AR 10044831, Operating And Service Manual

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Model 175A400
24 Rev C
3.2.3 A6–A7 100W 400MHz Module Assembly (Schematic 10039874)
The 100W 400MHz Module Assembly consists of RF matching circuits, an RF transistor (Q1), a bias control
circuit, and a fault detection circuit.
The RF input is fed to a 4:1 balun transformer composed of T1 and T2. The output signal of the 4:1
transformer is connected to the gates of Q1. The drains of Q1 are connected to a 4:1 balun transformer
composed of T3 and T4. Q1 has approximately 40 VDC applied to both drains at a total current of 7.5A. The
RF stage has approximately 21 dB of gain and an output compression point of 75W or greater.
The current through Q1 is monitored by U1. The output of U1 is fed to an op amp (U3) which has a reference
voltage on the non-inverting input. U3 compares the output of U1 to the reference voltage and generates an
error signal. This error signal varies the gate voltages of Q1 which in turn controls Q1’s current draw.
U5 is a comparator that monitors the output of U1 and generates a fault signal if the drain current varies
outside normal operating conditions. It also monitors the module temperature via a thermistor (R31) and
generates a fault signal if the module heatsink exceeds approximately 80 o C
3.2.4 A5 Two-Way Combiner
A5 is a Two-Way combiner with impedance matching. The combiner operates in a zero-degree
configuration. The combiner is designed to tolerate loss of input signals at full output power.
3.3 POWER SUPPLIES (PS1, PS2)
Power supply PS1 supplies a +5VDC housekeeping supply for the control system assemblies A12
Control/Fault Board and A14 Interface Board.
PS1 also supplies -15 VDC at 2.5 amps. PS1 is a switching supply that automatically sets the AC input
circuits to the correct connections for the line voltage 100-240 VAC input ranges 47-440 Hz.
The -15 VDC at 2.5 amp power supply is fed to the A1 Pre-Amp, and A6 – A7 Power Modules.
The +24 VDC at 7.5 amps power supply is fed to the B3 and B4 fan assemblies.
Power Supply PS2 is a single output supply which pr40VDC to the A6 and A7 power modules and
the A9 regulator assembly. Primary AC circuit protection is provided by the circuit breaker in the Power
Entry Module.