Elenco AM Radio Kit - Manual (Page 27)

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R4 - 1M
Ω Resistor
(brown-black-green-gold)
TP2 - Test Point Pin
(see Figure F)
Q2 - 2N3904 Transistor NPN
(see Figure A)
R6 - 1k
Ω Resistor
(brown-black-red-gold)
C5 - .02 μF or .022μF Discap
(marked 203 or 223)
With the power OFF, connect your equipment as shown
in Figure 26. Turn the power ON and adjust the RF
generator for 0.4Vpp at the cathode of D1. If necessary,
realign transformer T3 for maximum output while
adjusting the output of the RF generator to maintain
0.4Vpp. Slowly decrease the frequency of the RF
generator until the signal drops to 0.707 of its peaked
value or 0.28Vpp. Record the frequency of the RF
generator here:
FL=___________kHz.
Now increase the frequency of the RF generator past
the peak to a point where the signal drops to 0.707 of
its peak value. Record that frequency point 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. Your results
should be similar to the values shown in Figure 23.
BANDWIDTH TEST
FIRST IF AMPLIFIER
The operation of the first IF amplifier is the same as for
the second IF amplifier with one important difference.
The gain of the first IF amplifier decreases after the AGC
threshold is passed to keep the audio output constant at
the detector and prevent overload of the second IF
amplifier. This is accomplished by making the voltage
on the base of transistor Q2, lower as the signal strength
increases. Since the voltage from base to emitter is fairly
constant, the drop in voltage at the base produces a
similar drop in voltage at the emitter of Q2. This drop lowers the voltage across R6 and thus reduces the DC
current through R6. Since all of the DC current from the
emitter of Q2 must go through R6, the DC current in Q2
is therefore lowered. When the DC current in a transistor
is lowered, its effective emitter resistance increases. The
AC gain of transistor Q2 is equal to the AC collector load
of Q2 divided by its effective emitter resistance. Raising
the value of the effective emitter resistance thus lowers
the AC gain of Q2.
THEORY OF OPERATION
ASSEMBLY INSTRUCTIONS - FIRST IF AMPLIFIER
Figure 26
GND
TP10
Output Adjust
Generator
GND
TP10
Oscilloscope
.02
μF
Probe
SECTION 4