Sony HP-210 - Page 7

7
all
Stage/Control
“Frequency”
doubler
D301, D302
38-kHz
amplifier
Q305
STEREO-lamp
circuit
Q304, Q305
Multiplex
demodulator
D303, D304
D305, D306
Function
Signals developed at the collec=
tor of Q303 are transformer-
coupled to a full-wave rectifier
consisting of D301 and D302.
The output of this rectifier is
not filtered, resulting in two
positive pulses’ for each input
cycle. Thus the 19-kHz pilot-
carrier frequency is effectively
doubled by D301 and D302.
However, the waveform is not
sinusoidal at the base of Q305.
The 38-kHz pulses produced by
D301 and D302 are amplified by
Q305. The tank circuit at the
collector of Q305 is tuned to
38 kHz to restore these pulses to
a sinusoidal waveform for use
as a switching signal. This signal
is transformer-coupled to the
bridge-type demodulator to
supply sampling drive for the
demodulator:
The STEREO indicator lights
when an fm-stereo signal is
received. The emitter of Q305
is connected to the base of
Q304, which is normally cut off.
The circuit operates as follows:
When a composite stereo signal
is applied to the multiplex de-
coder, the 38-kHz pulses pro-
duced at the output of the
frequency doubler yield a higher-
average current flow through
Q305. This forces Q304 into
conduction, lighting STEREO
indicator lamp PL1.
The demodulator circuit em-
ploys four diodes in a balanced
bridge arrangement. This system
has the advantage of cancelling
residual rf components (38-kHz
signal, some 19-kHz signal, and
higher-order harmonics of these
frequencies). “L” and ““R” com-
ponents are developed at each
side of the bridge as the result
of demodulation, when the
receiver is operated in the stereo
mode. In the monaural mode,
diodes D303 and D304 are
Stage/Control
De-emphasis
circuit
C319, C320
Twin-T filter
C321, C325,
R335, R332,
R336, C324
(C322, C326
R334, R333,
R337, C323)
A-m Tuner
Local oscillator
Q404
Mixer Q401
I-f amplifier
Q402
Ceramic filter
CF401
Function
forward biased by supply volt-~
age through the STEREO lamp,
R321, R326 and R327, so these
diodes merely act as small resist-
ances. Under this condition, the
monaural signal is applied to
both “L” and “R” audio ampli-
fiers.
These capacitors provide the
roll-off at high audio frequencies
necessary to compensate for pre-
emphasis at the transmitter.
This filter eliminates the 38-kHz
carrier, thereby preventing
carrier leak.
This stage supplies injection volt-
age to the mixer via L402. The
circuit is a modified Hartley
oscillator with feedback applied
to the emitter of Q404 from the
tap on the secondary winding
of oscillator coil L402.
Incoming rf signals and !ocal-
oscillator voltage are hetero-
dyned in the base-emitter
junction of Q401 to produce
the 455-kHz output.
IFT401 is a transformer tuned
for 455 kHz. It develops the i-f
signal, and provides a path to
ground through bypass capacitor
C405 for the other heterodyne
products. The low-impedance
output winding of IFT401 pro-
vides link coupling to if
amplifier Q402.
This stage is basically an RC-
coupled amplifier. The selec-
tivity of the stage is determined
by a double ceramic filter
(CF401) in the interstage-coupl-
ing path. This filter provides
extremely sharp skirt selectivity
inside the pass band.