Elenco SEE AMFM108CK - Manual (Page 15)

SECTION 2
AM DETECTOR AND AGC STAGE
-14-
Figure 12
Switch
J2 - Jumper Wire
(use a discarded lead)
The purpose of the automatic gain control (AGC)
circuit is to maintain a constant level at the detector,
regardless of the strength of the incoming signal.
Without AGC, the volume control would have to be
adjusted for each station and even moderately strong
stations would clip in the final IF amplifier causing
audio distortion. AGC is accomplished by adjusting
the DC bias of the first IF amplifier to lower its gain
as the signal strength increases. Figure 12 shows
that the audio at the top of the volume control is
actually “riding” on a negative DC voltage when strong signals are encountered. This negative DC
component corresponds to the strength of the
incoming signal. The larger the signal, the more
negative the component. At test point five (TP5), the
audio is removed by a low pass filter, R36 and C32,
leaving only the DC component. Resistor R35 is
used to shift the voltage at TP5 high enough to bias
the base of transistor Q8 to the full gain position
when no signal is present. Resistors R35 and R36
also forward bias diode D4 just enough to minimize
“On Condition” threshold voltage.
The purpose of the detector is to change the
amplitude modulated IF signal back to an audio
signal. This is accomplished by a process called
detection or demodulation. First, the amplitude
modulated IF signal is applied to a diode in such a
way as to leave only the negative portion of that
signal (see Figure 12). The diode acts like an
electronic check valve that only lets current pass in
the same direction as the arrow (in the diode symbol)
points. When the diode is in conduction (On
Condition), it will force the capacitors C33 and C38 to charge to approximately the same voltage as the
negative peak of the IF signal. After conduction stops
in the diode (Off Condition), the capacitors will
discharge through resistors R36 and R42. The
discharge time constant must be small enough to
follow the audio signal or high frequency audio
distortion will occur. The discharge time constant
must be large enough, however, to remove the
intermediate frequency (455kHz) and leave only the
audio as shown in Figure 12.
ASSEMBLY INSTRUCTIONS
1/8”