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CIRCUIT DESCRIPTION
Tuner and I.F. Stages:
The tuner used on this chassis, is powered
by the +5V
supply. The CP, CL and CS models cover the VHF-L, VHFH
and UHF bands whilst the 'C' models cover UHF only.
During tuning, the mark/space ratio output from pin 1 of
ICO001 changes. This is applied to the base of Q001
via
RO39, and the resultant voltage at the collector is filtered
and applied to the VT terminal of the tuner.
Signal recognition is performed by |C201 in conjunction with
IC001, and is explained later in the Remote Control and
Tuning Circuit description.
Band switching: CP and CS models
IC001
performs
the
necessary
band
switching
in
conjunction with Q100.
"High" or "Low" outputs are obtained from pins 11 and 12
of 1C001, then applied to pins 3 and 4 of the tuner terminals.
These select the UHF or VHF-H bands.
When VHF-L band is selected, pins 11 and 12 of IC001
will
both
go
"Low".
Q100
will then
turn
off, and
approximately +5v is applied to the VHF-L terminal of the
tuner via R106.
Therefore, which of the tuner terminals BU, BH, or BL has
a +5v applied to it depends
on these "High" or "Low"
outputs.
UHF only: 'C' Models
A +5V is applied via R105 to the UHF (pin 3) terminal of the
tuner only.
The I.F. output from the tuner is applied to pins 45 and 46 of
IC201 via the SAW filter CP201.
Sound I|.F. stages:
C and CP models
The
sound
stages
consist
of !C201,
which
basically
performs the required demodulation,
and !C401, which is
the output amplifier.
The |.F. signal from the tuner is fed via saw filter CP201, and
applied to 1C201 at pins 45 and 46.
The composite
signal available from
pin 7 of I1C201
is
applied to the filter network of MF422, via L401A etc., then
fed to pin 5 of 1C201 via Q421 and C422.
Demodulation
is then performed
within the IC, with the
sound output being obtained from pin 50.
Sound I.F. stage: CS models
The sound stages consist of 1C4001, which performs the
change from |.F. to intercarrier, 1C201 which demodulates
the signal and !C401 which is the output amplifier.
The I.F. signal from the tuner is fed via the sawfilter MF4003,
and applied to IC4001 at pin 1.
The resultant intercarrier is available at pin 12 of IC4001 and
fed via the filter network consisting of MF422
(5.5MHz
intercarrier) and MF421
(6.5MHz intercarrier) and Q421 to
Pin 5 via C422.
Demodulation
is then performed
within the IC, with the
sound output being obtained from pin 50.
Audio
From pin 50 of !1C201 it is then applied to pin 3 of 1C401 via
Q401,
C421
and
R413,
amplified,
then
output to the
speaker from pins 6 and 8.
Should C417 on pin 2 of 1C401 or its output go short circuit
the base of Q440 will become "Low", and it will turn on. A
voltage will then be applied to the base of Q705, which then
turns off Q952, and as a result the +12v supply is removed,
thus preventing damage occurring to 1C401. How the +12v
is removed will be described in more detail later during the
horizontal circuit description.
Volume control is performed by the DC voltage applied to
pin 5 of IC201.
This is determined by the Pulse Width Modulated output
from pin 2 of IC001, which is then fed to IC201 after filtering
via RO17/R028 etc.
Under no signal conditions, or when the "SEARCH" routine
is initiated, pin 4 of 1C201 (ident) will go "Low". When this
happens,
internal
circuits
in |C201
prevent
any
sound
output from being obtained.
Demodulated sound is also available at pin 1 of IC201. It
is then connected
to pins 1 and 3 of the 21 pin Euro
socket, via Q453 network for gain and impedance/level
matching.
If desired, this output can
then be applied to external
equipment via an appropriate connecting plug.
Audio signals from external equipment can be applied to
the 21 pin Euro socket at pins 2 and 6 and these will be fed
via R403/R404 and C452 to Pin 6 of IC201.
Audio
signals can
also be applied to the front phono
sockets and these will be applied to pin 6 of IC201 via L151,
R406 and C452.
A separate switching voltage is also applied to pin 16 of
1C201 when external equipment is connected. The external
sound signal applied to pin 6 will then be switched to the
output of 1C201 {pin 50).
This
is then
further
amplified
by IC401,
as
previously
described.
Volume control of the external audio signal is obtained in the
same way as for internal sound, i.e., by the voltage level at
pin 5 of IC201.
(How the switching voltages are obtained for IC201, will
be
explained
in
the
Tuning
and
Contro!
Circuit
description).
Vision I.F. Stages:
The |.F. signal from CP201
is applied to pins 45 and 46 of
IC201. These pins supply an internal amplifier consisting of
three stages whose gain is controlled by the A.G.C. circuit.
The speed of response from this internal A.G.C. stage is
determined by C205, connected to pin 48.
The output from the internal amplifier is then input to a quasi
synchronous
demodulator
stage,
where
the signal
is
multiplied by its own carrier, i.e. the reference signal.
This reference signal is obtained from the input signal using
the tank circuitry of L202 etc. connected between pins 2
and 3 of the I.C.
The reference signal is limited by a clamping circuit, and is
then applied to the demodulator.
A control voltage from pin 47 of IC201 determines the RF
A.G.C. of the tuner.
The tuner take over point is determined
by the voltage
applied to pin 49, which
is governed
by the setting of
VR202.
The composite video finally emerges at pin 7 of 1C201.
Luminance Circuitry
The composite video signal from pin 7 of |C201 via buffer
transistor Q501,
is applied to the sound
rejection filter
MF501, (6.0MHz, 'C' model) (5.5MHz CP and CS models)
and on the CS models there is an additional filter in parallel
with MF501 marked MF502 which is for frequency 6.5MHz
as this model has dual sound.
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