Ericsson DIALOG - Wall Terminal Box for Connectivity

3.8 Wall terminal box
operating flux density. An additional diaphragm (9) of sheet iron
is included. and serves as a magnetic shunt, providing a further
increase in the transfer factor. A thin aluminium ring (10)
separates the magnetic shunt from the protective cap (11). The
receiver inset is sealed by a chromium plated brass cover (13).
The short ALNICO steel magnet ( 3) which has high coercive force
is. together with its pole pieces (4). cast into the receiver-well
by means of a corrosion-proof metal (2) which expands when
cooling. This method provides an extremely stable mechanical
anchorage and ensures at the same time that the correct air gap
is maintained between the pole pieces and the diaphragm. Grinding
of the pole pieces is unnecessary, which makes it possible for us
to avoid any risk of altering the magnetic properties.
The thermoplastic coil bobbins (5) have been designed to form
a wall in the space behind the diaphragm, creating two acoustical
chambers. The response curve is evened outby the same method
as applied in the transmitter, i.e. a hole (6) covered by a fabric
disc (7). This design is most effective in smoothing the response
curve over the high frequencies and thereby ensuring correct
reproduction of the weaker sounds.
Fig. 30 The receiver in parts and cross section
The receiver side of the telephone instrument has dry contacts,
screw terminals (14), as only a.c. flows through the contact
positions.
Fig. 31
The wall
terminal
box
1
2
3
19
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