Click here for history of the development of the Trimphone

Sales Circular 342/65 - Telephone 712

Sales Circular 121/68 - Telephone 722

Extract from Hansard

General fault finding on your phone

Trimphone Prototype

Diagram - N812.

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 1966, at extra rental cost, with a choice of three two-tone colour schemes: Grey-white, Grey-Green and two-tone blue.  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 1970.

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 baseplate with its polycarbonate. press bar projecting beneath the front edge of the cover.  The 4-way line cord enters the baseplate 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

Additional information

Model Mark Green Grey Blue Introduced Remarks
Tele 712 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.
Tele 712L  yy y Provision for one press button


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Last revised: December 22, 2018