AR 10044005, Operating And Service Manual

Page: 55 / 72

Model 5000A225A
Rev B 41
3.2.6 A8 Voltage Regulator Board
The A8 Voltage Regulator Board contains two adjustable positive voltage circuits employing Linear
Regulator ICs U4 and U5. U4 accepts a +37 VDC input and outputs the +26.5 VDC Drain Voltage for the A1
preamp stage. U5 accepts a +37 VDC input and outputs the +24.0 VDC supply voltage for the B1 Fan
Assembly.
3.2.7 Splitter, Attenuator
The output of the A10 Driver module is then connected to a 8-way splitter. The eight outputs from the splitter
are of equal gain, each to feed one of eight inputs to the main power amplifier units.
3.2.8 Main Power Amplifier Unit (MPA)
One MPA unit provides the rated output power. Refer to Schematic 10037906 for low voltage (200-240
VAC) or 10039680 for high voltage (380-415 VAC). The MPA consists of four liquid-cooled power amplifier
modules (A1-A4), power supplies, and a cooling system. The cooling system, including a pump, heat
exchanger, and a cooling fan, circulates a mixture of water and propylene-glycol coolant through the amplifier
modules. Each of the four amplifier modules has a gain of approximately 21 dB, and each has its output fed
to the front panel connectors of the MPA. The total output of the four modules in combination is sufficient to
meet the specified output power of 2500 watts. Primary power to the MPA is from the AC power distribution
unit. It is switched by the breaker on the MPA rear panel, as well as the contactor internal to the MPA which
is controlled from the driver/control unit. Fault detection signals are gathered from the four modules by the
switch breakout board A5 and fed to the driver/control unit. Control signals from the driver/control unit are
fed to A5 and distributed to the four amplifier modules.
3.2.9 Liquid-Cooled Module
The Liquid-cooled module (schematic 10038146) consists of two pair of push-pull transistors, for a total of
four transistors (Q1-Q4). The transistors are mounted on a copper plate through which liquid coolant is
circulated. Transistor bias is set and controlled by the automatic bias board which plugs into the module. The
bias levels and fault detection levels (over-current, under-current, and current limit) for each transistor are
factory-programmed into the bias board, and set to correct levels each time DC power is applied to the
amplifier. Current through each RF transistor is monitored by hall-effect sensors U1-U4 (corresponding to
Q1-Q4). The output signal of each hall-effect sensor is fed to the bias board through J1; the bias board
applies the appropriate bias voltage to each transistor based on its current as read by the sensors. The bias
board also reports faults to the control panel through the transmit board as described in the previous section.
Each bias board (one per module) also has four LED’s corresponding to each transistor to aid in fault location.
Thermal switch SW1 senses over-temperature locally and 1) sends a signal to the control panel to shut down
the amplifier and 2) as a failsafe and backup to the control panel, disconnects the +5V supply from the bias
circuitry. Power combination from the four transistor stages is achieved through a series of balun
transformers in the output section.