Click here for history of the development of the Trimphone
Sales Circular 342/65 - Telephone No. 712
Sales Circular 121/68 - Telephone No. 722
Extract from Hansard
General fault finding on your phone
Trimphone Prototype
Other colours - Public survey

Circuit diagram - N812.

Specification - Mark I is S(W) 2072 and Mark II is S(W) 2091.

The Trimphone started life in 1964 as the Telephone No. 712 Trimphone.  The (then) modern design incorporated the novel feature of dial illumination, tone calling and a unique handset.  The initial four letters of the name Trimphone stand for Tone Ringer Illuminated Model.  The Trimphone was designed by Martyn Rowlands.

The handset was coded 'Handset No. 8' and featured smaller transducers (Inset Receiver No. 13 and Transmitter No. 15) mounted adjacent to one another in the earpiece cavity.  The transmitter being coupled to the mouthpiece by an acoustic horn.  The transmission circuitry was based on that of the Telephone No. 706.  The hollow handset led to some embarrassing results when customers attempted to cover the mouthpiece by hand in order to make a confidential aside - the sound was still transmitted inside the handset!

The Trimphone was the first in the BPO range to use a tone caller which warbled at around 2000Hz modulated by ringing current.  The volume of the ringer gradually built up over the first few cycles of ringing current.  There is a volume control in the base of the telephone with LOUD, MEDIUM and SOFT settings (OFF setting was achieved by slackening off a screw on the tone ringer board inside the phone - engineers work).  Some people were able to mimic the sound of the tone ringer by simultaneously whistling and wobbling their lips... a vulgar habit which should be frowned upon.

This innovative design by STC, half the weight of the more traditional 700-type telephone, originated in 1961 when the Post Office decided it needed a luxury telephone to add to its range.  Towards the end of 1963 the Post Office settled on the design by STC, and in 1964 placed a contract for 10,000 units.  The first example of the Trimphone was presented in May 1965 by the Postmaster-General, Anthony Wedgwood Benn, to a newly wed couple in Hampstead in a ceremony marking the installation of the ten millionth telephone to be installed in Britain.  Production of the new telephone commenced in 1965, and an initial quantity of 1000 was offered to customers on a selective trail basis in the London North West Telephone Area in the same year, before becoming available throughout the country in 1968, at extra rental cost ("Modern Telephone Charge" of 1 on top of the standard rental cost), with a choice of three two-tone colour schemes: Grey-white, Grey-Green and two-tone blue. 

In a reply to a Parliamentary question about Trimphone numbers, the Postmaster General replied that around 36,000 had been installed by June 1968 and that monthly sales had averaged out at 4,350.  By 1980 there were 1.6 million in operation out of a total telephone population at that time of 27 million.

There was also some concern about the luminescent dial which glowed green in the dark.  This effect came from a small glass tube of tritium gas, which gave off beta radiation and made the dial fluoresce.  Although the radioactivity was equivalent only to that given off by a wristwatch it was felt wise to withdraw this facility as public concern over radioactivity grew.

Another problem with the dial version of the Trimphone was its light weight, 0.8kg compared with 1.4kg for the 700-type and 2.6kg for the 300-type telephone.  This led to the complaint that on slippery surfaces the telephone turned and slid whilst dialling.  The fix for this was to wet the feet and the phone stuck to the table!

An improved version, the Telephone No. 722, was introduced in 1968.

Source - Rob Grant

Notable differences between the 712 and the 722

712 has:-
Case fixed by 3 screws through the base.
Has a plug in  regulator.
Liable to have a dial with letters.
Earpiece fixed to handset by a single screw on the upper part of the handset.

Click the picture to hear a Trimphone warbler

An article by Alan Hollingdale

I was working as a draughtsman at STC in 1965 and arrived there just at the right time to be given an excellent project to work on.

I was handed a futuristic looking phone which was, due to it's shape, known as the Delta Phone.

The concept of the design had come from Lord Snowdon's office and all the drawings were marked PROTOTYPE and were unfit for major production work.  One of the first things that I had to do was to design a new cover fixing that could be removed by a single screw either on the rear or on the top.  The "rather strange" reasoning for this was that to remove the three screws from the underside, although it was envisaged that an engineer would hold the instrument in his hand to do so, it may scratch the customer's highly polished table if placed upside down to remove the screws.  Initially, a single captive metal screw was positioned low down at the centre of the rear but this proved to be rather fiddly in trials and so a single nylon screw was put in the centre of the top under the handset.  This then presented the problem of how to secure the front of the cover to the base.

Several ideas were worked up into trial models, a major criterion being that of ease of tooling/moulding.  Eventually a compromise was reached and is visible, or rather invisible, on all subsequent models.

On the later production model you will also notice that the ribs on the underside of the base were deemed an unnecessary cost and so it is now smooth.  During the year or so that I was working on the Delta phone several irksome modifications were made in order to reduce production cost.  I can recall on one occasion that I had just completed the layout of the tone board for the second or third time when I was presented with a different capacitor.  This particular 'cheap' component's terminals were a few thou further apart which meant redesigning the whole blasted thing again - all to save something like a halfpenny per thousand!

Along with all the other design changes it was decided to have two separate printed circuit boards keeping the tone generator separate from the main phone unit.  Another cost saving exercise was to do with the etching of the printed circuit boards.  Cost was not a consideration on pre-production units but when it came to the amount of copper to be etched away from the PCB's it had to be kept to a minimum.  The removal of too much copper would not only be wasteful but also the etching medium which would quickly become contaminated.

In 1966 I was also working on the push button variant although it was to be several years before it was offered to the public.

Well, that's about as much as my ageing grey matter can recall after so long!


Field Trial of the Trimphone Telephone No. 712
by F.E. TROKE (taken from POEEJ)

A new type of telephone incorporating several novel features and  having a very modern appearance is undergoing field trial.  The instrument has a handset of unique design and utilises a tone caller instead of a magneto bell.

IN accordance with its revised commercial outlook the Post Office is to offer an alternative telephone instrument.  The modern design, approved by the Council of Industrial Design, incorporates the novel features of dial  illumination, tone calling, and a unique handset features which give rise to its name, Trimphone, from the initial   letters of Tone Ringing Illuminated Model.

The Handset
The whole design concept of the new telephone arises from the unique handset (coded Handset No. 8), which  in turn is based upon the light-weight headset used by operators and known as Headset No. 1.  The small   transducers (Inset Receiver No. 3T and Transmitter  No. 15) are mounted adjacent to one another in the earpiece cavity, the transmitter being coupled to the mouthpiece by an acoustic horn.  The handset parts are moulded in acrylonitrile butadiene styrene (ABS), a tough thermoplastic with a good surface finish that is currently used for Telephone No. 706 mouldings.  The mouthpiece grid is located by a lug and attached by a screw that is then obscured by a polypropylene button.  This button also has the function of preventing chafing where the mouthpiece rests on the body of the telephone.  The acoustic horn, of toughened polystyrene, is fixed within the handset by adhesive; its lower end is sealed to the mouthpiece cavity, and at the top end the transmitter is held by four lugs, the interface being sealed by a neoprene washer.  The horn insert provides acoustic coupling between the mouthpiece cavity and the transmitter, equalising the frequency response in the same way as the horn in the Headset No. 1.

The receiver is retained in position by a metal plate and a rubber ring seal between the receiver and the earpiece.  Two lugs, which are an extension of the metal plate, clip the earpiece on to the handle by engaging behind two moulded bosses; a special tool, which can be inserted in the joint-line, is required to release these clips.  A light-weight helical cord, with four conductors and a p.v.c. covering, connects the handset to the body of the telephone.

The Telephone Body
t712.jpg (8117 bytes)The cover of the telephone body, moulded in ABS, is attached to the toughened-polystyrene base-plate moulding by three screws, which are inserted from the under-side.  The gravity-switch bar, moulded from smoke-tinted polycarbonate. to match the dial finger-plate, passes freely through two holes in the cover and is attached by a pivot rod to a bell crank.  The gravity-switch spring-set is mounted on a metal bracket attached to the cover; this bracket is extended to form two knife-edge bearings for the bell crank, and a helical spring between the two parts keeps them in close contact and provides the restoring force for the gravity-switch bar.

The dial is mounted on the base plate and protrudes through a close-fitting hole in the cover; to permit alignment it is flexibly mounted by a three legged P.V.C. moulding.  The pulse mechanism is identical to that of the Dial No. 21, but the body is modified to contain a luminescent tube behind a translucent number ring.  A thin coating of aluminium is vacuum deposited on the surface of the cavity within which the tube is fitted; the coating provides a highly efficient reflector to make the best use of the light emitted by the tube.  This is a sealed glass tube that has a fluorescent coating on the inner surface and is filled with a small quantity of tritium, a low-intensity radioactive gas (an isotope of hydrogen).  The low-energy Beta radiation energises the fluorescent coating and is then absorbed by the glass.  The secondary radiation (Bremsstrahlung) which then arises has been confirmed by both the Post Office Radiological Officer and the Radiological Protection Service to be much less than the recommended maximum for luminous wrist watches.  The tube is expected to have a useful life of at least 10 years.  The illumination, although unnoticeable under normal levels of incident light, is ample to enable the telephone to be located and used in the dark.  The fingerplate, transparent to avoid masking the low-level illumination as well as being an attractive design feature, is moulded in smoke tinted polycarbonate., a tough thermoplastic.

For those installations requiring a single press-button, a micro-switch with a change-over contact can be mounted at the front of the base plate with its polycarbonate. press bar projecting beneath the front edge of the cover.  The 4-way line cord enters the base plate at the rear edge, whereas the handset cord enters at the side, conventionally from the left, but it may be transferred to the right if preferred.

The Tone Caller
tonecal1.jpg (10736 bytes)Beneath the dial is mounted the printed-wiring board of the tone caller, which is used instead of the more usual magneto bell; it emits a pleasantly-modulated tone, the volume of which is adjustable.  The tone-caller circuit consists of a single-stage transistor oscillator tuned to about 2,000 c/s, the basic waveform being modulated by the ringing frequency.  The output feeds a modified rocking-armature receiver that is positioned by the circuit board above an orifice in the base.  The diode Dl acts as a half-wave rectifier of the incoming ringing current, resistor R1 and capacitor C1 smooth the waveform, resistor R3, with other resistors in the circuit, controls the bias applied to the transistor, and the frequency of oscillation of the circuit is determined by capacitor C3 and the inductance of the receiver.  Thermistor TH1, diode D2 and capacitor C2 provide a threshold to guard the circuit against false operation by random pulses on the line.  Thermistor TH2 in parallel with resistor R4 delays the build up of the volume if the LOUD or MEDIUM settings of the volume control are used, and resistors R5 and R6 attenuate the output for MEDIUM and SOFT settings of the volume control.  The knurled edge of the control knob projects through a slot in the rim of the baseplate so that it is just visible beneath the edge of the cover.  Instead of a bell on/off switch, a locking screw can be withdrawn from the volume control, permitting the knob to be turned to an OFF position.  The shunt resistor R2 is incorporated to improve the performance of an additional magneto bell, which may be connected in series with the tone caller if required.  Click here for instructions on the on/off screw

Circuit Arrangement
The circuit of the Telephone No. 712 is the same as that of the basic Telephone No. 706, incorporating the Induction Coil No. 31, and Regulator No. 1A, but, to economise in space, three 0.9uF capacitors of metallised polyester film encased in polypropylene are used instead of the larger 1.8 + 0.9 uF unit used in the Telephone No. 706.  The same 19 terminals are provided to facilitate connection of the new telephone as an alternative to the Telephone No. 706 in extension plans in accordance with standard arrangements.

It is essential that the transmission performance of the new telephone should be at least as good as that of the Telephone No. 706. Exhaustive tests at the Post Office Research Station, using loudness comparisons by trained crews, measurements of pure tone sensitivity/frequency characteristics, and conversation tests in which subjects are permitted to hold the handset as they wish, show that the performance of the new instrument is comparable to that of the Handset No. 3.

Production of this new telephone commenced in the early part of 1965 and an initial quantity of 1000 is to be accepted by the Post Office.  These will be offered to selected customers on a trial basis in order that the validity of the radical design can be proven in use by members of the public. The first contract will be completed later in the year with any modifications which the field trial may show to be necessary.  It will then become freely available with a choice of three two-tone colour schemes: grey-white, grey-green, and two-tone blue.

The new telephone has been developed for the Post Office by Standard Telephones and Cables, Ltd., under the British Telephone Technical Development Committee procedure.

An unusual Telephone No. 712 in clear plastic - used for promotional purposes


Part I gives a description and installation information. Part II describes the maintenance procedure.

The Telephone No. 712L MARK I was the subject of a restricted field trial which took place mainly in the LTR North West Area and is described in Works Specification s(w) 2072.  The MARK II telephone described in this specification differs from the MARK I telephone mainly in two respects. It has a more sensitive tone caller whilst the receiver earcap is now held on by a nylon screw.  The MARK II telephone will be available in the following colours OFF WHITE; GREY GREEN; BLUE.

Advantage has been taken of modern design techniques to achieve a lightweight telephone of small dimensions.  The telephone weighs only 2 lbs. and measures 7.5" long, 4.25" wide and 4" high.  The size of the body has been reduced by substituting a tone caller for the magneto bell.  The handset extends from the front to the back of the set, overlying the dial which is illuminated from within by a phosphorescent light which glows sufficiently bright to locate the set in the dark and allow the dial characters to be read.  A plastic cradle bar across the middle of the set performs the dual function of gravity switching and acting as a carrying handle whilst he telephone is in use.  The instrument incorporates a modified 706 circuit.

The novel design of the handset owes its origin to the Headset No. 1, using the same small transducers; Receiver Inset No. 3T and Transmitter Inset No. 15.  The resulting slim handset tapers from the earpiece to the mouthpiece and weighs only 5.5 oz. and can be held comfortably between finger and thumb.  The receiver inset is held in position by means of a spring clip which is screwed to the earpiece moulding by four screws. The terminals of the receiver extend beyond the clip plate and a polythene membrane is clamped beneath the clip to insulate the terminals from the transmitter, should they touch.

The transmitter inset is fitted adjacent to the receiver under the earpiece and the mouthpiece is coupled to the transmitter by means of an acoustic horn. The transmitter is fitted between the lugs of .he horn adaptor, the frame terminal being positioned in the corner of the opening to avoid its fouling either the earpiece or the clip.

The ear-piece is located by a tongue which slides inside the body moulding and held in place by a non-captive nylon screw.  An extra light coiled handset cord (Cord, Inst. No. 4/101AX Colour 7.5") has been chosen to prevent drag on the handset or the telephone. The mouthpiece is in the form of a grid which covers the opening to the acoustic horn.

This consists of a conventionally wired modified 706 circuit with a jack-in regulator.  There is no change to the transmission circuit but dialling now takes place via one winding of the induction coil the brown wire of the dial being connected to T3.

The gravity switch assembly is screwed to the inside of the cover and the springset is operated by a lever mechanism which pivots on a knife-edge.  The lever is acted on by the plastic cradle bar which is hinged to the ends of the lever.

The dial is supported above the tone caller by a three-legged pliable moulding which provides a tight fit on three pillars projecting vertically from the base through spring collets in the printed wiring board of the tone caller. The tone caller is also located by a fixing screw.

The cover which is shaped at the top to accommodate the handset is held firmly to the base by three fixing screws.

This is a transistor oscillator which emits bursts of warble tone which can be varied in intensity by means of a volume control knob the knurled edge of which projects slightly from one side of the base near the front

The control knob has four settings as follows:-
1 - OFF - No tone.
2 - SOFT - Soft warble tone of constant intensity.
3 - < - Soft warble tone gradually building up to an intensity similar to that obtained for the loud position.
4 - LOUD - Loud warble tone of constant intensity.

The OFF position is not normally available to subscribers with only one telephone.   However when the telephone is to be used as an extension instrument, e.g. in a Plan 1A, where the "Bell off" condition is required this can be achieved by:-
(a) removing the screw which holds, the red coloured spacer on the volume control.
(b) reversing the spacer and using the other hole.
(c) replacing the screw so that it does not project through the printed board to limit the movement of the control knob.

This dial has a mechanism similar to that for the Dial Auto No, 21.  The body moulding has been enlarged to accommodate the Betalight which is used to illuminate the dial. The Betalight consists of a sealed glass tube with an inner fluorescent coating and is filled with Tritium gas, the radioactivity of which causes it to glow (Radiation is much less than the recommended maximum for luminous wrist watches).  This tube is mounted behind the translucent character ring beneath the dial finder plate. The label protector can only be removed by using an Extractor No. 29.  A screwdriver cannot be used.  Care should be taken when replacing the protector to locate one of the slots, on the protector skirt, with the lug in line with the Figure 9.
Reference should be made to Engineering Instructions, TELEPHONES, Stations, A 3215 which details the precautions to be observed when handling and storing these items.

The telephone is fitted with a new extensible line cord - CORD, INST, No. 4/104 AX COLOUR 54" - which enters the base, at the rear of the instrument, via a keyed hole and can be locked by twisting the cord so that the splines in the brass ring are about 45 degrees out of' line with the keyway.  The conductors are terminated on a centrally placed terminal strip after having passed through the lower slot in the regulator board.

The handset cord - CORD, INST, No. 4/101 AX COLOUR - enters the middle left-hand aide of the base and terminates on the same terminal strip.  This cord also enters the handset by a keyed entry hole and the conductors travel the length of the handset body to the connections on the transmitter and receiver insets.

The following cords are being made available for use with the telephone.
(a) CORD, INST, No. 4/102 AX COLOUR 180".
(b) CORD, INST, No. 4/102 AX COLOUR 300".
(c) PLUG No. 420 GREY - 1A (This is a PLUG No. 420 fitted with a cord similar to the extensible line cord).

One non-locking press button unit may be fitted to the front of the telephone by:-
(a) removing the cover.
(b) removing the press button aperture blank.
(c) locating the switch on the two pillars which are positioned just behind the aperture.

The unit is coded Switch No. 13A-1 and is complete with a micro switch with one changeover springset and a button.

The telephone may be used in most circumstances to replace a Telephone 706 with or without press button in the standard 700 range of DEL and extension plan arrangements. It can also be used to replace a Telephone No, 710 with two buttons where the facilities required are Bell ON-OFF (see para. 6) J1d a simple make, break or changeover contact. The circuit of the telephone and the wiring of the add-on unit are shown on Diagram N 812. Only one Thermistor must be left connected in the wiring circuit even if there axe a number of tone callers and bells in series. The thermistors not required should be removed and Terminals T16 and T17 strapped. The Thermistor has tags which are smaller than those fitted on the normal Thermistor No. 1A-1. If necessary a Thermistor No, IA-1 may be fitted providing the prongs of the tags are first slightly squeezed together. No additional Thermistor should be fitted on shared service lines.

The limits on the number of bells that may be joined in series as laid down in E.I. TELEPHONES, Stns., D 1001 will still apply to tone callers and to a mixture of tone callers and. bells. However since when using one or more tone callers a Thermistor is always required difficulty may arise on long lines where more than four calling devices are fitted and the number of bells exceed the number of tone callers. In these cases a local ringing converter may have Lo be fitted.

The telephone cannot be fitted with add-on gravity switch springsets but can be used as an extension on a 14-wire PBX if a 2 wire/4 wire conversion unit is available.

An instruction card (it 3717 Trimphone) is available.  This contains notes to help the subscriber use the new telephone.

First supplies will be restricted to certain specified Areas. For this reason the Telephone 712L and the Dial Auto No. 30LA, will remain E control
items and will only be issued to these Areas.  Inquiries about availability should be passed to the Telephone Manager (Sales).

Dated 1966

Additional information

Model Mark Green Grey Blue Introduced Remarks
Tele 712L Mk 1y yy 5/641000 units field trailed in the London Telephone Region (LTR).
  Mk 2y yy 3/66Handset No. 8A & Tone ringer No. 2A fitted. Superseded the Mk 1.


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Last revised: January 03, 2021