Elenco SEE AMFM108CK - Manual (Page 20)

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DYNAMIC MEASUREMENTS
AC GAIN
Connect your test equipment as shown in Figure 18.
Figure 18
TP15
GENERATOR
Hz
TP15
.001 μ F
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 4
Vpp 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 18.
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 .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 .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 16. Turn the power OFF.