Article on the 300 type
300 type comparison chart
Development of the Bakelite Wall Telephone

Quick History
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'.


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
A 1010

Issue 2
June 1950


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:-

  1. D.E.L. and P.B.X. extensions without re-call facility. Provision is not made for plunger keys on this pattern.

  2. 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.

  3. 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
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 A 5201.

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 plunger.
Early chassis.  Note the flat plate gravity hook switch plate is used only with the old style plunger (see picture above for plungers).

Main parts
Each instrument has four main parts, viz:-

  1. Hand-set
  2. Moulded case
  3. Chassis
  4. Base-plate and directory tray
  5. 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:-

  1. 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).

  2. 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).

  3. 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 fitted parts.

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 dial retaining 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 gongs.

Chassis used with DC ringing.  The bell was used for intercommunication.
Chassis fitted with AC ringing

Base Plate
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 base plate.
Drawing No. SCA17 - 300 type complete case.
Drwaing No. SCA20 - 300 type case.
Drawing No. SCA47 - 300 type complete case (improved switch hook buttons).

Vol. XXXI - April, 1938

The New Combined Hand Microtelephone and Bell Set
Part 1

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 No. 25) intended primarily for wall fitting could also be mounted under the telephone. Experience has shown, however, that even with such an admittedly makeshift arrangement 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.

General Design
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 dial aperture.

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 telephones.

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 varieties 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.

There 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.

Key Units
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 dummies.

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.

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