Elenco SEE AMFM108CK - Manual (Page 50)

-49-
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 44. If the signal seen on the scope
increases, this means L3 needs less inductance. To
remove 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”. Increase the 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.
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 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”. Increase your 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 an
alignment tool or screwdriver, adjust the FM antenna
trimmer (see Figure L on page 23). If the signal
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.
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.
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.