Marantz 2275 - FM Tuner Circuitry; Audio Muting and MPX Stereo Decoding

current of the RF amplifier through the AGC amplifier incorporated in the IC. A part of the DC
component is also fed from J157 to the signal strength indication amplifier H154. The output
appearing at the collector of H154 is level adjusted by R178, indicated on the signal strength
meter MOO2.
2.1 Suggestions for AM Tuner trouble shooting
Check for a broken AM bar antenna. Next, attempt to tune stations by rotating the fly-wheel
tuning knob slowly and observe the AM signal strength meter for deflection. If the signal strength
meter gives a deflection at several frequencies received, a probable failure exists after the AM
detector stage. Next, connect an oscilloscope to J161 and check for audio signals with the tuning
meter deflected. If the signal strength meter does not deflect, check the local oscillator circuit.
Normal loca! oscillator output voltage at the hot end of the oscillator tuning capacitor is about
1.5 to 3 volts, varying with the tuning capacitor position. When measuring the local oscillator
output voltage use an RF VTVM, no common circuit tester will give a correct indication, due to
loading. |f the local oscillator output voltage is normal, check all voltage distribution in the AM
circuits by using a DC VTVM and compare the measured values with those given in the schematic
diagram.
3. FM TUNER
The FM Tuner section in the Model! 2275 is divided into four functional blocks: FM Front
End, |F Amplifier & Detector, Muting Control and Multiplex Stereo Decoding Circuit.
An FM signal from an FM antenna is fed to antenna coil L101 from the balun coil. The
signal is then applied to the FET RF amplifier which in turn feeds its output to the FET Mixer
H102 through a triple tuned, high selectivity circuit. The FET Mixer converts its input signal into
a 10.7MHz intermediate frequency and amplifies it at the same time. H103 is the local oscillator,
the output of which is injected into the source of the FET Mixer. The injection voltage is
approximately 700mV. The 10.7MHz front end output is fed to the IF amplifier from J105.
The IF amplifier unit consists of eight IF amplifier stages, one AGC amplifier stage and an
audio buffer amplifier stage. Eight ceramic filters are also used to obtain high selectivity, and four
symmetrical diode limiter stages are also employed for best limitting characteristics, improved
capture ratio and good AM suppression.
A part of the FM Front End output is fed to and rectified by the AGC amplifier H209, and
the rectified output is fed back to the gate of the FET RF amplifier from J208 to decrease the
gain with increased signal strength.
The signal required for multipath indication is obtained from five IF amplifier stages through
coupling capacitors C211, C214, C223, C252 and C228 respectively and is rectified by four full
wave rectifiers diodes H221 through H228. These AM components of the FM signal are mixed
and amplified by transistors HSO1 and HSO2 and the output is again rectified to obtain DC
current required for actuating the signal strength meters, which is used for multipath indication
as well.
The IF signal is fed to the Detector Amplifier H208. The detected audio output is fed to the
buffer amplifier H210 and its output is fed to: (a) the noise amplifier H310 through resistor
R378 and capacitor C333, (b) the QUADRADIAL OUTPUT Jack on the rear panel through
resistor R379, and (c) the MPX stereo decoding IC (H321) through R301 and H301.
The DC current developed at the third winding of the discriminator transformer is directly
cnnected to the FM center tuning meter.
3.1 Audio Muting and Stereo mode auto-selecting circuit
100% solid state muting is incorporated in the Model 2275. Three inputs control the muting
function. The first is related to signal strength, the second to the noise level 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.