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
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
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
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
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
Click the picture to hear a Trimphone warbler|
An article by Alan Hollingdale
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
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
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
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.
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.
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
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
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
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.
telephone has been developed for the Post Office by Standard Telephones and
Cables, Ltd., under the British Telephone Technical Development Committee
An unusual Telephone No. 712 in clear
plastic - used for promotional purposes
5/64||1000 units field trailed in the London Telephone
3/66||Handset No. 8A & Tone ringer No. 2A fitted.
Superseded the Mk 1.|
Tele 712L|| ||
y|| ||Provision for
one press button|