87
C4 is 470 nf and link D is omitted for KT1
Series telephones, whilst C4 is 220 µf and R6 is
omitted for the KT4 Series.
88 C1 is used to trim the value of C2. If C2 is 2.4
µf, C1 is fitted. C1 is omitted if C2 is 2.7 µf.
(Figure 6)
KT1 and KT4 Mk2 Version,
with Discrete Timed Break Recall
89 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
90 The A and B wires (nominally 48V or 60V d.c.
lines) are routed via link L and the polarity bridge D6,
D7, D9 and D10. The polarity bridge, including surge
suppressor, D8, a 130V Zener diode, also provides
transient protection.
91 The message waiting option comprises R26,
PLD, SKD, PLF, SKF 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.
9 2 The bell capacitor, C 1 4 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.
93
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 contacts. The tone caller capsule
is fitted with an adjustable volume control (atten
uator), which should be turned clockwise (when
looking at the base of the telephone) to increase
volume.
94 When a Texas TCM1 512P IC (IC2) is used for
the tone caller, R22 and XL2 are omitted. Com
ponents R18, R21, R25, D13 and C12 are omitted
when IC2 is a Ferranti device. C 11 is then changed
to 22 µf, 40V.
95 With SKG bridging pins 2 and 3 of PLG, the Dar
lington pair, TR3 and TR5, are connected in series
with the line. These are normally turned on by means
of base bias from R12. When the recall key, S14, is
pressed, this bias is momentarily removed by C9 and
3513 300 05070
11
the line is broken as TR3 and TR5 turn off. This im
mediately establishes a supply across the program
mable unijunction transistor (PUT) circuit centred on
TR7 or its functional equivalent formed by TR2, TR4
and R 1 0. The supply is limited to 2 2V by Zener diode
D 14. As the supply is established, C 1 5 begins to
charge via R24, until the voltage across C1 5 reaches
the gate potential at R15/R16 junction. TR7 is off
and thus TR6 is on, ensuring that the Darlington pair
are held off. When TR7 fires after the delay set by
R24 and C1 5, TR6 is turned off, which releases the
clamp from TR3 and TR5. These transistors turn on
once again, reconnecting the line. The value of R24
can be changed to meet the various time break
periods required, (see Figure 8).
96 For the timed break recall option, SKG is fitted
across pins 2 and 3 of PLG, SKJ is fitted across pins
1 and 2 of PLJ and SKK is fitted across pins 2 and 3
of PLK. Links J and K are omitted. If PLG, PU and
PLK are not fitted, then fit links J and K. For earth
loop recall, SKG is fitted across pins 1 and 2 of PLG.
SKJ is fitted across pins 2 and 3 of PU and SKK is
fitted across pins 1 and 2 of PLK. Links J and K are
again omitted. On register recall telephones, links J
and K are fitted, whilst SKG, SKJ, SKK, PLG and PLK
are omitted.
97 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 tele
phone with a KBX 1 0 or KBX 1 00 system, fit SKB to
pins 2 and 3 of PLB.
98 In the circuits associated with IC1, R5 is fitted
only when an alternative D 1 is used. 03 and D5 are
not required for initial units, but the positions are
provided for possible future developments.
99 TR 1 provides current amplification during
speech periods as it is configured as a current source
in series with the loop. Capacitor C3 maintains the
positive supply rail for IC 1 during the interruption of a
timed break recall.
1 00 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.
101 The signal from the line is obtained via an
attenuator R8 and R4 and is fed into the earphone
amplifier via C 5 where it is summed with the side
tone signal from the microphone amplifier to provide
sidetone cancellation. The earphone output stage
consists of a large enhancement transistor with an
active driven load to provide the drive required by a
600 ohm earphone.
