Zenith J584W - Page 22

FIGURE 5 — AM RF/OSC/MIX/IF IC BLOCK WITH EXTERNAL COMPONENTS
8. One Integrated Circuit for Phase Locked Loop (PLL)
Multiplex Detector (IC301) with one variable resistor
adjustment (R302) .
RF/IF/MPX
As in the "G and H Line Wedges", the new "J Line Mini 
Wedge" (Chassis 3WJR52) continues the use of separated
RF and IF circuitry for FM and AM. This is illustrated by
the RF/IF/MPX schematic in Figure 2, as well as the Block
Diagram in Figure 3.
AM RF/OSCILLATOR/MIXER/IF
Most noticeable change in the "J Line" AM circuitry is in 
troduction of a single Integrated Circuit to handle RF/Oscil-
lator/Mixer/IF functions on the AM band (See Figure 4).
Figure 5 illustrates the same circuit redrawn to identify
the various functions interconnected within the Integrated
Circuit (IC101), and also includes external circuitry. RF
signal voltage developed across the AM Antenna (L101) is
connected to IC1 01 's RF Amplifier, via pin 12, where it is
amplified and fed to pin 13. Amplified RF is coupled, from
pin 13, via C105, to pin 1 and then to the internal Mixer.
Also at pin 13 is a 455 kHz trap (L102) which is designed
to remove any 455 kHz signals present at the output of
the RF Amplifier. The Local Oscillator, with an external
tank circuit at pin 2, is connected internally to the Mixer.
Output of the Mixer at pin 14, is coupled to the 1st IF Trans 
former (T102). This transformer is not only double tuned,
but also contains a Ceramic Filter within the transformer
can.
NOTE: When aligning IF circuits containing ceramic filters,
in either the AM or FM IF circuit of this chassis, it
is important that the alignment be done at the center
frequency of that filter. To do otherwise will reduce
the circuit performance. See additional comments
under FM-IF Ceramic Filters.
AM IF signals from T102 are'coupled back to the chips IF
Amplifier, via pin 7, where it is amplified. The signal then
leaves the IC at pin 6 going to AM Detector Transformer
T103 and AM Detector Diode CR101. Signal at pin 6 is
sampled, via C109, to be used in the internal AGC Amplifier
whose input is connected at pin 5. Output of the AGC Ampli 
fier is then internally coupled to the RF Amplifier providing
overload protection. A Zener Diode is also located within
IC101 (connected at pin 3). Capacitor C110 is the AGC
bypass.
When B+ is applied to the AM circuitry, via the bandswitch,
a portion of that voltage is applied via R109 and R301, to ,
forward bias CR301, placing a positive voltage on pin 14 of ’
Multiplex chip IC301. This will turn-off the Voltage Controlled
Oscillator (VCO) in IC301, thereby preventing possible "birdies"
when the receiver is in the AM mode.
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