AR 10009284, Operating And Service Manual

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Model 40T26G40A
6 Rev A
Amplifier gain is set by the voltage-controlled variable gain attenuator. The control head determines the
output of a digital-to-analog converter (DAC) on the HPA interface board. The output of the DAC controls
the attenuator’s loss.
The emergency bypass board (A24830-001) is mounted behind the front panel. It is provided with a circuit
that increases the attenuation so that reflected power is limited to a level on the order of 10 watts. In
emergency bypass operation (see section 3.7), the gain control signal is provided locally by means of a
potentiometer on the emergency bypass board. The foldback circuits remain on line during emergency bypass
operation.
2.3 DESCRIPTION OF THE POWER SUPPLY (A22826-352)
The TWT’s power supply is of modular construction. Low-voltage power for logic and control of the entire
power supply assembly is provided by the low-voltage power supply module (A23687-001). In addition, this
module provides Direct Current (DC) power for the HPA interface and emergency bypass boards. Control
logic and TWT protection circuits are contained in the HPA Logic and Control Assembly (A16485-488).
The Heater Power Supply Module (A25963-000) powers the TWT DC heater. In addition, the millimeter
wave TWT has a grid modulator (A23684-352).
The high-voltage power supply consists of the following: the Power Factor Correction Module (A23683-100)
converts line voltage to DC for the high-voltage switching supply. Switching transistors are on the Power
Board Assembly (A23710-000), and switching is controlled by the Pulse Width Modulation (PWM) Board
(A10017-488). The high-voltage transformer and rectifiers are contained in the High-Voltage
Diode/Capacitor Assembly (A23707-488). The high-voltage DC is filtered in the HV Filter Assembly
(A21461-488).
Low-voltage interconnects between the power supply modules are through a motherboard. It is installed in a
finned heat sink assembly to which the modules are bolted. The heat sink is cooled by the incoming cabinet
air. The Heat Sink/Motherboard Assembly is A23709-000.