TELEPHONE No. 712 | ||||||||||||||||||||||||||||||||||
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here for history of the development of the Trimphone Sales Circular 342/65 - Telephone 712 Sales Circular 121/68 - Telephone 722 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
Rowlands.
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. 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! Notable differences between the 712 and the 722 712 has:-
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 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. INTRODUCTION The Handset 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 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 Circuit Arrangement PERFORMANCE FIELD TRIAL ACKNOWLEDGEMENT An unusual Telephone No. 712 in clear plastic - used for promotional purposes
Additional information
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Last revised: December 22, 2018FM |
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.
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. 