ERICSSON BULLETIN
No. 23 PAGE No. 27


New Magneto Telephones
Certified for Use in Coal Mines
Types N2976 and N2986
JULY 1951

The main principles of design of magneto telephones developed by the company or use in coal mines have not materially changed in the last thirty years, but improvements have been made from time to time as a result of experience and advances in technical knowledge. In conformity with this progressive policy, two new telephones have been developed to replace the present standard sets N2974 and N2984.

Fig. 1
Telephone N2980 (1921)
Fig. 2
Telephone N2984 (1934)

The new instruments are coded N2976 and N2986, and, like their respective predecessors, are practically identical except that the former has a hand micro-telephone and the latter a fixed transmitter with separate receivers, consequently, other details of their construction given herein may be regarded as applying to both.

To give some idea of the changes which have been made in the appearance of the ironclad magneto mine telephone over the last thirty years, illustrations of equivalent instruments manufactured in 1921, 1934 and 1950 respectively, are shown in Figs. 1, 2 and 3. Nothing will be gained from a detailed comparison of the telephone in Fig. 1, with those in the other illustrations, but in view of the widespread use and popularity of the N2984 instrument, it will be compared with its superseding equivalent
N2986.

Two of the primary requirements of telephones for use in coal mines are that they shall be capable of withstanding very rough usage and shall be well protected against damage from knocks, falling roof material, etc. This originally led designers to favour a rather too massive construction, with the result that a certain amount of difficulty was created for the installer and the maintenance electrician, owing to the considerable weight and size of the instruments. These points have been borne in mind in the designing of the new sets, as the following comparison reveals

Old Type, N2984
Weight (w/out batteries) 92 lb.
Approx. dimensions 20.75” x 12.5” x 10”

New Type, N2986
Weight (w/out batteries) 64 lb.
Approx. dimensions 19.5” x 11.5” x 8”

Fig. 3
Telephone N2986 (1950)
Fig. 4
The New Telephone with Door Closed

The reductions have been made without prejudicing the strength, safety or efficiency of the instruments ; indeed, by using improved types of components and modifying the design, increased speech efficiency has been achieved.

CERTIFICATION
The new instruments are certified intrinsically safe for use in methane (firedamp) by the Ministry of Fuel and Power Certificate T/TEL/84, dated 3.10.49.

GENERAL CONSTRUCTION
The construction of the cast iron casework follows the familiar lines of the earlier sets, consisting of a robust body attached to two vertical wrought iron mounting straps and having an inner and outer door. A cable terminating chamber is screwed to the bottom of the case, and, like the inner door, has special screws to prevent unauthorized interference with the vital parts.
The outer door, fitted with a slam catch, has a flanged edge which presses hard against waxed rope embedded in the grooved front rim of the case, making a weatherproof protection for the speaking equipment and generator crank when the door is shut (Fig. 4).

The interior construction is revealed in Fig. 5 and it will be observed that the whole of the apparatus, with the exception of the batteries and terminals, is mounted on the inner door which is gate-hinged and thus constitutes a comparatively light, removable apparatus unit, when the wires from the

battery chamber have been disconnected at the screw terminals below the ringer. This is a very advantageous innovation from the point of view of the maintenance electrician, as the unit can be readily transferred to the workshop if necessary. In this connection it should be remarked that on omnibus circuits, no break in service between other stations occurs when a telephone apparatus unit is taken from the case.

APPARATUS (N2986 DOUBLE RECEIVER TYPE)
On the inner door, and insulated from it by a moulded bakelite ring, is fixed a modern carbon granule microphone which is protected from damage by a hygienic, domed mouthpiece of perforated stainless steel. Experiments were conducted to determine the position of the microphone and the form of protective device which would give the best results. This involved studies of microphone reaction to speech from persons of different heights and “telephone habits,” in order to ascertain the most efficient arrangement for the average user.

Fig. 5—Interior View

The transmitter housing is extended upwards and forms a strong support for the receiver arms pivotted on either side. These move in unison and carry 40-ohm inset receivers set at an angle from the vertical plane in order to provide a more comfortable fitting for the head, while the horizontal distance between them can be varied as necessary. The cast iron cradles formerly used, to protect the receivers from damage when the arms are released from the raised position, are replaced in the new instrument by resilient rubber blocks.

Below the transmitter is the generator crank which has a bearing designed for long service and a large aluminium handle.

At the rear of the inner door are two compartments; the lower one has a domed aluminium cover and contains a modern single-magnet “alnico“ generator with a non-inductive shunt across the armature winding, while the upper compartment, shown open in Fig. 6 (not shown), has a brass coverplate hinged at the bottom. Mounted on the outside of the plate are a heavy-duty ringer and a 5-way bakelite terminal strip which projects through to form the connecting point for the internal wiring. On the inner face of the plate are an anti-sidetone induction coil, a blocking capacitor, and a safety capacitor for the ringer, while inside the chamber are the transmitter, the receiver switch, and a 2-way bakelite terminal block for the receiver connections which are made in P.V.C. insulated flex threaded down the arms. The switch springs are operated by a cam when the receivers are raised.

The interior of the main case (Fig. 5) is horizontally bisected by a metal wall, the lower portion housing two BSS.397, type DR2, dry batteries, and the terminal blocks. As batteries are liable to cause corrosion if allowed to become exhausted, their isolation is a desirable feature of the new design. They are supported on projections formed in the casting, so that they will not be affected, should condensation cause water to form in the bottom of the telephone. A similar precaution is taken with the line terminals which project from the terminating chamber through a thick bakelite strip in the battery compartment.

In order that ringing signals shall be clearly audible, the upper section of the case is open at the back, except for a perforated steel plate provided to protect the ringer from damage.

The line terminating chamber, similar to that on N2984, has three entries, one being fitted with a plug and two with glands for armoured cable. Plug and glands are interchangeable so that the external leads can enter the telephone from either side or from below. Glands suitable for 0.75“ screwed conduit can also be provided.

APPARATUS (N2976, MICRO-TELEPHONE TYPE)
It has been stated that the N2976 type instrument is the same as N2986 except that the twin receivers and fixed transmitter are replaced by a hand micro-telephone.

Fig. 7
Telephone N2976

This necessitates some modification of the inner door construction, as may be observed from Fig. 7.

The micro-telephone has been designed to withstand severe handling ; accordingly, the casing is a very strong aluminium alloy casting, as is also the removable mouthpiece which protects the modern inset transmitter. In order to avoid having cold metal in contact with the ear, the inset-type receiver earpiece is made of tough bakelite which projects well above the screwed brass ring clamping it to the diaphragm.

A specially shaped micro-telephone hook, and two phosphor-bronze springs attached to a block fixed on the inner door, behind the transmitter, ensure a secure seating for the handset, while a moulded guard protects the receiver from damage. The microtelephone switch inside the upper compartment of the door is operated by a lever and spring device and the wired connections to the handset are in the form of a tough, rubber-sheathed cord secured at both ends to prevent strain being placed on the conductors.

PROTECTIVE FINISHES
in view of the severe conditions under which these telephones are often used, nothing less than the highest grade finishes were considered suitable, therefore all components, windings, etc., are tropicalized. Steel parts are given a special anti-corrosive zinc plating, and non-ferrous parts are heavily nickelled.

The main casing is rust-proofed and light grey enamelled (in contrast to the black exterior of the N2974/2984 telephones) thereby making the instrument more easily discernible below ground, while the receiver arms on N2986 and the handset on N2976 are finished black.

CIRCUIT DIAGRAM
A diagram of the wired connections is supplied with every telephone and is in the form illustrated in Fig. 8, which shows the circuit for N2986.

 

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Last revised: June 24, 2003