Zenith J584W - Page 29

FIGURE 14 - MUL TiPLEX DECODER WA VEFORMS
In the demodulator the Sum (L+R), difference (L — R) and
38kHz are combined to derive the L and R outputs.
Diode CR301 is connected to the AM B+ point at pin 14.
When AM is turned on, a positive voltage forward biases CR301 ,
applies a positive voltage to pin 14, turning off the VCO. This
prevents “ birdies" from appearing on the AM band.
To reduce the possibility of “ pop" in the speaker system when
switching to FM or FM Stereo, three components (CR302,
C314 and R314) have been added at composite input pin 2
of IC301. When the mode selector switch is moved to the
FM or FM Stereo positions FM B+ (13.1V) is connected,
via R314, to C314. As capacitor C314 charges, the voltage
at the + side of C314 will slowly increase from 0 volts. 3.2
volts always appears on the anode side of CR302 (anode
connected to pin 2 of IC301). When FM is first turned “ on",
the voltage at the cathode of CR302 will be less than 3.2
volts causing the diode to be forward biased. While forward
biased the composite signal at pin 2 will effectively be shunted
to ground, muting the FM audio. As the voltage on the +side
of C314 increases past 3.2 volts, diode CR302 will become re-
verse biased; opening the shunt path allowing the composite
audio to pass to IC301 for processing. This will effectively
mute pops or related noise occurring during switching into
FM or FM Stereo.
Manual Stereo/Mono switching occurs as follows. When the
bandswitch is in Phono, AM, FM Mono or Tape, 13.1 volts
from the bandswitch will be connected, via R305, to pin 9 of
IC301. This pin is at the output the Amplitude Detector
Filter connected between pins 8 and 9. In the above position
approximately 5.7 volts will appear at pin 9, effectively cutting
off IC301's internal automatic stereo switching, causing it to
go to mono operation. When the bandswitch is in the FM
Stereo position voltage will not be applied to pin 9, resulting
in approximately 2.5 volts appearing at pin 9, allowing a
19kHz pilot signal to control the internal switching, passing
stereo to the output.
A regulator located with-in IC301 is connected to pin 1 (the
+13.1 volt supply) and supplies most stages, while the un 
regulated +13.1 volts powers certain circuits.
Stereo Indicator Lamp (DS301) is now part of a solid state
switching circuit. The base and collector of Q301 are connected
to the 13.1V regulated and the 24V supplies respectively.
When IC301 detects the presence of a stereo pilot, pin 6 of
IC301 will be enabled, completing a path through Stereo In 
dicator Lamp DS301, R315, the emitter/collector circuit of
Q301 and R313 to the 24V supply. With the connections
shown, transistor Q301 will function as a series pass regulator
providing DS301 with a fixed current. R313 is current limiting
for Q301 while R315 is current limiting for D3301.
Typical waveforms as found at IC301 terminals are shown in
Figure 14.
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