Elenco Electronics AM/FM-108CK Assembly And Instruction Manual page 24

The "difference frequency" is used as the intermediate
frequency in AM radios.
contains an IF transformer (T6) tuned only to the
difference frequency.
frequencies except those near 455kHz.
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
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)
Note: Mount the tuning gang
capacitor to the foil side of
the PC board.
The collector of Q7 also
This transformer rejects all
T6 also
C28 - .1μF Discap (104)
Figure J
Resistance
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.
resistance of the coil. Your
readings should match the
approximate values as shown.
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.
3 Wire
measurements
White
Measure the
Black
Red
-23-
R31 - 56kΩ 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 - .01μF Discap (103)
Q7 - 2N3904 Transistor
(see Figure C)
R32 - 12kΩ Resistor
(brown-red-orange-gold)
R33 - 3.3kΩ Resistor
(orange-orange-red-gold)
C29 - .02μF Discap (203)
or .022μF Discap (223)
4 Wire
White
}
R=9 - 11Ω
}
R=1 - 1.5Ω
Black
Red
Green
1/8"
}
R=9 - 11Ω
}
R=1 - 1.5Ω