Fm Radio Highlights - Elenco Electronics AM/FM-108TK Assembly And Instruction Manual

OSCILLATOR ALIGNMENT
Remove the clip lead and set your generator at 88MHz
modulator at 1kHz, 22.5kHz deviation and minimum
voltage output. Tune the radio until a 1kHz signal is
seen on the scope. It may be necessary to increase the
amplitude of the generator. Rotate the dial until the
white pointer is aligned to 88MHz. Using the "magic
wand" place the brass end near L3 as shown in Figure
45. If the signal seen on the scope increases, this
means L3 needs less inductance.
inductance, carefully spread apart coil L3. Pulling apart
or pressing together coil L3, a small amount will have a
great effect on the coil's resonant frequency. Place the
iron end of the "magic wand" near L3. If the signal seen
on the scope increases, it means L3 needs more
inductance. To add inductance carefully press together
coil L3. Repeat these steps until the signal decreases
for both ends of the "magic wand".
frequency of your generator to 108MHz. Tune the radio
until a 1kHz signal is seen on the scope. Rotate the dial
until the white pointer is aligned to 108MHz. Place the
brass end of your "magic wand" near L3. If the signal
on the scope increases, it means that L3 needs more
capacitance. Adjust the FM oscillator trimmer on the
gang (as shown in Figure L on page 23) until the 1kHz
signal is at a peak. Place the iron end of the "magic
wand" near L3. If the signal increases, it means that
coil L3 needs less capacitance. Adjust the FM oscillator
trimmer on the gang until the 1kHz signal is at a peak.
Repeat these 2 steps until the signal decreases for both
ends of the "magic wand". Since adjusting both the
oscillator coil L3 and the oscillator trimmer will effect the
frequency of oscillation, it is advisable to repeat this
procedure until the oscillator alignment is optimized.
This process sets the FM oscillator range at 98.7MHz
to 118.7MHz.

FM RADIO HIGHLIGHTS

1. The FM broadcast band covers the frequency
range from 88MHz to 108MHz.
2. FM signals are usually limited to line a sight.
3. Audio signals up to 15kHz are transmitted on the
FM carrier.
4. The amount that the RF carrier changes frequency is
determined by the amplitude of the modulating signal.
RF ALIGNMENT
Set your generator at a frequency around 90MHz
modulated at 1kHz, 22.5kHz deviation and minimum
voltage out. Tune your radio until a 1kHz tone is heard.
Place the brass end of your "magic wand" near RF coil
L1. If the signal on the scope increases, it means that
coil L1 needs less inductance. Carefully spread apart
the coil L1 to reduce its inductance. Place the iron end
of the wand near L1. If the signal increases, it means
To remove that coil L1 needs more inductance. Carefully press
together the coil L1 to increase its inductance. Repeat
these steps until the signal on the scope decreases for
both ends of the "magic wand".
generator to a frequency near 106MHz. Tune your
radio until a 1kHz tone is heard. Place the brass end of
your "magic wand" near L1. If the signal increases, it
means that the coil L1 needs more capacitance. With
Increase the an alignment tool or screwdriver, adjust the FM antenna
trimmer (see Figure L on page 23).
increases, this means coil L1 needs less capacitance.
Carefully adjust the FM antenna trimmer until a peak is
seen on the scope. Repeat these steps until the signal
on the scope decreases for both ends of the "magic
wand". Since adjusting both the RF coil L1 and the
antenna trimmer will effect the gain of th RF stage, it is
advisable to repeat this procedure until the RF amplifier
alignment is optimized. This process sets the RF stage
to "track" the FM oscillator stage.
This concludes the alignment of the FM radio section.
If no stations are heard, verify that FM signals are
present in your location by listening to another FM radio
near the Superhet 108. If the FM section is still not
receiving, go back and check each stage for incorrect
values and for poor soldering.
5. The number of times the carrier frequency changes
in a period of time is exactly equal to the audio
frequency.
6. The change in frequency is called the deviation and
is limited to 75kHz for monaural FM.
7. The bandwidth assigned for FM is 200kHz.
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