Zenith J584W - Page 24

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FIGURE 8 - CHASSIS 3WJR52 FM IF SCHEMA TIC
Although that method was effective, handling could be
improved if protection were included within the FET package.
A schematic of the internal configuration of such a package is
shown in Figure 7. Back-to back zener diodes connected from
each gate are diffused into the FET while it is being con 
structed. When a voltage of sufficient value is developed
across the zeners, they will conduct, bypassing voltage tran 
sients which approach the gate breakdown voltage. This
protects the gate structure, while allowing the FET to retain
the wide dynamic signal range capability.
Even though gate protection has been included in the design
of most MOS devices, certain precautions should be observed
while handling either MOSFETS or MOS Integrated Circuits:
A. Do not generate static.
B. Keep relative humidity above 60%.
C. Do not have rugs (especially nylon) in the service area.
D. Do not use nylon or polyester pants, shirts or jackets.
E. Do not wear rubber gloves. Cotton is recommended.
P. Bo not insert MOS device^ Ih fdSM hflldfiFS.
G. Leave MOS devices in their protective carriers (if supplied)
until used in a circuit.
H. Benches and soldering irons should be grounded.
FM - AFC
Oscillator stability is important, therefore it is desirable to
provide Automatic Frequency Control (A.F.C.) which is
guided by a voltage directly related to oscillator frequency
shift. This is accomplished by taking a DC voltage from pin 7
of IC201 (the IF, Limiter and Detector) and feeding it back,
via R210 and R13, to voltage controlled diode CR1 (See
Figures 2, 6 and 8). This diode is connected across the oscil 
lator tuned circuit and acts as a frequency controlling device.
If the oscillator shifts frequency, it causes a change in detector
output voltage which is fed back, changing the diode capaci 
tance of the oscillator circuit, automatically adjusting the
oscillator frequency to compensate for the original oscillator
frequency shift. There is a possibility that some component
may fail in the oscillator circuit, shifting the frequency beyond
the +/ — .4 Megahertz control range of the diode. In addition,
an AFC disabling switch has been provided, should it be
desired to receive a weak FM station within the AFC pull-in
range of a strong FM station. In the AFC-OFF position a
fixed DC voltage is applied to the AFC line from B+, via the
AFC Reference Adjust (R21 1) and AFC Switch SW2.
FM-IF
This chassis incorporates several technological advances seen
in "Wedge" chassis 12WGR59 and 12WHR29 FM-IF (See
Figure 8) including the use of;
1. 10.7 MHz Ceramic Filter (Y201). |
2. Integrated Circuit for gain, limiting and quadrature
detector (IC201).