Technical Description; General; Functional Description; Frond End - Marantz 4270 Instructions Manual

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 DESCRIPTION
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 R F 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 I F 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 F ET 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