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Set the RF generator at 455kHz, modulation of 400Hz
80% and minimum voltage out. Set the oscilloscope to
read .1 volts per division and turn the power ON.
Increase the amplitude of the generator until the
oscilloscope shows a 400Hz sinewave 5 divisions or .5
volts pp. With an alignment tool or screwdriver adjust
T6 for a peak. Reduce the generator amplitude so that
5 divisions are maintained. Adjust T7 for a peak and
reduce that amplitude again if necessary. Repeat these
steps to optimize the IF alignment. This process aligns
the IF amplifiers to 455kHz.
After the IF alignment is complete, lower the frequency
of the generator until the voltage drops .707 of its
peaked value or .35Vpp. Record the frequency of the
lower 3dB corner here:
Fl = _________kHz.
Increase the frequency of the generator past the peak
until the voltage seen on the scope drops .707 of its
peaked value or .35Vpp. Record the frequency of the
high 3dB corner here:
Fh = __________kHz.
The bandwidth of the IF is equal to BW = Fh - Fl. The
IF's bandwidth should be around 6kHz. Turn the power
OFF and remove the short from the collector of Q7
to TP6.
Calculate the bandwidth: ___________kHz.
OSCILLATOR ALIGNMENT
Set the RF generator at 540kHz, 400kHz 80% AM
modulation and a low level of output. Turn the power
Generator
Hz
TP15
Wire loop
close to
antenna
Figure 26
ON and set the volume control to a comfortable level.
Turn the tuning knob counter-clockwise until the white
pointer is aligned at the 540kHz marking on the dial.
With an alignment tool or screwdriver adjust L5 until a
400Hz tone is heard. Adjust L5 for a peak on the
oscilloscope. Adjust the amplitude of the RF generator
to maintain a level of .5 volts peak to peak or less. After
peaking L5, set the generator frequency to 1600kHz.
Turn the tuning knob clockwise until the white pointer is
aligned to the 1600kHz marking on the dial. With an
alignment tool or screwdriver, adjust the AM oscillator
trimmer on the back of the tuning gang until a 400Hz
tone is heard. Adjust the trimmer for a peak on the
oscilloscope. Refer to Figure 25 for the location of the
AM oscillator trimmer. Repeat these steps to optimize
the oscillator alignment. This process sets the oscillator
range at 955kHz to 2055kHz.
ANTENNA ALIGNMENT
With the power turned OFF, connect your test
equipment as shown in Figure 26.
AM Antenna
FM Antenna
AM Oscillator
AM Antenna
Battery
-29-
Figure 25
AM Oscillator
FM Oscillator
FM Oscillator
FM Antenna
Oscilloscope
TP15
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