PAGE No. 3

The Latest Post Office Type Auto-C.B. Table Telephones
July 1937

Since the article entitled “New Bakelite Telephones“ appeared in Bulletin No. 8 of January 1936, the Ericsson Company in collaboration with the engineers of the British Post Office, have developed an improved type of table telephone with an outer appearance very similar to the table telephones described in the article referred to above. When the new telephone is used on automatic common battery systems it is equipped with a finger dial impulse switch and appears as shown in figure 1, while figure 2 shows the telephone as used on manual common battery systems.

Figure 1 Figure 2

The telephone is available in four standard colours, namely, black, Chinese red, ivory and jade green. The simple lines and pleasing proportions, together with the choice of colour, will ensure aesthetic satisfaction in any surroundings.

The body is made in highly polished moulded material and the external metal fittings are finished bright chromium plate, so providing hard wearing properties and easy cleaning facilities when in service.

A further advantage from the user’s point of view is the sliding instruction tray incorporated in the metal base plate, illustrated in figures 1, 2, 3 and 4. This metal tray with moulded knob and front strip, is provided with a pair of hinged flaps as shown in figure 4. The upper flap is made from sheet metal and forms a shield for the transparent flap beneath. For the British Post Office the metal flap is marked with the “publicity badge“ on its outer surface only, the inner surface is left unmarked.

If desired the flap can be chemically engraved on one or both surfaces to display any permanent markings and instructions.

The lower flap is made from transparent sheet material doubled on itself to form a holder for a printed instruction card, which is approximately 3.8" square and can be printed with information of a variable nature, such as, exchange codes used in multi-office exchange areas, see illustration figure 3. Since the transparent flap allows both sides of the card to be seen, advantage can be taken of this facility to display instructions printed on both sides of the card when necessary. The two flaps are attached to a spring-controlled double hinge which forms a clip for an alphabetically indexed writing pad, to be seen in figures 3 and 4. The writing pad should prove a great boon to the telephone user, as it can be made into a private directory of the more frequently called subscribers, and is thus always to hand when required. Renewal of the pad is a matter of a few seconds only, as it is a simple matter to insert a new one between the hinged flaps and the sliding tray.

Figure 4

Figure 3

The base plate with sliding tray illustrated in figure 4, is readily detached from the telephone after unscrewing the four corner screws of the captive variety. Removal of the base gives immediate access to the ringer and its gongs, and to the condensers, induction coil and cord connection block, as seen in figure 5. Incidentally, when the base is removed, it is possible to adjust the ringer and gongs, or replace the cords without removing the chassis from the moulded body.

The chassis, figures 5 and 6, is made from sheet metal and carries all the electrical components with the exception of the finger dial impulse switch. The components are all standard British Post Office types, and furthermore the gongs, ringer, induction coil, cradle-switch, springset and micro-telephone are all interchangeable with those used on B.P.O. Telephone No. 162 and bell set No. 25.

Figure 5 Figure 6

On the underside of the chassis as seen in figure 5, are the cord connection block, anti-side-tone induction coil, dual condenser, ringer and two gongs. The upper side as seen in figure 6 is occupied by the cradle-switch lever, cradle springset, the connection block for the dial connecting cord, and a suppression unit the purpose of which is described hereafter.

The two connection blocks are moulded in black bakelite. The terminals are retained in the blocks by a half twist on the soldering tag portion, and they are provided with 4BA connection screws and cupped washers.

The two gongs are fixed to short pillars on the chassis by means of 2BA screws. Between each gong and the pillar is a spring steel locking washer which effectively prevents the gong becoming loose under working conditions. The two gongs are made from different thicknesses of material, giving to each a different tone. The tones have been specially chosen to provide a more distinctive and pleasing signal than is the case when gongs of the same tone are employed. Adjustment is provided by having the fixing hole eccentric with the outer rim of the gong.

The ringer is of very simple construction and is easy to adjust. The polarizing magnet is of cobalt steel rod, having the armature pivoted at one end, whilst the other end fits into the iron yoke, which is slotted in such a manner that it forms an effective clamp for the magnet. When adjustment is necessary the clamp is slackened off by loosening one screw only, the magnet is then free to move into or out of the yoke. The two coils are wound to 500 ohms each and have the windings terminated on metal tags fixed in one of the end cheeks. Since these tags are also used for connecting the telephone wiring, it is an easy matter to connect the coils either in series or in parallel. In the instruments here described the coils are connected in series, thus giving a total resistance of 1000 ohms.

The cradle switch mechanism shown in figure 6, consists of a pivoted spring controlled lever carrying an insulated plunger for opening the contact springs. The contact springs are fitted with twin contacts of pure silver. The lever is normally actuated by two metal plungers, which project through the cradle portion of the telephone body. These metal plungers are depressed by the microtelephone when at rest in the cradle, and they slide freely in metal bushes attached to the telephone body only, thus facilitating the removal and replacement of the chassis. Incidentally the chassis is retained in position by means of three captive screws similar to those on the base plate. The switch mechanism is so designed that any adjustment of the contact springs can be made with the chassis removed from the telephone body, with the sure knowledge that correct operation of the contact springs will take place when the chassis is screwed into place.

The cord connection block is equipped with 13 terminals, some of which are connected together with metal straps to provide means for altering the circuit conditions. The terminals numbered 4, 5 and 6 are normally used for the micro-telephone connections. One end of the desk cord is connected to this block, the other end being terminated on a 4-way moulded terminal block. The standard length of the desk cord is 4 feet 6 inches. A 3-way desk cord is used and by removing a strap on the 4-way terminal block, an extension bell can be connected in circuit.

When the telephone is required on a manually operated common battery system, a dummy, as shown in figure 2, is fitted in the dial aperture. When working to an automatic system the dummy is replaced by the appropriate finger dial impulse switch as shown in figure 1. The switch is connected in circuit by means of a 5-way flexible cord attached to the 5-way connection block on the upper side of the chassis. A metal strap is fitted across terminals 4 and 5 on the 5-way connection block, when the telephone is used for manual working. 

This strap is removed when the impulse switch is connected for automatic working. A spring clip is provided at one end of the 5-way terminal block for holding the flexible cord away from the impulse switch springs. This ensures the chassis being returned to the telephone body without the cord interfering with the free movement of the impulse switch. The aperture in the telephone body is formed to take the standard B.P.O. dial impulse switch.

The usual circuit diagram is pasted inside the base plate in the recess beneath the sliding tray, as seen in figure 4. A reproduction of this diagram for the Auto & C.B. telephone is shown in figure 7, It will be seen that several new features are provided in this circuit. The anti-sidetone induction coil is provided with six windings, and its introduction is noteworthy for the fact that the transmission efficiency is somewhat better than the standard fixed by the British Post Office for telephone No. 162 and bell set No. 25. The average efficiency of transmission is approximately 1 db. better than the standard, and reception is equal. The side-tone reduction is greatest on the longer subscribers loops, this giving two advantages; firstly, as the side-tone is correspondingly reduced the user tends to speak louder, thus making it better for the more distant user; secondly, the absence of side-tone makes it much easier to receive the distant user’s conversation.

There is a further point to be noted, namely, the 50 ohm non-inductive winding on the induction coil, which in conjunction with the 2 micro-farad condenser forms an effective absorption combination across the impulse contacts of the finger dial switch during impulsing.

There is also a 0.5 micro-farad condenser connected across the transmitter for the primary purpose of reducing radio interference to a minimum.

The two condensers (2 micro-farads and 0.5 micro-farad) are accommodated in one metal case, the outer dimensions of which are 1 inch by 1 inch by 3.2 inches. Although the bulk is so much smaller than previous types, the normal electrical properties have been maintained. The reduction in size has helped considerably towards the compact arrangement of the components on the chassis.

Provision is also made for fitting a radio interference suppression unit across the finger dial impulse contacts. A view of this auxiliary component is shown in figure 8. It is also to be seen attached to the terminal strip in figure 6. This component consists of a small compact form of capacity inductance and resistance, mounted on a Bakelite base carrying terminals which allow the unit to be inserted in the circuit without disturbing the permanent wiring. When this suppression unit is connected in circuit it absorbs the high frequency component of the dialling impulses, so reducing interference from this source to an absolute minimum. The unit is not a permanent feature and is only intended for use when interference is troublesome, normally it is not required.

The micro-telephone shown sectioned in figure 9 is the modern moulded type, fitted with a 3-way cord of normal length 3 feet 6 inches. The connections from the receiver to the transmitter chamber are of heavy gauge insulated copper wire soldered to screwed brass terminal inserts and are embedded in the moulded handle. The receiver is of the inset type, the fixing screws also serving as electrical connections. The polarizing magnet is of the cobalt chrome steel type, so reducing the weight to a minimum. The inset transmitter is the highly efficient type previously described in the Ericsson Bulletin No. 9 dated July 1936. The transmitter connections consist of a springy centre pin and a double contact flat spring, which automatically complete the circuit when the inset is placed in position. The moulded earpiece screws on to the receiver inset case and clamps the diaphragm. The mouthpiece is fixed by means of a “secret” bayonet clip arrangement, and a special key is provided for unlocking. This device provides an effective means of preventing unauthorised interference.

There is also available a local battery type of telephone. The circuit for this is reproduced in figure 10. This telephone differs from its common battery counterpart as follows: the anti-side-tone induction coil has only three windings instead of the six required in the normal C.B. telephone; the cradle switch springset is provided with 3 additional contact springs; a 5-way desk cord is fitted, terminating on an 8-way moulded terminal block. All the components are interchangeable with those used on the C.B. telephone. The instrument is intended for use on extra long C.B. and auto direct exchange lines, and for C.B.S. and magneto systems. A local battery for providing transmitter current is needed with each telephone; in addition, a separate hand generator is required on magneto systems for providing signalling current.

From the foregoing description it will be appreciated that everything possible has been done to make this instrument thoroughly up-to-date and attractive to the user. Also that particular attention has been given to the main essentials, namely, improved speech transmission and reception, accessibility of components, robust construction, easy maintenance, pleasing appearance and lastly, the convenience of the user.

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Last revised: October 14, 2019