Service manual

RC2000 Service Manual Chapter 4 Common RC2000 Questions 30
Research Concepts, Inc; 10679 Widmer; Lenexa, Kansas; USA 66215 WWW.RESEARCHCONCEPTS.COM
The AGC input circuit consists of a voltage divider implemented with a 2.4 and 1 Meg ohm resistors
followed by an op amp voltage follower (see Figure X). The maximum common mode input voltage of the
op amp is approximately 3 volts (worst case). If the input to the voltage divider exceeds 10.2 volts. the
voltage at the input of the op amp will exceed the common mode voltage. To extend the range of AGC
input a resistor can be placed in series with the input. For example, if a 1 Meg resistor is placed in series
with the input, a 13.4 volt input voltage will result in 3 volts at the input of the op amp.
In the event that the AGCsignal from the receiver is very noisy, it is possible to place an additional filter
section in series with the AGC input. The filter section consists of a series resistor and shunt capacitor. A
good starting point for component values is 20K ohm for the resistor and 2.2uF for the capacitor. This
combination will result in a time constant similar to that in the RC2000 and assumes a low AGC port output
impedance. The component values will vary with the output impedance of the receiver AGC port, and
some experimentation may be necessary. If the output impedance is very high, it may be possible to do
away with the series resistor altogether.