1 02 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 the
voltage reference multiplier circuit.
1 03
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 dur
ation of the pushbutton operation. There is a 70ms
'silent' period at the end of each tone, when no tones
or speech can be sent.
1 04 Links D and F are not currently required but
provision for their incorporation caters for future
development. Link Eis fitted only when tone levels of
-6 to -8dB are required.
105 For KT1 Series telephones, link C is omitted,
C4 is to be 0.68 µf if the microphone is to DNT2002-
002-0199, Issue 6, or 0.47 µf if the microphone is
to DNT2002-002-0199, Issue 7. For KT4 Series
telephones, R6 is omitted, and C4 is 0.22 µf.
106 C 1 is used to trim the value of C2. If C2 is 2.4
µf, C 1 is fitted. C 1 is omitted if 2. 7 µf is used for C2.
KT1 and KT4 (1503) MF Version
with Discrete Timed Break Recall
{Figure
7)
1 07
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
108 The A and B wires {nominally 48V or 60V d.c.
lines), are routed via link N and the polarity bridge,
D6, D7, D9 and D10. The polarity bridge, including
surge suppressor, D8, a 1 30V Zener diode, also
provides transient protection.
109 The message waiting option comprises R23,
SKG, PLG and l.e.d. D22. {PLJ to be 7-way). This
option is most useful for example, in 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, as a reminder that
a message is waiting to be collected.
12
11 0 The bell capacitor, C 14, and PLF/SKF are
fitted in installations not using wall socket cap
acitors. The circuit provides protection from transient
spikes, and is used as an anti-tinkle device to prevent
other telephones ringing during dialling operations.
111 The tone caller capsule XL3 may be 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 clock
wise {when looking at the base of the telephone) to
increase the volume.
1 1 2 The tone caller circuit is centred on the TMC
1503 integrated circuit. Diodes D19 and D20 are
designed for future development and are not
currently used. Link P is fitted on all current versions.
Components SKK, PLK and R24 are omitted and link
Q
is fitted for British Telecom versions. Conversely,
for commercial applications, Link
Q
is omitted and
SKK, PLK and R24 are fitted. R24 in circuit will
reduce volume of tone caller by 1 OdB. When a bell is
required in place of the tone caller, components C 1 0,
D11 to D14, D19, D20, IC2, links P,
Q
and R, R10,
R14,R16,R20,R24,PLK,SKK,XL2,XL3,BC1 and
BC2 are all omitted.
11 3 TR3 to TR6 and associated components are
fitted when timed break recall is required. When SKC
is bridging pins 2 and 3 of PLC, the Darlington pair,
TR3 and TR4, 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 C 11, {with link S
omitted), and the line is broken as TR3 and TR4 turn
off. An alternative circuit for TR3 and TR4 is TR7 and
D21, with R 1 3 being changed to 2 20k {instead of
1 OOk), and D1 5 is omitted, but link S is provided to
discharge C 1 1 . The recall operation immediately
establishes a supply across TR6. The supply is
limited to 22V by Zener diode D18. As the supply is
established, C 1 5 begins to charge via R22, until the
voltage across C 1 5 reaches the gate potential at
R17/18 junction. TR6 is off, thus TR5 is on, ensuring
that the Darlington pair {or alternatively TR7) is held
off. When TR6 fires after the delay set by R22 and
C 1 5, TR 5 is turned off, which releases the clamp
from the Darlington pair. These transistors turn on
once again, reconnecting the line. The value of R22
can be changed to meet the various timed break
periods required {i.e. 70ms, 125ms or 280ms) as
shown in Figure 8.
114
For alternative recall {on chip) connections for
a timed break recall, SKE is fitted across PLE pins 2
and 3, and for an earth loop recall, SKE is fitted
across PLE pins 1 and 2. The alternative recall
{discrete) connections for a timed break recall are
3513 300 05070
