Elenco AM/FM Radio Kit - Manual (Page 29)

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DYNAMIC MEASUREMENTS
AC GAIN
Connect your test equipment as shown in Figure 28.
Figure 28
Set the generator at 455kHz, no modulation and
minimum voltage output. Set the oscilloscope at 1
volt per division. The scope probe must have an input
capacitance of 12pF or less or it will detune T8. Turn
the power ON and slowly increase the amplitude of
the generator until 4 volts peak to peak are seen on
the scope. With an alignment tool or screwdriver,
tune T8 for a peak on the scope while readjusting the
generator’s amplitude to maintain 4Vpp at the
oscilloscope. After T8 is aligned, move the scope
probe to the base of Q9 and record the peak to peak
amplitude of the signal here:
Vb=________ Vpp.
Turn the power OFF. The AC gain of the second IF
amplifier at 455kHz is equal to 4/Vb and should be
greater than 100. If your value is less than 50 check
components R39, R40, R41, C36 and C37. Also
make sure that Q9 is properly installed. Turn the
power OFF.
BANDWIDTH
Reconnect your test equipment as shown in Figure 28.
Turn the power ON and adjust the generator for 4
volts peak to peak at TP3. Realign T8, if necessary,
for maximum output while adjusting the output of the
generator to maintain 4Vpp at TP3. Slowly decrease
the frequency of the RF generator until the signal at
TP3 drops to 0.707 of its original value or 2.8Vpp.
Record the frequency of the RF generator here:
Fl = _______kHz.
Now increase the frequency of the generator past the
peak to a point where the signal drops to 0.707 of its
peak value. Record that frequency here:
Fh = __________kHz.
By subtracting the frequency of the lower 3dB corner
from the frequency of the higher 3dB corner you get
the bandwidth of the second IF amplifier.
Calculate the bandwidth by (FI–Fh)
Bandwidth = __________kHz.
Your results should be similar to the values shown in
Figure 26. Turn the power OFF.
GND
TP15
Generator
GND
TP15
Oscilloscope
.001 μ F