Elenco Electronics AM/FM-108CK Assembly And Instruction Manual page 21

DYNAMIC MEASUREMENTS
AC GAIN
Connect your test equipment as shown in Figure 18.
Generator
Hz
TP15
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 50pF 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 re-adjusting 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.
.02μF
Probe
Figure 18
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:
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:
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.
-20-
Oscilloscope
Fl = _______kHz.
Fh = __________kHz.
TP15