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
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
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
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
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
POST OFFICE ENGINEERING DEPARTMENT
WORKS SPECIFICATION S(W) 2091
TELEPHONE NO. 712L (TRIPHONE) MARK II
DESCRIPTION AND INSTALLATION
1. SCOPE OF INFORMATION
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.
4. HANDSET (HANDSET No. 8A)
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.
5. THE TELEPHONE SET
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
The cover which is shaped at the top to accommodate the
handset is held firmly to the base by three fixing screws.
6. TONE CALL (TONE RINGER No. 2A)
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
(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.
7. DIAL (DIAL AUTO NO. 30LA)
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
(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).
9. ADD-ON UNIT
One non-locking press button unit may be fitted to the front of the
(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 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.
11. INSTRUCTION CARD
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).
5/64||1000 units field trailed in the London Telephone
3/66||Handset No. 8A & Tone ringer No. 2A fitted.
Superseded the Mk 1.|