Realistic trc-448 Service Manual page 8

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f
-11Q207
•
T209
D224
D225
Q214
VR206
VR207
C279
L208
D209
D210
777
V
C90
R4
Q15
C104 R280
D28
AUTOMATIC MODULATION CONTROL CIRCUIT
The Automatic Modulation Control (AMC) circuit consists of Q14, D26, D27 and D24. The Mic input
signal is fed to pin 4 of the microphone jack and then through R62, C84 to the input terminal (pin 5) of
1C-3 where it is amplified and delivered through C90 and VR4 for further amplification by Q15, Q232,
Q233, Q234 and Q235. The Audio amplifier/Modulation amplifier drives T216, whose secondary in-
corporates C104, through D228 and R280, which couples a portion of the signal to AMC detector diodes
D27 and D28. D26 (an 8-volt zener) is connected to the output of D27/28; when the detected DC voltage
from D27/28 exceeds 8 volts, D26 conducts. This applies a DC voltage to the base of Q14, thus decreasing
its collector impedance. Notice that the combination of Q14 and VR4 automatically sets the desired audio
signal level processed by the audio amplifier circuitry. VR5 is adjusted to seta modulation level of less than
100%.
VR4 is adjusted to set a modulation level in the SSB mode. IC-3 is a silicon integrated circuit which func-
tions as a audio amplifier with built-in automatic voice-operated gain adjustment. It is designed to provide
an essentially constant output signal for a 60 dB range of input.
.
MIC JACK
7
4
IC3
R62
4
Q14
AUTOMATIC LIMITER CONTROL (TRANSMITTER)/AUTOMATIC GAIN CONTROL (RECEIVER)
The Automatic Limiter Control (ALC) circuit consists of D209, D210 and Q214. The RF output signal is
detected by 0209 and D210. This applies a negative DC voltage to the gate of Q214, thus decreasing its
source voltage. This source voltage controls Q207's (7.8 MHz amplifier) base bias voltage, thus decreasing
its base voltage. This automatically sets the desired RF output level processed by the RF amplifier circuitry.
V R207 is adjusted to set a RF power level of less than 12 watts PEP.
Q214 is also used as an automatic gain control (AGC). AGC circuit consists of Q214, D221 and D222.
The amplified IF signal is detected by D221, D222, D224, and D225. This voltage is used for fast-attack
AGC. D221, D222 and C279 are used for slow-release AGC. Thus, the circuit functions in the same way,
except for the delivering of source voltage. VR206 is adjusted to set a "0 - reading on the S-meter.
D26
D27
VR5
K K
T216
D228