Marantz 2275 - Page 7

coupled audio amplifier, where the signals are amplified and fed to output terminals J311 and
J313. From these terminals, the audio signals are fed to the TAPE MONITOR OUT jacks through
the function switch. Figure 1 presents an internal block diagram showing the functions of the
PLL MPX stereo decoding IC HA1156. The input stereo composite signal, amplified by the audio
amplifier, is delivered to the phase detectors PD-1 and PD-2. A part of the stereo composite signal
is also delivered to the stereo decoder section. The VCO (Voltage Control Oscillator) produces a
free run oscillation of approximately 76kHz with the time constant determined by capacitor
C305 and resistors R311 and R312 connected on the outside to pin (4 . The VCO output has its
frequency divided down to 19kHz through the two frequency divider stages (DIV-1, DIV-2), and
is returned to the phase detector PD-1, which contains two input terminals designed to produce
an output in proportion to the product of the two input signals. The signal fed to one PD-1 input
is a 19kHz square wave formed through frequency division of the 76kHz VCO output signal by
the two frequency divider stages DIV-1 and DIV-2, and the 19kHz pilot signal included in the
stereo composite signal, as a reference signal is fed to the other PD-1 input. Therefore, the output
of PD-1 which has gone through the low pass filter, LPF-1, provides DC output voltage in
proportion to the phase variance between the two inputs. This DC output voltage is amplified by
the DC amplifier, and is supplied to the 76kHz VCO as a contro! voltage. This means that the
output frequency and phase of the VCO have been phase-locked to the input pilot signal. The
38kHz sub-carrier reproduced by the PLL, as stated above, is delivered through the stereo switch
to the stereo decoder section as a switching signal, thus driving the decoder stage. One of the
inputs of PD-2-is given the 19kHz resulting from the frequency division completed by DIV-1 and
DIV-3, whereas the other input receives the 19kHz output contained in the composite signal, and
the output is provided with a DC component in proportion to the amplitude of the pilot signal.
This DC output is furnished through LPF-2 to the trigger amplifier which drives the stereo
indicator lamp. and stereo switch. Therefore, insufficient supply of the pilot signal results in
failure to light the stereo indicator and to turn on the stereo switch located in the path of the
38kHz switching signal, thereby avoiding a wrong stereo operation.
H303 located off pin is a switching transistor for automatic monaural-stereo mode
selection. When the intensity of an incoming signal from an FM station is weaker than a
predetermined level, H303 is turned on and pin is grounded, thereby developing a condition
for monaural reception. For forced monaural operation, switching the MODE switch to
“MONO”, turns on H303 with the positive bias voltage applied to the base, and pin (@) is
AUDIO 19KHz
AMPLIFIER
STEREO
COMPOSITE
SIGNAL
INPUT
LPF-1 76KHz 38KHz
+2 +2
(DIV-1) (DIV-2)
STEREO
SWITCH
PHASE
DETECTOR
(PD-1)
DC
AMPLIFIER
19KHz, <90°
TRIGGER
19KHz
STEREO
INDICATOR
LAMP
LPF.2
PHASE
DETECTOR
(PD-2)
DECODER
AUDIC OUTPUT
STEREO COMPOSITE
SIGNAL
38KHz SQUARE WAVE
Figure 1. Block Diagram of the HA1156