Table of Contents
Advertisement
Set the generator at 600kHz, 400Hz 80% modulation,
moderate signal strength. Set the oscilloscope to read
.1 volts per division. Turn the tuning knob fully counter-
clockwise and turn the power ON. Slowly turn the tuning
knob clockwise until a 400Hz sinewave is seen on the
scope. Adjust the volume control to a comfortable level.
If a station exists at 600kHz, then lower the frequency of
the generator and repeat the previous steps. With the
"magic wand", place the brass end near the antenna coil
as shown in Figure 23. If the signal on the scope
increases, it means that the antenna coil needs less
inductance. To add more inductance, carefully slide the
antenna coil along it's ferrite core in the direction shown
in Figure 23. Repeat these steps until the signal seen
decreases for both ends of the "magic wand". Increase
the frequency of the generator to 1400kHz and turn the
tuning knob clockwise until a 400Hz sinewave is seen
on the scope. If a station exists at 1400kHz, increase
the frequency of the generator and repeat the previous
AM RADIO HIGHLIGHTS
1. The number of vibrations (or cycles) per second
produced by a sound is called the frequency, and is
measured in hertz.
2. The distance between peaks of sound waves is
called the wavelength.
3. Sound waves are produced as a certain number of
vibrations per second. The more vibrations per
second, the higher the frequency; the fewer
vibrations, the lower the frequency.
4. Waves of very high frequency are called radio
waves and travel great distances through the air
without the use of wires.
5. Carrier waves are radio waves used by broadcast
stations to carry audio waves.
DC VOLTAGES
The voltage readings below should be used in
troubleshooting the AM section.
position).
Q7
B
1.5
E
1.1
C
8.8
Q8
B
1.4
E
.7
C
8.8
Q9
B
1.7
E
1.1
C
9.0
Q10
B
.7
E
.06
C
3.3
(Switch at AM
TP1
3.9
Q11
B
4.4
E
3.9
C
8.4
Q12
B
3.3
E
3.9
C
.6
Q13
B
8.4
E
9.0
C
3.9
Q14
B
.6
E
0
C
3.9
steps. Place the brass end of the "magic wand" near the
antenna coil. If the signal increases, it means that the
antenna coil needs less capacitance.
antenna trimmer for a peak. Refer to Figure 25 for the
location of the AM antenna trimmer.
adjustment of both the antenna alignment is optimized.
This process sets the AM tracking of the Superhet 108T.
Once the antenna is properly aligned, carefully apply
candle wax or glue the antenna coil to the ferrite rod to
prevent it from moving. Proceed to the FM assembly
section.
This concludes the alignment of the AM radio section.
If no stations are heard, verify that AM signals are
present in your location by listening to another AM radio
placed near the Superhet 108T. If the AM section is still
not receiving, go back and check each stage for
incorrect values and for poor soldering. Proceed to the
FM assembly section.
6. The process of adding the audio waves to the radio
waves is called modulation, and the process of
removing the radio wave from the audio wave is
called demodulation, which is performed in an AM
radio by the detector.
7. The amount of signal picked up by the antenna will
depend on the power of the signal transmitted and
the distance the signal travelled.
8. Rectification is the process of removing half the
signal, while filtering is the process of smoothing
that signal.
9. Heterodyning is the process of mixing two signals
(the incoming RF signal and the RF signal from the
local oscillator) to produce a third signal (the IF
signal).
Test Conditions
1. Volume set to minimum.
2. Switch to the AM position.
3. Connect side of capacitor C29 (that goes
to L4) to TP15 with a jumper wire.
4. Battery voltage = 9V
5. All voltages are referenced to circuit
common.
6. Voltage readings can vary +10%
-30-
Adjust the
Since the
Advertisement
Table of Contents
