Elenco AM Radio Kit - Manual (Page 32)

Figure 29
DYNAMIC MEASUREMENTS
With the power turned OFF, connect the oscilloscope to
the circuit as shown in Figure 29. Set the oscilloscope for
a vertical sensitivity of 1 volt/division and turn the power
ON. The oscilloscope should display a low voltage sine wave. The frequency of the sine wave should change
when capacitor C1 is turned. If your circuit fails this test,
turn the power OFF and check components Q1, C1, C2,
C3, L1 and L2.
OSCILLATOR CIRCUIT
GND TP10
Oscilloscope
FINAL ALIGNMENTS
With the power turned OFF, connect the RF generator and
the oscilloscope to your circuit as shown in Figure 30.
Short TP2 to the collector of Q1 with a clip lead to “kill”
the local oscillator. Set the RF generator at a frequency
of 455kHz, modulation of 400Hz 80%, minimum
amplitude output. Set the oscilloscope to read 0.1Vpp
and turn the power ON. Increase the amplitude of the
RF signal until the oscilloscope registers 0.5Vpp. Align
transformers T3, T2 and T1 for the maximum AC reading
on the oscilloscope. Decrease the amplitude of the
signal from the RF generator to restore 0.5Vpp on the
oscilloscope. Repeat the last two steps until no change
in the peak at the oscilloscope is noticed. After IF alignment, lower the frequency from the RF
generator until the reading on the VOM drops to 0.707
of its peaked value. Record the frequency of this lower
3dB corner here:
Fl=____________kHz.
Increase the RF generator frequency past the peak to
the upper 3dB corner and record that frequency here:
Fh=____________kHz.
The bandwidth of the IF amplifiers is BW=Fh - Fl. IF
bandwidth should be between 1 to 2kHz. This bandwidth
will widen as the AGC is approached.
IF BANDWIDTH
Figure 30
GND
TP10
Output Adjust
Generator
Oscilloscope
.02
μF
Wire lead
or clip lead
Probe
-31-
GND
TP10
If you do not have an RF generator, go to the Final Alignments with No Test Equipment Section.