Zenith J584W - Page 26

FIGURE 10 - CHASSIS 3WJR50 TUNING METER CIRCUIT
RF Transistor Q1. When the IF signal level reaches sufficient
amplitude, Q1's G2 voltage is reduced from +4.6 volts, thereby
lowering the gain of Q1 .
FM - QUADRATURE DETECTOR
Output of the amplified and limited IF signal in IC201 appears
at pin 8 and is coupled, via L204, to the Quadrature Detector
at pin 9. Connected externally between pins 9 and 10 is the
quad coil (L203). Voltage appearing across L203 is a function
of the signal frequencies appearing at the ends of L203.
Signals at these pins will be 90 degrees apart, or in quadra 
ture, resulting in the circuit being called a quad detector.
Signals at the center frequency will produce equal voltages
at pins 9 and 10, while signals off center frequency will
create different voltages on pins 9 and 10. Being a differ 
ential circuit, only the difference of the signals appearing
at pins 9 and 10 will be amplified. Identical signals on those
pins will not be amplified.
AM/FM - TUNING METER - CHASSIS 3WJR50
As has tan notedi Sllffii? in console models)
has the same circuitry as Chassis 3WJR52 with one signi 
ficant difference. The difference being that Chassis 3WJR50
has a tuning meter which is common to both AM and FM.
Figure 10 is a partial schematic of the tuning meter circuit.
Tuning meter (M1) is maximum reading on AM and zero
center on FM.
When Chassis 3WJR50 is in the AM mode, meter signal will
be via AM Detector CR101, R106, R105, Bandswitch SW1-E6
and 1, meter M1 and the low side of the AFC/Meter Adjust 
ment R211 to ground. In FM or FM Stereo the meter signal
will be via pin 7 of FM-IF (IC201 ), R209, Bandswitch SW1-E5
or 4 and 1, meter M1 and the low side of the AFC/Meter
Adjustment R21 1 to ground.
Meter Adjustment R21 1 is adjusted for zero center as part of
the FM IF alignment. As mentioned previously when we dis 
cussed ceramic filters, FM alignment of this chassis requires
that you set your generator to the frequency of the ceramic
filters, then align the IF transformer (T201), the Quad De 
tector (L203), then Meter Adjustment 211 (See Figure 10).
FM-MULTIPLEX
Chassis 3WJR52 makes use of a circuit called Phase Locked
Loop (PLL), which can be compared to a thermostat controlled
heating system. Figure 11 A illustrates a basic heating system in
which the furnace will generate heat. A thermostat will sense
the temperature and compare it with the thermostat's manual |
setting. The thermostat will turn the fuel supply valve on or
off, connecting or disconnecting the fuel source to the furnace,
controlling heat generation from the furnace. This system
functions in a closed loop.
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