Pin 4 - Earth Loop Recall (return)
Pin 5 - A wire
Pin 6 - Not used
72 The A and B wires (nominally 24V, 48V or 60V
d.c. lines) are routed via the polarity bridge, D6, D7,
D9 and D10. The polarity bridge, including surge
suppressor, D8, a 130V Zener diode, also provides
transient protection.
7 3
The message waiting option comprises R 1 6,
PLH, SKH and l.e.d. D1 9. 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.
7 4 The bell capacitor circuit, C 11 and PLE/SKE 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.
7 5
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.
76
The tone caller circuit is centred on a TMC 1 503
integrated circuit. D 1 7 and D 1 8 are provided for
future development and are not currently used. Links
K and L, although currently fitted, allow for future
development. Components SKF, PLF and R1 5 are
omitted, and link M is fitted for British Telecom
versions. Conversely, for commercial applications,
link M is omitted, and SKF, PLF and R 1 5 are fitted.
R 1 5 in circuit will reduce the volume of the tone
caller by 1 OdB.
7 7 For the timed break recall option, IC 1 must be
TMC 161 7. Links C, E and
J,
PLD and SKD are
omitted. For earth loop recall, TR2, link H, PLD and
SKD are omitted. On the basic version with no recall
facility, the locations for TR2, link H, link
J,
PLD and
SKD are left vacant. For alternative recall, IC 1 must
be a TMC 1 61 7 chip, and links C, E, H and
J
are
omitted. Switch S 13 is operated by the R (recall)
key.
7 8 Link B is not fitted on commercial versions of
KT1 and KT4. For 200
x
200 ohm feeding bridge
systems, fit SKC to pins 1 and 2 of PLC. For some
overseas installations using 400
x
400 ohm feeding
bridge systems, and when using the KT 1 or KT 4
telephone with a KBX 1 0 or KBX 1 00 system, fit SKC
to pins 2 and 3 of PLC.
10
79
In the circuits associated with IC 1, D3 and D5
are not required for initial units, but the positions are
provided for possible future developments.
80 Most of the line current flows down the base of
TR2 (if fitted, see paragraph 77) and through TR1.
TR2 is then saturated in the on state. TR1 provides
current amplification during speech periods as it is
configured as a current source in series with the loop.
TR1 and TR2 are switched off during timed break
recall if this option is installed. Capacitor C3 main
tains the positive supply rail for IC 1 under such cir
cumstances. If TR2 is fitted, R9 is required to provide
a small bias current which is used to start up the
circuit.
81 The signal from the microphone is amplified by
about 24dB by the IC microphone amplifier which is
one 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.
82 The signal from the line is obtained via an
attenuator R7 and R5 and is fed into the earphone
amplifier via C8 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 active driven load to provide the
drive required by a 600 ohm earphone.
83
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.
84
Operation of any of the twelve keypad push
buttons causes the circuit to change from speech to
tone mode. The on chip oscillator oscillates at the
parallel resonance of XL 1 which is 560kHz. The
microphone and receive paths are switched off, but a
small signal is fed from the tone generators to the
earpiece, (confidence tone). The duration of any tone
is a minimum of 70ms, but will remain for the dura
tion of the pushbutton operation. There is a 70ms
'silent' period at the end of each tone, when no tones
or speech can be sent.
85 For the locking version option, S 1 5, SKA and
PLA are fitted, link A is removed. This prevents
outgoing calls from being made by disabling the clock
signal to the tone generators when S 1 5 is open
circuit.
86 Link G is not currently required, but provision for
its inclusion is for possible future development. Link F
is fitted only when tone levels of -6 to -8dB are
required.
3513 300 05070
