Elenco SEE AMFM108CK - Manual (Page 22)

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DYNAMIC MEASUREMENTS
AC GAIN
Connect your test equipment as shown in Figure 19.
The scope probe must have an input capacitance of
12pF or less, otherwise it will detune transformer T7.
Using a clip lead, short TP3 to R38 as shown. This
short prevents the AGC from lowering the gain of the
first IF amplifier. Set the generator to 455kHz, no
modulation, and minimum voltage output. Set the
scope to read 1 volt per division and turn the power
ON. Increase the amplitude of the generator until
approximately 4Vpp is seen on the scope. Retune
the IF transformer T7 to maximize the 455kHz at
TP4. After tuning T7, adjust the generator amplitude
in order to keep 4Vpp at TP4. Now move the scope
probe to the base of Q8 and record the peak to peak
level of the 455kHz signal here:
Vb=___________Vpp.
The AC gain of the first IF amplifier is equal to 4/Vb.
The AC gain should be greater than 100. DO NOT
TURN THE POWER OFF, GO TO THE NEXT TEST.
AGC ACTION
Move the scope probe back to TP4 and adjust the
generator for 4Vpp if necessary. Remove the clip
lead shorting TP3 to R38. The AGC should reduce
the signal level at TP4 to approximately .8 volts. Turn
the power OFF.
SECTION 5
In a superheterodyne type receiver, the radio wave at
the antenna is amplified and then mixed with the
local oscillator to produce the intermediate frequency
(IF). Transistor Q7 not only amplifies the RF signal,
but also simultaneously oscillates at a frequency
455kHz above the desired radio station frequency.
Positive feedback from the collector to the emitter of
Q7 is provided by coil L5 and capacitor C31. During the heterodyning process the following four
frequencies are present at the collector of Q7.
1. The local oscillator frequency, OF.
2. The RF carrier or radio station frequency.
3. The sum of these two frequencies, OF + RF.
4. The difference of these two frequencies, OF – RF.
AM MIXER, AM OSCILLATOR, AND AM ANTENNA
Figure 19
Short TP3 to R38 as shown below.
TP15
Generator
Hz
.001 μ F
TP15