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TM 11-5820-890-30-5
CHAPTER 5
AMPLIFIER, RADIO FREQUENCY AM-7238/VRC
MAINTENANCE INSTRUCTIONS
Subject
Section
Page
Principles of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
5-1
Repair Parts, Special TooIs, TMDE, and Support Equipment . . . . . . . . . . . . . . . . . . . . . . . II
5-4
Troubleshooting Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III
5-4
Maintenance Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV
5-32
Preparation for Storage or Shipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V
5-39
Section I. PRINCIPLES OF OPERATION
Subject
Para
Page
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1
5-1
Overall Functional Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2
5-1
Input Control Signal Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3
5-2
RF Signal Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4
5-3
Feedback and Output Signal Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5
5-3
5-1.
INTRODUCTION.
The power amplifier increases the 4-W output of the RT to 50 W for long range communication. It has no controls
or indicators. When properly installed in a vehicular radio, operation is automatic.
The power amplifier separates into three parts:
Case, Amplifier, RF 6A1 (power amplifier case).
Circuit Card Assembly, Decoder Control 6A2 (decoder control).
Electronic Component Assembly - Amplifier, Radio Frequency 6A3 (power amplifier heat sink).
The power amplifier case contains filters, electronic switches, and the three external connectors. The decoder
control provides the signal interface to the RT. The power amplifier heat sink contains the RF amplifier that
provides the RF gain.
5-2.
OVERALL FUNCTIONAL DESCRIPTION.
Figure 5-1 is a block diagram of the power amplifier. During high power transmit, the RF signal from the RT is input
at connector J2. It is routed through an RF switch to the amplifier. The amplifier increases the power level to
50 W. The amplified signal is routed through one of three filters to reduce harmonic and spurious signals. From
there, it goes through a second RF switch and out the ANT connector (J1).
During low power transmit, the RF signal is routed from J2 through the two RF switches to J1. During receive, the
same path is followed in reverse.
Control signals from the RT identify the frequency band and the RF path required. The decoder control activates
the high voltage switch drivers based on the control signals from the RT. The switch drivers select the switch
positions that determine the RF path.
Feedback signals are provided during high power transmit. Internally, an automatic level control (ALC) signal is
used to control the RF gain. If the VSWR is too high, the RF gain is reduced and the sidetone disable line to the RT
is activated. The decoder control also drives the RT SIG display.
5-1
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