Article on the 300 type
300 type comparison chart
Development of the Bakelite Wall
Around 1930 the Swedish Ericsson company decided that a new telephone shape was required which would have a global
appeal - they considered that the world was their marketplace (and quite rightly still do so!). The job was given to a
company engineer of Oslo's Elektrisk Bureau (an Ericsson subsidiary) called Johan Christian Bjerknes. He decided to use
Bakelite, but found that he needed help with the design. The leading Norwegian artist Alf Rolfson was at the time too
busy with another project and so the job was given to Jean Heiberg.
Jean Heiberg (1884-1976) was a Norwegian painter who had studied in Munich and in Paris, latterly under Matisse. At the
time he had just returned from Paris to take up a post as professor in Oslo's National Academy of Arts.
His final design had hard lines and sharp edges (possibly influenced by the 1930's Cubist movement) and retained
elements of the neo-classical stylobate which dominated his first plaster drafts.
L. M. Ericsson put this phone into production in 1932 as the 'DBH1001'
(shown to the right) and later the Swedish Telephone Co. (Televerket) adopted it as standard.
The Prince of Wales (Eldest son of King George V) visited the Stockholm Exhibition in October 1932 where the telephone
was displayed. He admired it and selected it for use in his home. The approval of this fashionable man-about-town did
much to promote the telephone's success beyond Scandinavia.
Around that time the BPO wished to introduce a telephone with integral bell. They developed the Telephone
No. 332 in conjunction
with Ericsson Telephones Ltd (UK). By 1937 there were six presses in England stamping out the Norwegian painters
designs. This also came in a standard colour Black, although again other colours were available including Lacquer Red,
Jade Green and Ivory - and many other colours have been spotted - but beware of sprayed cases! The 300 Type Telephone used
the Telephone No. 164 Handset and had new internal equipment, mounted on an
internal chassis. The line cord was rear entry, but many feel the that
side entry would have been a better option. A variety of different instruments based around this design were
produced to suit varying purposes.
The 300 type series contained an integral bell, catered for the provision of 1, 2 or 3 keys (with interchangeable
labels) and was easier to manufacture because a single moulding for the case was used rather than several for the '200
type'. Ease of fitting and subsequent reduction of fault liability was a by-product because there was no need to drill
holes in walls for mounting Bellset etc and no wiring to connect for the external Bellset.
Because of the shape of the case the phone was sometimes referred to as
the "Cheese Dish" phone.
The 300 series was not intended as a replacement of the '200 type' instrument as it was still thought that a separate bell
mounted centrally in a household would be more desirable (indeed the 200 type was used into the 1960's for certain
applications!). The 300 type was thought to be primarily suitable for business use.
All of the main manufacturers produced 300 Type sets, both for the BPO and their own private systems. Another common
manufacturers code seen on the handsets of these instruments is 'AEP' - these were manufactured not in England but in
Portugal by 'Automatic Electric Portugal'.
ARTICLE BY ROB GRANT
Suggested further reading:-
- The Conran Directory of Design' edited by Stephen Bayley.
- Telephone Design Notes for Students - Design Museum.
- IPOEEJ Vol. 31 page 1 - see below.
300 type components
Picture by Paul Follet
A modified extract from:-
Post Office Engineering Instructions
COMBINED SETS (ONE-PIECE)
These telephone sets (known as '300-type') consist of a handset, chassis (including a magneto or trembler bell), base with directory tray,
keys (where required) and dial (if needed), contained in single
moulding. This moulding is available in black and the three standard colours, viz., ivory, jade-green and Chinese-red to form a single unit.
These sets are available in three patterns for use as indicated below:-
D.E.L. and P.B.X. extensions without re-call facility. Provision is not made for plunger keys on this pattern.
P.B.X. extensions with ' operator recall', certain Extension Plan arrangements, and shared service (separate metering) auto. exchange lines. Provided with one plunger-type key, included in the telephone, as issued.
Extension Plan arrangements. Has facilities for fitting one, two or three plunger-type keys, are required.
As will be seen from the illustrations, the layout of the apparatus provides accessibility to all parts of the instruments, for inspection and maintenance purposes.
The gravity-switch springs are of the twin-contact type, similar to those incorporated in
Post Office standard telephone-type relays.
The chassis is secured to the moulded case by means of three screws, which are accessible on removing the base of the telephone.
As all the apparatus, except the key plungers, the dial and the gravity-switch plungers, is mounted on the chassis, accessibility to all parts is obtained when the chassis is removed.
Mark numbers on the base advise that:-
Mark 1 - has a receiver, Inset No. 1L in the handset.
Mark 2 - has a receiver, Inset No. 2P in the handset.
Mark 2A - has a receiver, Inset No. 2P in the handset and switch bracket
and plungers of an improved design (see next paragraph).
Gravity Switch hooks
The gravity-switch plungers and switch bracket on the original design of this type of instrument were
liable to stick in service: this fault requires treatment as detailed in A 5254.
To overcome this fault, a new type of chassis and gravity-switch plungers have been developed; these are shown
below. The new chassis has a switch bracket fitted with rollers on which the new shrouded-type gravity-switch plungers operate.
This new-type chassis cannot be used with the old-type plungers nor can the old-type chassis be used with the new-type plungers. For maintenance replacement of the moulded case, see
|The chassis shown above is the later
improved version and also shows the two types of plungers used on the 300 type
telephones. The plunger the left is the new style, whilst to the
right is the old style. This chassis will only use the new style
|Early chassis. Note the flat
plate gravity hook switch plate is used only with
the old style plunger (see picture above for plungers).
Each instrument has four main parts, viz:-
- Moulded case
- Base-plate and directory tray
- Dial (if fitted in an Auto area)
The hand-set is the standard 'Telephone No. 164'.
The case is a one-piece moulding, the hand-set cradle being part of the case, as distinct from the detachable cradle employed for earlier types, see par. 14. Three varieties exist, one of which is shown in
the picture to the right, and are as follows:-
For D.E.L.s and P.B.X. extensions WITHOUT 'operator re-call' facility.
Plain undrilled and available in black and the three standard colours, for use when a plunger key is not required
(Part No. 1/SCA/20).
P.B.X. extensions WITH 'operator re-call' facility, shared-service working (separate metering) auto, exchange lines and certain extension plan arrangements. The case is drilled for one key plunger and the associated standard label-fixing screws, for use in C.B. or auto. areas when one non-locking key is required
(Part No. 2/SCA/20).
Extension Plan arrangements, where one, two or three key plungers are required.
The case is drilled for three key plungers (see picture to right) and associated standard label-fixing screws.
It is available in black and the three standard colours. When the instrument is issued by the Supplies Department, the key-plunger drillings are closed by means of dummies, which are removed, as required, at the installation office or fitting centre before the telephone is handed to the fitter.
The picture to the right shows the case (Part No. 3/SCA/20) as issued for maintenance-replacement purposes, i.e., without
A standard GPO dial is fitted or a dial blank if the telephone was used on
the C.B. system. It is unnecessary to remove the chassis to gain access to the dial, since there is space between the bell-gongs for unscrewing the
screw. Remove the retaining screw, turn the whole dial anticlockwise and then pull
away from the case.
The chassis (shown below) is practically identical for each type of telephone, the only difference being in the apparatus which is mounted on it.
The chassis is fixed to the telephone case with 3 screws, one either side and
one near the cord entry. The Telephone model number can be found printed on the Chassis under the bell
|Chassis used with DC ringing. The
bell was used for intercommunication.
|Chassis fitted with AC ringing
The base-plate with tray (D62076), which is fitted to each type of telephone, incorporates a container for certain traffic literature and the necessary instructions to the subscriber.
The internal wiring diagram is either pasted or directly-printed on the base-plate so that it is disclosed when the tray is withdrawn.
Four captive fixing
screws (later fitted with shake proof/serrated washers) - situated in the extreme corners of the base-plate - secure the base to the
telephone. Adjacent to each screw is a rubber foot. Some phones were not fitted with the drawer, in which case a
blank plate was fixed to the front edge of the base-plate. The
telephone model number is printed on the outside of the base.
Tray with drawer
Drawing No. 63022 -
Telephone components parts.
Drawing No. 63023 - Chassis components.
Drawing No. 63250 - Chassis components
(improved switch hook buttons).
Drawing No. 90319 - Telephone
parts (Mk 3A).
Drawing No. 90373 - Chassis components
(improved switch hook buttons).
Drawing No. SBA12 - 300 type
Drawing No. SCA17 - 300 type complete
Drwaing No. SCA20 - 300 type case.
Drawing No. SCA47 - 300 type complete case
(improved switch hook buttons).
THE POST OFFICE ELECTRICAL ENGINEERS' JOURNAL
Vol. XXXI - April, 1938
The New Combined Hand Microtelephone and Bell Set
by C. A. R. PEARCE
A new telephone is described in which the telephone instrument and bell set are combined in the same moulded case.
The advantages claimed are ease of fitting and reduction of fault liability. Provision has been made for one, two or
three press buttons to be mounted on the instrument, thus permitting a variety of extension and interconnection
facilities to be given.
There has recently been developed by
the Post Office Engineering Department, working in conjunction with Messrs. Ericssons Telephones, Ltd., a new standard
subscriber's set in which the bell, condenser and induction coil are mounted as an integral part of the telephone
instrument and not as a separate bell set as hitherto.
When the Telephone No. 162 was introduced in 1929 it was considered that the demand for a combined set, i.e. one in
which the telephone and bell set form a single unit, would be insufficient to justify the development of a special piece
of apparatus and consequently the only provision for such an item was the arrangement whereby the standard bell set (Bellset
25) intended primarily for wall fitting could also be mounted under the
telephone. Experience has shown, however, that even with such an admittedly
as this, approximately half the micro-telephone instruments are fitted as combined sets and doubtless with the new
design an even greater proportion of the demands will be for this class of instrument.
There is, however, a serious
disadvantage in combining the bell with the telephone in residences where the bell has to be heard in all parts of the
house. In such instances the best position for a bell is in the hall, whereas the telephone may be anywhere that is
convenient. For this reason the new set is regarded as being primarily suitable for business subscribers and plan number
installations, although its use is not, of course, restricted to these spheres.
The advantages of combining the telephone apparatus in a single unit are, first, the economy of a single case and
chassis as compared with the variety of mouldings which make up the Telephone No. 162, and, second, the ease and
simplicity of fitting. With a single unit the fitter has, on direct exchange lines, to connect only two wires to the
block terminal and need not open up the instrument to do so. Thus the new instrument materially reduces the possibility
of errors in fitting. Its use will also generally reduce the disfigurement to walls which is occasioned by this
operation, since the block terminal can frequently be located on a window-frame or skirting board and the fitter thus
saved the plugging of the walls. A very prolific source of faults is bad fitting and the combined set is well suited for
fitment by relatively unskilled staff.
Although the external shape is based on that of an instrument designed by the Swedish Ericsson Co., the internal
arrangement is entirely new. The design of the case is centred around the use of a plastic moulding and the result is
considered to be an extremely sturdy construction and ideally adapted for production in large quantities and in colours.
The mould from which it is produced is practically a straight 'draw-out', the only loose part being that which forms the
It will be appreciated that an important factor in the success or failure of any design of telephone instrument is
the reaction of the general public to its external appearance. It is thought that the new set will be popular on this
account as it has already received very favourable comment from many acknowledged authorities on design and was
especially selected by the Duke of Windsor when Prince of Wales during his visit to the Stockholm Exhibition in October,
1932, for use in his own private residence.
The original design did not include the
sliding tray in the base and it can hardly be claimed that this addition has improved the general appearance of the set.
This tray was designed as a convenient holder for the list of 'dialling codes' in those areas where a subscriber obtains other exchanges
by the dialling of two or three routing digits, but it has been decided to provide it on all telephones as a container
for a subscriber's private directory.
The shape of the
Handset Micro-Telephone (H.M.T.) rest has been designed
so as to facilitate the removal and replacement of the hand microphone and it is anticipated that the use of the button
type of 'switch-hook,' in lieu of the pattern used in the Telephone No. 162, will obviate the
sticking trouble experienced with the latter class of instrument. Another important feature of the case design
is its convenient width for gripping when lifting although it must be conceded that the weight of the set does to some
extent mitigate against any great ease in this respect.
With the exception of the dial all the various
components are mounted on a single chassis plate. The condenser is the only component for which it has been
found necessary to introduce a special variety on account of the new set. Advances in the technique of condenser
manufacture have made it possible to produce a 2 + 0.1 uF condenser which is little none than half the size of the 2 + 0.1 uF pattern
at present used in bell sets. This new item will he the standard size for use in all future designs of bell sets and
The layout is such that all the main component
s with the exception of the switching springs and dial are revealed
and made accessible for maintenance operations by the removal of the under-base, while the removal of three further
screws frees the chassis from the case and enables any maintenance work on these two items to be easily carried out.
Every effort was made to render the dial directly removable from the front of the instrument but this has not been found possible
without spoiling the appearance of the set.
The circuit of the CB and Auto type is simply the standard anti-sidetone arrangement (Coil
Induction No. 22) which has been described elsewhere. The local battery
circuit remains a sidetone arrangement and is practically the standard three winding induction coil circuit as at
present used when a Telephone No. 196 is connected to a Bell Set No. 31. Although
the local battery set will eventually disappear as subscriber's instrument it will still presumably be necessary for
private wires, etc., and it is likely that for these uses a local battery anti-sidetone induction coil will be
introduced, but no entirely satisfactory design of coil is yet available. Space has been left, however, to permit of the
use of any design of anti-sidetone coil which can be envisaged at the moment.
A feature of both C.B. and LB. circuits, which is novel to subscriber's
sets in this country, is the shunting of the transmitter by a 0.1 uF. condenser.
This condenser, besides preventing the rectifier action of the transmitter from causing interference between the
telephone and nearby radio sets and vice versa, also has the effect of reducing the likelihood of the transmitter
becoming packed by current surges occasioned by automatic switching, etc. A filter for preventing the dialling impulses
from causing radio interference has also been designed and it has been arranged that, when required, it can be connected directly to the dial strip terminals.
There are in all some seven main
of the new instrument, excluding the sub-varieties necessitated by director and non-director working and the several
colours. Patterns of both C.B. and L.B. types incorporating a trembler bell will be available for plan number working
and these instruments will include a radio interference suppressor for dealing with the disturbance created by the bell
contacts. This same suppressor will also act as a spark quench for the contacts and, in addition, reduce the disturbance
created by the use of the bell when connected directly across C.B. and auto exchange lines.
Controls for Extension Instrument Installations, Plan Number Working
One of the main features of the new instrument is the facility for accommodating on the telephone the controls for
plan number working. This facility has for some time past been a feature of the telephone instruments used by certain
continental administrations but the design of the British Post Office apparatus has not until now been such as would
permit of the easy inclusion of switches and keys, etc. The whole of the facilities necessary for the existing plan
numbers (excluding Nos. 5 and 7) as well as many others are provided by means of three key units, two of which are
identical except that for economy one is equipped with just sufficient switch springs to meet the more popular of the
plan number arrangements, whereas the other is equipped with enough springs to deal with all anticipated demands. The
third key is fitted with one make contact only and is mounted and wired as an integral part of one particular variant of
telephone, important uses of which will be as an extension instrument for P.B.X's, where the press button will be used
for transferring calls.
are several distinct advantages both to the subscriber and to the Post Office which are obtained by locating switching
apparatus as part of the telephone. For the subscriber it simplifies operation and for the Post Office it simplifies
fitting and enables the instrument to be prepared at the fitting centre complete with keys, terminal block, etc., ready
for wiring. The labour economies possible from such an arrangement are obvious.
The convenience of this form of control and the fact that it may be linked
with the switch-hook operation (switch hook release feature) has made possible considerable improvements in plan number
facilities and it is anticipated that a complete revision of these will be undertaken
in the near future together with the
introduction of a simple house exchange system catering for tip to three extensions with exchange line facilities. The
switch-hook release feature is of particular importance because of the
facility which it affords of returning conditions to normal at the completion of a call thereby obviating
mis-operation due to forgetfulness in this respect on the subscriber's part.
A further economy to be introduced with this set is the standardisation of the label, it having been found possible
to meet all the envisaged demands for plan number working by ten varieties of engraved label. Up to the present the
labelling of keys on plan number installations has been carried out with mechanically engraved xylonite labels mounted
adjacent to time keys. The new labels are a chemically engraved metal type of
rather neater appearance than the old pattern and cheaper to produce.
The two key units are of similar appearance of which one consists of three plunger-type keys
operated by press buttons located in front of the handset. The keys may be interlocked in various manners, e.g. all
three can be left quite independent of each other to act as separate press buttons or it can be arranged that the first
when depressed shall be locked in the operated position and only released from this position by the operation of the
second or third. Alternatively the first or third may be locked down when operated and released by the replacement of
the telephone on the rest, and so on.
These facilities are all controlled by means of time
very simple control plate shown in the picture to the right. The key when operated is held in position by the lip of the
spring latch plate meeting time projection on the key in ratchet and pawl fashion. The depression of a second similar
button will, of course, force out the latch-plate and release the first button. To render any key non-locking,
therefore, it is only necessary to prevent the latch-plate from overlapping time key projection. This is done by filling
the lip with a portion of the control plate. The arrangement is such that the control plate may be mounted in a
variety of positions on the latch-plate. In each of these positions it provides one form of interlocking
between the three keys.
Switch-hook release is provided by removing a second control plate
from the key frame where it is located for convenience and attaching it to time switch-hook mechanism of the telephone.
The sloping face forming the underside of the top projection on the plungers has been provided to cater for cases where
the subscriber inadvertently depresses two locking buttons simultaneously and where the conditions are such that there
is no other means of releasing them. To obviate this the top lip is designed so that if either button is depressed
beyond its locked position it will force the latch plate outwards and release time second button.
The key unit switch springs incorporate twin contacts and arc mounted
vertically to reduce fault
liability. When one or two keys are not required the buttons are replaced by
Click here for more information on 300 type key units
Click here for Lamp Fittings
The new telephones and keys offer what are, in the writer's opinion, unique facilities for
mechanical switching associated with the telephone and facilities, moreover, which could only otherwise be provided by
complicated lever type keys or by press buttons with relays, either of which arrangements would indubitably be
considerably more expensive than time new system, and it is considered that time new apparatus represents a real advance
in telephone instrument design.