Portsmouth were changed to the Telephone No. 700 and after one month all users were asked to express opinions on the performance of the new instrument. Some 75 per cent of the users classed the reception as too loud on internal calls while some 50 per cent also found it too loud on external calls.

The laboratory test was conducted under more controlled conditions; the conclusions reached were similar to those of the field trial but were capable of being expressed more precisely. The following estimates were made in terms of loudness sensitivity, this criterion being used rather than articulation because it is the loudness of the received signal which is troublesome; articulation at these high levels always approaches 100 per cent:-

1. The maximum loudness sensitivity which a telephone connection should provide is +31dB relative to a metre-air-path (monaural listening to a talker at a distance of 1 metre in echo-free surroundings).
    

2. If possible, side-tone loudness sensitivities should not exceed +25dB relative to the metre-air-path.

Approximate ranges of loudness are given in Fig. 19, which shows that the greatest loudness sensitivity using Telephones No. 700, i.e. two telephones connected directly via a transmission bridge providing full feeding-current, exceeds the maximum desirable by some 10dB. In fact, a similar connection employing 300-type sets is also some 2dB too loud. At the other end of the scale, minimum loudness sensitivity is reached when two telephones, each on its maximum local line for the particular local exchange transmission bridge, are connected together by a junction having a transmission loss of 27dB. Thus the economic loudness range for telephone calls in the United Kingdom network at present extends over some 37dB, from 21dB below the minimum preferred to about 4 dB above the maximum preferred, i.e. to the maximum desirable.

Fig. 19

For all telephones, local-line planning ensures that sensitivities are not less than -6dB relative to a metre-air-path by setting an appropriate limit for each gauge of conductor according to the sensitivity of the instruments available, e.g. 660ohms T.E.R. for the 300-type set and, now, 1,000ohms T.E.R. for the 700 type set, when each is connected to a 50volt transmission bridge.  That the maximum desirable sensitivity has been exceeded by 2dB using 300-type sets is of little account. However, the excess of 10dB for the Telephone No. 700 is more serious and it was decided that some inexpensive form of regulation should be developed giving approximately 6dB sending, 4dB receiving and at least 7dB side-tone loudness reductions when the telephone is connected to very short lines with, if possible, appropriate reductions for medium lines. The original design objective, that the telephone must be usable on 1,000ohm T.E.R. lines, was still to be met.

Other Considerations
A further need for sensitivity regulation to avoid discomfort for the telephone user is that, with very sensitive instruments, the risk of a subscriber receiving an acoustic shock due to the inevitable pulses of energy fed on to the line by switching operations is greater. There are also engineering reasons:-

1. The increasing number of the new telephone sets in the network will cause the mean speech voltage appearing at the input of broadband amplifiers feeding carrier systems to rise, thereby increasing the risk of inter-channel crosstalk.
    

2. Near-end crosstalk, which becomes greater for sets of increased sensitivity, can at least be reduced on the short lines.
    

3. Without sensitivity regulation, the large difference in performance between telephones on long and short lines may be noticed by subscribers, so giving rise to complaints from those on the longest lines.
    

4. "Howling" can occur with modern high-sensitivity transducers having poor side-tone suppression when the handset is placed on a desk or table, due to. the acoustic coupling between the transmitter and receiver.
    

5. There is a loss of secrecy when the received level is high.

Choice of Method of Regulation
Of the methods of reducing sensitivity on short lines described in the earlier article, it was decided to adopt that of including a line-current-sensitive regulator within the telephone. The principal reason for this was that it largely overcomes the problem of the private-branch exchange (P.B.X.) extension, which on some calls is on a short line to the P.B.X. only, yet on others may be connected via a long line to the public exchange. The problem can be solved by combinations of attenuated telephones and P.B.X. connecting links but each installation would need individual treatment and thus create administrative difficulties. The decision to use automatic regulation was further influenced by success in developing an entirely new, comparatively inexpensive device free of the inherent defects of earlier types.

The New Transmission Circuit
The small, inevitable, long-line loss of the regulator and the slight loss in electrical efficiency of the induction coil, compared with that of the Coil, Induction No. 30, are compensated for by emphasis on design for long lines and by a slight change of transmission circuit Y ratio.  The Telephone No. 706 is thus equal in transmission performance to the Telephone No. 700 on long lines and the 1,000ohm T.E.R. limit is still attained. For the Y ratio of 3.3, winding ratios of 1 to 67 (winding 1: winding 2), and 2 to 86 (winding 1: winding 3) are used. The windings are connected into the circuit in series-aiding and their sequence on the bobbin has been chosen for maximum overall transmission efficiency. The induction-coil core is designed for maximum inductance at 30ma, for which the small air-gap of 1-2 mils obtained by using a butt joint is adequate. The balance network is of asymmetrical pye configuration and includes the 1.8uF d.c. blocking capacitor. The network design allows the use of 15 per cent tolerance capacitors and 10 per cent preferred value resistors.

Automatic Regulator
The regulator is basically a variable-loss network of low d.c. resistance inserted in series with the transmitter with connections to two other points (PI and P2) in the transmission circuit. It provides a.c. shunt paths between each of these points and the junction between the transmitter and the induction coil windings, the value of the shunt being determined by the magnitude of the line current. A detailed description of its operation is being published elsewhere.

The shunt path to point PI has an impedance of 60-70 ohms and that to P2 an impedance of 30-40 ohms when the telephone is connected to a short line with full feeding-current, say 95 ma. These values produce sending and receiving losses relative to the Telephone No. 700 of about 6 dB and 4 dB respectively. The circuit and the loss ratio of the two shunt paths is also such that side-tone is reduced by some 10db or more according to the impedance of the line. On long lines, having a line current of 30-40ma, the impedance of both paths rises to several kilohms and therefore the regulator losses are very small.

The resistance of a rectifier element connected in the forward direction falls rapidly when the voltage across it is increased from zero. The change in line current is made to produce a change in the bias voltage applied to a pair of such elements by passing the current through a low-value resistor, R1, as shown in Fig. 21(a) opposite. If a single rectifier element were used in a shunt path, noticeable distortion would arise because the element resistance would vary according to the instantaneous value of the speech voltage. This is overcome by connecting two elements in a "push-pull" arrangement, as shown, to ensure similar treatment for alternate half-cycles and thus reduce distortion. By d.c. driving the elements to a much lower impedance than is required, and adding a series resistor R2 to give the correct total value, distortion in the shunt path is further reduced since part of the path is then linear.

This is necessary to avoid d.c. paths within the telephone circuit shunting each section of a push-pull pair to a different degree, thus producing unequal d.c. bias. The circuit becomes that of a Wheatstone bridge in which the voltages at points 3 and 5 are equal, and thus d.c. resistance paths across these points have no effect. The resistance of the bell is not catered for by this arrangement and therefore a small degree of unbalance does exist, but the bell resistance (1,000ohms) is sufficiently high for the effect to be negligible.

In Fig. 21(b) above, a set of oppositely-poled parallel-connected rectifier-elements has been added. This is necessary so that the circuit can cater for either direction of line polarity. The second shunt path is added in Fig. 21(c) above, with its appropriate resistor R3 necessary to a just the path impedance to the required value. In the diagram R1 is shown as a resistance lamp. The characteristic of the lamp is such that it has a total resistance of 36 ohms at 76mA, falling to 10ohms at 30mA. It can be seen in Fig. 14 and is used in place of a linear resistor for two reasons:-

1. On long lines its resistance, and therefore its  unwanted transmission loss, is low.
    

2. It provides a greatly increased voltage on short lines for biasing the rectifiers.

The required resistance/voltage characteristic of each section of a push-pull pair of rectifier elements is obtained from two 0.25in. diameter selenium disks in series and the rectifier assembly is mounted in a nylon tube containing 16 plates, tapped at every two and interconnected as in Fig. 21. The rectifier assembly can be seen in Fig. 14.

TRANSMISSION PERFORMANCE OF TELEPHONE No. 706
Full-scale subjective tests are costly and will be delayed until sufficient telephones are available from production lines for average instruments to be selected. Information given here is based mainly on a combination of subjective loudness and pure-tone assessments. So far, in this article, discussion has centred around the very-short-line, and very-long-line performance of the telephone. It is equally important that the performance should have the optimum line length on all intermediate lines.  Ideally regulation should provide a constant and sufficient speech voltage applied to the junction terminals of the exchange transmission bridge whatever the composition of the local telephone circuit and the characteristics of the talker, and a constant sound pressure in the ear of the listener whatever the speech voltage applied to the junction terminals, i.e. the local telephone circuit should be transmission equalized. Thus considering Fig 22(a) and (b) opposite, the ideal local-telephone-circuit sending and receiving sensitivity characteristics are lines at the appropriate sensitivity level parallel to the line-resistance axis. For any given non-regulated telephone the local-telephone-circuit sensitivity falls with increase of line resistance, as shown typically for the Telephone No. 700. Because its maximum sensitivity is fixed, over the economic line-resistance range of the instrument, the sensitivity must fall below ideal on the higher-resistance lines. The more practical "best economic" characteristic may be predicted as shown by the dotted lines, the ideal and best economic characteristics being identical on the lower-resistance lines.

Cost, of course, precludes the provision of the ideal condition. If the best economic characteristic is provided, then the loudness level on receiving for any connexion will never exceed the maximum desirable and in fact local calls between all subscribers whose lines do not exceed about 500 ohms resistance will have the same overall sensitivity. The actual performance is shown by the full line to be about + or -1db from the best characteristic for the particular junction and transmission bridge, the performance being of this order for other types of junction.

FUTURE DEVELOPMENT
Telephone design is a continuous process in which all related developments rarely come to fruition together. Changes which will probably be made to Telephone No. 706, and which have been anticipated in the design, include replacement of the Transmitter-Inset No. 13 by a new transmitter and modification to the dial to improve its appearance and to reduce its cost. Use of Telephone No. 706 in the more complicated extension plans is also under consideration and a multi-key plinth may be added to it for this purpose. This also has been anticipated in the design to the extent of providing the necessary fixing holes and cable entries in the base. In the transmission field an accurate assessment of the transmission performance, using the latest subjective techniques, remains to be completed.

CONCLUSIONS
An aesthetically pleasing telephone has been designed which it is confidently believed is the most sensitive, yet best regulated, telephone in the world today. By its use the cost of line plant and the maintenance of telephone service will be considerably reduced and it is expected that when in quantity production the new telephone will cost no more than the existing instrument.

ACKNOWLEDGEMENTS
The new telephone is the product of close co-operation between the Post Office and the whole telephone industry but particular credit is due to the design staff of Ericsson Telephones, Ltd., who were largely responsible for the physical design, and Siemens Edison Swan, Ltd., who designed the printed-wiring interior and base. The authors wish to thank colleagues in both the Post Office and the design departments of the manufacturers for help in producing this article and to thank Ericsson Telephones, Ltd., for permission to reproduce certain of the illustrations. The manufacturers of the new telephone are: The Automatic Telephone and Electric Co., Ltd., Ericsson Telephones, Ltd, The General Electric Co., Ltd., Phoenix Telephone and Electric Works, Ltd., The Plessey Co., Ltd., Siemens Edison Swan, Ltd., Standard Telephones and Cables, Ltd., and The Telephone Manufacturing Co., Ltd.

References
The New, 700-Type Telephone. P.O.E.E.J., Vol. 49, p. 69, July 1956.
The Rocking-Armature Receiver. P.O.E.E.J., Vol. 49, p. 40, April 1956.
B.S. 1006, Part 2. Determination of Fastness to Daylight of Coloured Textiles (British Standards Institution).
Patent Application No. 25782158.
Introduction and Application of Transmission Performance Ratings to Subscribers' Networks. I.P.O.E.E. Printed Paper No. 198, 1949.
Transmission Circuits for Telephonic Communication (The Library Press, 1924).
Some Principles of Anti-Side-Tone Telephone Circuits. P.O.E.E.J., Vol. 48, p. 208, Jan. 1956.
Patent Application No. 39958157.
The Design of an Automatic Sensitivity Control for a New Subscriber's Telephone Set: The British Post Office 700-Type Telephone. Proceedings L.E.E. Paper No. 2867E.

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Officially introduced in 1960

How to remove the case from a Telephone No. 706 or 710

1. Remove handset and slacken the two screws underneath the handset.
2. Lift the rear of the case upwards and forwards.
3. Once clear of the dial, pull the case away from the front of the base plate to remove completely.

Re-fitting is the process in reverse, except you may have to move the casing dial ring to locate it over the dial.

Additional information - Model types

Last revised: January 17, 2024