Repeat this step until the pointer is aligned to the
station’s frequency. Adjusting both the oscillator coil L3
and the oscillator trimmer capacitor will effect the
oscillator’s frequency, so it is advisable to repeat this
procedure until the FM oscillator alignment is
optimized. This process sets the FM oscillator range at
98.7MHz to 118.7MHz.
RF ALIGNMENT
Press together L1 and L2. Spread apart coil L1 so that
it resembles Figure 46. The gaps or spaces should be
between 1/32” and 1/16” wide. This procedure sets the
tracking of the RF section. Use the special coil spacer
provided to gap the coil as shown. Carefully slide the
coil spacer between the coils to get the spacing shown
in Figure 46.
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 placed
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.
Approx.
1/16” gap
Approx.
1/16” gap
Figure 46
Figure 47
Turn the power ON. Slowly increase the amplitude of
the generator until a 1kHz signal is seen on the scope.
Keep the generator at a low level of output to prevent
the IF sections from limiting. With an alignment tool or
screwdriver, adjust T1 for a peak on the scope. Reduce
the amplitude of the input signal if necessary. Adjust T2
for a peak and reduce the amplitude of the input signal
if necessary. Repeat these steps until the IF alignment
is optimized. This procedure aligns the FM IF amplifiers
to 10.7MHz.
ALIGNMENT WITH RF GENERATOR AND OSCILLOSCOPE
IF ALIGNMENT
Spacer
Top View
L1
L2
L1
L2
Switch to the FM section. Connect your RF generator
and oscilloscope to the circuit as shown in Figure 47.
Set your RF generator at 10.7MHz modulated at 1kHz
deviation with minimum voltage output. Set the scope
to read 50mV per division. With a clip lead, short the
base emitter junction of Q2. This short “kills” the local
oscillator.
-49-
Short the base of Q2 to the
emitter (as shown below).
TP15
Generator
Hz
Battery
TP15
Oscilloscope
