Elenco SEE AMFM108CK - Manual (Page 23)

-22-
The “difference frequency” is used as the
intermediate frequency in AM radios. The collector of
Q7 also contains an IF transformer (T6) tuned only to
the difference frequency. This transformer rejects all
frequencies except those near 455kHz. T6 also
couples the 455kHz signal to the base of Q8 to be
processed by the IF amplifiers. The antenna and the
oscillator coils are the only two resonant circuits that
change when the radio is tuned for different stations.
Since a radio station may exist 455kHz above the
oscillator frequency, it is important that the antenna
rejects this station and selects only the station 455kHz below the oscillator frequency. The
frequency of the undesired station 455kHz above the
oscillator is called the image frequency. If the
selectivity of the antenna (Q factor) is high, the image
will be reduced sufficiently.
The oscillator circuit must also change when the
radio is tuned in order to remain 455kHz above the
tuning of the desired radio station. The degree of
accuracy in keeping the oscillator frequency exactly
455kHz above the tuning of the antenna is called
tracking accuracy.
White
Black
Red
White
Black
Red
Green
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}
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R=9 - 11
Ω
R=1 - 1.5
Ω
R=9 - 11
Ω
R=1 - 1.5
Ω
4 Wire 3 Wire
C28 - .1
μ
F Discap (104)
R31 - 56k Ω Resistor
(green-blue-orange-gold)
C30 - 150pF Discap (151)
L5 - AM Oscillator Coil
(Red Dot)
J1 - Jumper Wire
(use a discarded lead)
TP7 - Test Point Pin
(see Figure A)
C31 - .01 μ F Discap (103)
Q7 - 2N3904 Transistor
(see Figure C)
R32 - 12k
Ω
Resistor
(brown-red-orange-gold)
R33 - 3.3k
Ω
Resistor
(orange-orange-red-gold)
C29 - .02
μ
F Discap (203)
or .022 μ F Discap (223)
Figure J
Resistance measurements
will be used to check the
configuration of the coil. Slide
one holder off the ferrite core
of the antenna assembly.
Then slide the coil off the the
ferrite core. Measure the
resistance of the coil. Your
readings should match the
approximate values as shown.
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ASSEMBLY INSTRUCTIONS
L4 - AM Antenna w/ holders
(see Figures J & K)
C1 - Tuning Gang Capacitor
2 Screws M2.5 x 3.8mm
(see Figure L)
Knob (dial)
Screw M2.5 x 7.5mm
Label AM/FM
(See Figure M)
1/8”
Note: Mount the tuning gang
capacitor to the foil side of
the PC board.