Kenwood KR-4070 Service Manual page 9

CIRCUIT DESCRIPTION
Description of block diagram
The 38 kHz signal obtained by VCO is fed, as a switching
signal. to the synchronizing detector circuit through the
stereo auto switch. Since the 38 kHz signal is an ideal
square wave signal, it does not include even number har-
monics and it will not produce a beat with SCA signal (67 +
7 kHz), thus eliminating the-need for SCA filter.
When a monaural signal or a weak stereo signal is received,
the stereo auto switch cuts off the 38 kHz signal to prevent
the deterioration of S/N of demodulated audio signal.
Functions of stereo auto switch
The output of DIV-1 is partly fed to DIV-3 and is divided into
19 kHz. It is then fed to the one side of the input of PD-2.
The 19 kHz signal of DIV-3 is phase locked by the 19 kHz
pilot signal during stereo reception, so that a DC voltage
proportional to the amplitude of the pilot signal is developed
in the PD output.
in monaural broadcase, there is no pilot signal and therefore
no voltage is developed. Also. in stereo broadcase, if the
signal is weak, it does not reach the trigger level so the
following functions are not effected
The DC signal is fed to the trigger circuit and when its level is
above the predetermined threshold, the stereo switch turns
on to send the 38 kHz switching signal to the synchronizing
detector and the stereo lamp tights.
Low pass fitter
Composite
'signal tN
VCO killer
When a +B voltage is applied to the front end (FM position),
the diode Dg4 is reversely biased and thus the No. 16 pin is
disconnected from the ground. However, in the case of the
mode other than FM, the +8 voltage disappears, so the
No. 16 1s grounded through the diode and resistor to prevent
VCO being operated, thus the S/N is not degraded.
Similarly, when the No. 2 pin of ICg1 is grounded, the
differential amplifier is unbalanced. This reduces the output
of the No. 6 pin so that the meter is not operated.
AM UNIT
The RF signal from the bar antenna is fed to the No. 2 pin of
ICg2 where it is amplified. This signal is fed from the No. 4
pin and is applied to the No. 7 pin and the tuning circuit with
a variable capacitor. The amplified signal is mixed with the
signal from the local oscillator composed of Tg6 and a
variable capacitor and is converted into IF output by CFg3 so
as to be fed to ICg2 where it is amplified. The signal
amplified passes through the IFT of Tg7 and is then fed to
ICg2, and the detected output is taken out of the No. 12 pin,
while the AM S-meter output is taken out of No. 15 pin of
ICg2.
Fig. 5 AM BLOCK DIAGRAM
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Oetermines the
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SEPARATION Stereo auio
ADJ cucuit
Fig. 4 LA3350 BLOCK DIAGRAM