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Marantz Model 4270 Manual

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TECHNICAL
DESCRIPTION
GENERAL
Figure 14 is a block diagram of
the
model
4270
Stereo 2 + Quadradial 4 Receiver showing main
functional elements
and
input
and
output
signal
routing.
PHONO INPUT jacks are provided
for
the
front
pair of channels.
FUNCTIONAL
DESCR
IPTION
FRONT
END
FM
antenna
signals are applied
through
a balun
transformer
to
the
antenna
coil which drives a
field-effect transistor RF amplifier.
The
signals
from
the
RF amplifier are fed
through
the
double-
tuned
RF
tank
circuit
to
the
FET
Mixer stage,
which is also fed by
the
signal generated by a
local
intermediate
frequency. Careful
attention
to
its thermal and electrical characteristics has
minimized drift,
thus
obviating
the
necessity
for
AFC.
The
1O.7MHz converted signal is
then
fed
to
a phase-linear ceramic IF filter, followed
by
the
limiter. It is
then,
in
turn,
processed
through
an FM discriminator.
The
output
of
the
FM discriminator is fed
to
a
buffer
amplifier
which
then
drives
the
multiplex
demodulator.
IF STAGES
The
IF section consists of six transistors and
three
stages of dual
element
ceramic filters.
The
characteristics of these filters are ideal in
that
the
200KHz
passband is phase-linear, with sharp
cutoff
slopes. This exceptional phase linearity
assures
the
elimination
of
a major source of high-
frequency
distortion
and
a loss
of
stereo separa-
tion.
The
sharp
cutoff
slopes provide improved
selectivity, permitting reception
of
closely spaced
channels.
LIMITER
The
Model
4270
utilizes symmetrical
diode
limit-
er circuits consisting
of
high-performance Gold
Bond Hot Carrier
type
diodes and IF limiter
amplifier with a very small
dynamic
symmetrical
aperture, eliminating
the
need
for
an AGC circuit
which introduces low
frequency
distortion. Un-
desirable
amplitude
Modulation (AM signals, AM
noise, AM distortion) are removed
from
the
IF
signal within
the
limiter.
FM
STEREO
DEMODULATOR
The
stereo
composite
signal
obtained
from
the
buffer amplifier is first led
to
the
FET muting
circuit;
then
to
the
phase locked loop stereo
demodulator
IC
circuit
and
decoded into
both
left and right channel signals. Each left
and
right channel signal is
then
applied
to
the
16KHz
low pass filter (LPF)
and
de-emphasis networks
to
remove
the
undesired switching carrier signal
in
the
audio
signals. Next, each
audio
signal is
applied
to
the
audio
amplifier consisting of
NPN-PNP direct-coupled transistors
and
amplified
to
the
required signal level, of
about
755mV
RMS.
Finally, each amplified signal is led
to
the
SELE-
CTOR Switch.
The
multiplex
stereo
demodulator
circuit
consists
of a phase locked loop IC and is
equipped
with a
separate
automatic
Stereo/Monaural switching
circuit.
The
circuit checks
the
input
signal
intensity and
actuates
the
stereo
demodulator
and stereo indicator lamp automatically, when
the
input
signal is of sufficient strength
to
provide
high
quality
stereo
reception. When
the
input
signal intensity is insufficient
for
this
purpose,
the
stereo
signal is automatically changed
to
a
monaural signal
to
insure reception with a high
qual ltv signal-to-noise ratio.
MUTING CIRCUIT
In
the
absence of an FM carrier, all FM receivers
produce
inherent
noise.
The
muting
circuit
elim-
inates this noise, providing noise-free
tuning
from
station
to
station.
A muting circuit, consisting of a two-transistor
noise amplifier
and
a three-transistor (including
FET) switching circuit, has been incorporated
in
the
Model
4270.
The
muting
circuit
perfectly
mutes
out
all
the
inter-station noise and also com-
pletely mutes
out
the
side slope spurious response
of
the
unit.
The
circuit
has been designed
to
minimize annoying
"pop"
noise
for
velvet
smooth
tune
-in and
tune
-out.
AM
TUNER
The
AM
tuner
portion
of
the
4270
is composed
of
one
IC circuit (including RF amplifier, local
oscillator, mixer, IF amplifier,
detector
and a
signal strength indication amplifier) and
one
transistor amplifier
to
amplify
the
detected
audio
signals. A three-section variable
capacitor
im-
proves spurious response ratio.
The
ceramic filter utilized in
the
AM IF amplifier
comes with higher selectivity
and
wide bandwidth
for interference-free hi-fi reception.
18
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