Specifications

4–13
Principles of Operation
4.11 RF Amplifier
The two RF power amplifier modules are mounted on a combiner board, heat sink,
slide rail assembly which slides into the main chassis at the rear, and is fastened to
the back panel with six screws. RF power, DC power, and control voltages enter the
PA assembly through a 72–pin edge connector that it slides into at the front of the
chassis.
The amplifier is built around two Phillips BLF278, dual-power MOSFETs rated for
50 volts DC and a maximum power of about 300 watts. When biased for class B, the
transistor has a power gain of 20 dB. (It is biased below class B in the transmitter.)
Input transformer, T1111, is made up of two printed circuit boards. The four-turn
primary board is separated from the one-turn secondary by a thin dielectric film.
R1112–R1117 are for damping. Trim pot R1111 sets the bias.
Output transformer, T1121, has a one-turn primary on top of the circuit board and a
two-turn secondary underneath. Inductors L1121 and L1122 provide power line
filtering.
The amplifiers are surrounded by a 50 Ω impedance, input/output combiner board
which takes the 15 watts input and divides it equally to each power amp. Then the
output from each amplifier is combined to form a single output.
4.12 Chassis
The AC power supply components, as well as the bridge rectifier and main filter
capacitor are mounted on the chassis. Changing the jumpers on the AC
distribution board (located beside the transformer assembly on the bottom of the
transmitter), configures the power transformer for 100, 120, 220, or 240 VAC; see
section 2.2 for switching and fuse information. The board also includes MOV
voltage-surge suppressors and in-rush current limiters as well as a 12 volt power
supply for the PA assembly cooling fans.
The main energy-storage/filter capacitors are located between the two power
regulator boards. The DC voltage across each capacitor will be 65 to 70 volts when
the carrier is on.
Shock hazard!
Do not attempt to short the capacitor
terminals. A bleeder resistor will
discharge the capacitor in
approximately one minute after
shutdown.
WARNING