(Figure 2)
KT1 and KT4 Basic LD (Loop Disconnect) Version
1 4 The PCB is connected to the line via line cord
terminal pins as follows:
Pin 1 - via optional message waiting lamp
circuit
Pin 2 - B wire
Pin 3 - Bell input
Pin 4 - Earth Loop Recall (return)
Pin 5 - A wire
Pin 6 - Not used
15 The A and B wires (nominally 24V, 48V or 60V
d.c. lines) are routed via Link L and the polarity bridge
04, 05, 07 and 08. The polarity bridge, including
surge suppressor, 06, a 1 30V Zener diode, also
provides transient protection.
16 The message waiting option comprises R1 5,
PLG, SKG, PLH, SKH and lamp LP1. This option is
most useful in say, an hotel, where the operator has
received a call for a guest, but has been unable to
contact him. The operator can remotely activate the
message waiting lamp, which is directly connected
to the line cord, to act as a reminder that a message
is waiting to be collected.
1 7 The bell capacitor, C 1 0 and PLC/SKC, is fitted
in installations not using wall socket capacitors. The
circuit provides protection from transient spikes, and
is used as an anti-tinkle device to prevent other
telephones ringing during dialling operations.
1 8 The tone caller capsule is fitted to the base of
the instrument and connected to the PCB when the
instrument is correctly assembled via BC 1 and BC2,
which are tin plated bridge contacts. The tone caller
capsule is fitted with an adjustable volume control
(attenuator), which should be turned clockwise
(when looking at the base of the telephone) to
increase volume.
19 When a Texas TCM1 512P IC (IC2) is used for
the tone caller, R22 and XL2 are omitted.
Components R14, R19, R20, 017 and C11 are
omitted when IC2 is a Ferranti device. C9 is then
changed to 22 µf, 40V.
20 For the timed break recall option, links G and H
are fitted; and links
J
and K are removed. Switch S 1 2
is operated by the R (Recall) key. For the earth loop
recall option, links
J
and K are fitted and links G and H
are removed. All of these links and S 1 2 are omitted in
the basic version.
21 Link A is not fitted on commercial versions of
KT1 and KT4. For 200 x 200 ohm feeding bridge
systems, fit SKB to pins 1 and 2 of PLB. For some
overseas installations using 400 x 400 ohm feeding
bridge systems and when using the KT 1 or KT 4
6
telephone with a KBX 1 0 or KBX 1 00 system, fit SKB
to pins 2 and 3 of PLB.
22 For the locking version option, additional
components D 1 9, S 1 3, PLE, PLF, SKE and SKF are
fitted. This circuit prevents outgoing calls being
made.
23
In the circuits associated with IC 1, R8, C 1 2,
C 1 4 to C 1 7, 02 and D 1 6 are not required for initial
units, but the positions are provided for possible
future developments.
24 Most of the line current flows down the base of
TR2 and through TR1. TR2 is then saturated in the
on state. TR 1 provides current amplification during
speech periods as it is configured as a current source
in series with the loop. TR 1 and TR2 are switched off
during impulses and line breaks. Capacitor C2
maintains the positive supply rail for IC 1 under such
circumstances. R9 provides a small bias current
which is used to start up the circuit. XL 1 is a 560kHz
ceramic resonator, providing a clock signal for the
internal logic of IC 1 .
2 5 The signal from the microphone is amplified by
about 24dB by the IC microphone amplifier, which is
of a special low noise design. This amplifier also
provides a d.c. level-shift function which is applied,
together with the amplified microphone signal to the
base of TR1.
26 The signal from the line is obtained via an
attenuator R7 and R3 and is fed into the earphone
amplfier via C7 where it is summed with the sidetone
signal from the microphone amplifier to provide
sidetone cancellation. The earphone amplifier output
stage consists of a large enhancement transistor with
an actived driven load to provide the drive required by
a 600 ohm earphone.
2 7 Both the microphone and earphone amplifiers
have a line displacement gain which is controlled by
the AGC control system. This circuit takes a voltage
from the d.c. control block which is similar to that
found on the base of TR 1 and compares it with a
voltage reference multiplier circuit.
28 When a pushbutton key is depressed, the circuit
changes into the dial mode of operation and the
oscillator is started. The microphone amplifier is
disabled and its d.c. level-shift function is taken over
by the dial mode d.c. regulator. The earphone
amplifier is ramped to a low current state by the anti
ear click circuit which is a clock driven integrator. The
button depression is verified by an anti-bounce
circuit, and if accepted, the circuit will impulse out
the required number. During impulses, the base of
TR1 is pulled down, thus switching off TR1 and TR2,
since the latter's base current is also removed. For
the duration of the impulse, the circuit relies on the
charge stored in C 1 to maintain operation (hence the
3513 300 05070
