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AUTOMATIC RF AGC ADJUSTMENT SYSTEM
The Automatic RF AGC Adjustment System employed in the VB7C chassis
replaces the mechanically adjusted RF AGC control used in conventional
systems. The primary difference between this system and conventional
system is the addition of the RF AGC A/D input circuit and the adjustment
program incorporated in the CPU (C-003).
The advantages of this system include improved productivity and increased
accuracy of the RF AGC adjustment during production. This is due to the
computerized and digitized adjustment procedure which allows remote
operation.
Note:
The automatic RF AGC adjustment system requires special equipment
and test signals. For this reason, this system should be used only for
production.
An additional adjustment mode is provided in the CPU (C-003) for
service. To enter the service menu, press and hold the MENU key
while connecting the AC power. Then select “NO. 03 RAD” (RF AGC
Delay) and adjust the data with the remote control. To turn off the
service menu display, press the MENU key again. Refer to the specific
Service Manual for your model for the complete RF AGC Adjustment
procedure.
The automatic RF AGC adjustment system is composed of the RF AGC A/D
(Analog/Digital) input circuit, the automatic adjustment program in the CPU
and the Signal Processor. The fundamental operations are described below.
Fundamental Operation
The BUS data for the RF AGC Delay control with the 6-bit control data up to
64 steps is output from IC801, the CPU. The BUS data for the RF AGC
control is coupled to the BUS Interface circuit within IC101 the Signal
Processor. The BUS Interface circuit transfers the 6-bit control data to the RF
AGC Delay Control Register within IC101. The 6-bit control data changes for
the RF AGC Delay control are the same as those shown for the RF AGC
voltage output from pin 6 of IC101 and coupled to the AGC input terminal of
A101, the Tuner.
Since the RF AGC voltage required for the Tuner to control the AGC gain is
also connected to the RF AGC A/D input circuit, the RF AGC voltage supplied
from pin 6 of IC101 is detected and input to the CPU, pin 28 (RF AGC A/D
input).
The RF AGC A/D input voltage is corresponding with the antenna input level
as shown below. The RF AGC A/D input circuit has been designed to apply
the maximum rating RF AGC A/D input voltage to pin 28 of the CPU when the
maximum RF AGC voltage is supplied from pin 6 of IC101. The CPU (C-003)
has been programmed to automatically adjust the RF AGC voltage between
69/128 Vcc (2.70V) and 85/128 Vcc (3.32V) at a specified antenna input level.
