Marantz 2240 - FM Tuner; Audio Muting and Stereo Mode Auto-Selecting Circuit

REBRaR Pa BEBE Z.
3. FM TUNER
The FM Tuner section of Model 2240 is divided into four functional blocks: FM Front End,
IF Amplifier and Detector, Muting Control, and MPX Stereo Decoding Circuit. FM signals
induced by a FM antenna are led to FM antenna coil L101 through a balun coil. These signals
are then applied to the FET RF amplifier which in turn applies its output to the next FET
Mixer H102 through the double tuned high selective circuits. The FET Mixer convert its input
signal into 10.7 MHz intermediate frequency and amplifies it at the same time. The H103 isa
loca! oscillator and its output is injected into the source of the FET Mixer, the injection voltage
_ is about 700 mV. The 10.7 MHz front end output is led to the next IF amplifier unit through a
coaxial cable.
The IF amplifier unit consists of five stages of IF amplifier and one stage of AGC amplifier.
Three pieces of dua! elements ceramic filters are also used to obtain high selectivity, four stages
of symmetrical diode limiters are also employed for the best limiting characteristics, improved
capture ratio and good AM suppression.
A part of FM Front End output is applied to the AGC amplifier H201 and its rectified
output is fed back to the gate of FET RF amplifier to decrease.the gain with increased signal
strength.
The IF signal sufficiently amplified through every stage of IF amplifier is finally applied to
the detector amplifier. The detected audio output is led to the buffer amplifier H208 and its
buffered output is led to; (a) noise amplifier H310 through resistor R378 and capacitor C333,
(b) QUADRADIAL jack on the rear panel through resistor R379, (c) MPX stereo decoding IC
(H321) through R301 and H301.
3.1 Audio Muting and Stereo Mode Auto-Selecting Circuit
The muting circuit consisting of all solid-state electrical switching has been incorporated in
the Model 2240. Three inputs contro! the muting function. The first is related to signal strength,
the second to the noise condition at the detector and the third is derived from the DC
component of the detector output. These inputs are properly matrixed and gated to provide
muting free from noise and transients.
The first input of DC voltage obtained by rectifying a part of IF output signal from the
H205 and H206 is applied to the base of H308 and turns on it, if the 1F output is greater than
predetermined level (muting threshold level). When the H308 is turned on the H309 is turned
off, allowing the emitter-collector resistance increasing and the collector voltage rises about 9V.
The increased collector voltage increases the gate bias voltage and turns on the switching FET
H301, decreasing the source-drain resistance to near zero ohm and allowing the audio signa
applied to the source to flow to the pin 2 of decoding IC through the source-drain path.
When the input signal is lower than predetermined level, the DC output obtained is small
and can not turn on the H308 thus the H308 keeps its turn-off stage and this makes H309 turn
on, decreasing the collector voltage and turning off H301. Thus no audio signals can pas
through the FET. This is the fundamental principle of the muting operation but for mor
elaborate muting operation the second and the third inputs are necessary.
The second input is used to protect the muting operation and MPX stereo beacon lamp
from misoperation due to undesirable noises. The high frequency noises included in th
detected audio signals are separated by a small capacitor C333 and amplified by the nois
amplifier transistor H310 and its output is rectified by the two diodes. The rectified DC output
is proportional to the noise components in the audio signals.
When there are excessive noises in the audio signals such as obtained with a station
incorrectly tuned in, the rectified DC output turns on the transistor H311, decreasing th |
emitter-collector resistance to zero. This means the collector of H309 is short-circuited to th: |
ground, therefore the H301 is turned off and any audio signals having excessive high frequency
noises can not go through the FET’s source-drain path.