LX Series Digital
Chapter 4, Circuit Descriptions
Power Amplifier Assembly
Volume 2, Rev. 1
4-1
Chapter 4
Circuit Descriptions
Power Amplifier Chassis Assembly
The RF from the exciter/driver assembly
connects from the RF Output “N” Jack J25,
through a RG-55 cable, to the PA RF Input
“N” Jack J200 or J201, located on the rear
of the PA chassis assembly. In systems
with multiple Power Amplifier Assemblies,
the RF output from the exciter/driver is
split and then fed to each of the PA RF
Input Jacks. The RF Input is wired
through UT-141 cable to the OSP port
J111, J121, J131 or J141 on the PA
chassis assembly, one for each 250W PA
module slide in assembly. Jack J1 on the
250W PA module assembly connects to
the OSP port when the module assembly
is slid into the PA slot. In the 250W PA
module, the RF is amplified and connected
to the PA RF Output Jack or J2, located on
the rear of the PA Module assembly. Jack
J2 on the PA module connects to the J112,
J122, J132 or J142 port, on the main
chassis assembly, when the module
assembly is slid into place. In a 125W
Digital system the output at J12 connects
directly to J203 the 7/16” Jack that is
located on the rear of the PA chassis
assembly.
The RF outputs of the PA module
assemblies are combined in a 2 way
combiner in a 250 Watt system or in a 4
way combiner in a 500W system. The RF
output jack of the PA chassis assembly is
the output of the combiner at the 7/16”
Jack J205 that is located on the rear of
the PA chassis assembly.
There is one On/Off Circuit Breaker, two
maximum, which is mounted on the rear
of the power amplifier chassis assembly.
The breaker supplies AC to each power
supply assembly. Each Power Supply
Assembly supplies voltages to two power
amplifier assemblies.
4.1 (A4) Power Amplifier Module
Assembly
The Power Amplifier Module Assembly
contains (A1) a UHF Phase/Gain Board
(1303213), (A2) a 150W Driver Pallet,
Dual Output (1303293), (A3 & A4) UHF RF
Module Pallet Assemblies (1300116), (A5)
a 2 Way UHF Combiner Assembly
(1303208), (A6) an Amplifier Control
Board (1303682) and (A7) a Temperature
Sensor IC.
4.1.1 (A1) UHF Phase/Gain Board
(1303213; Appendix B)
The RF input from J1 on the PA assembly
connects to J1 on the Phase/Gain Board.
The UHF phase/gain board provides the
circuits that adjust the gain and the phase
of the RF signal for the PA amplifier
assembly in which it is mounted. The input
signal connects to the gain circuit through
the capacitor C13. The gain circuit consists
of U1, R16, CR4, R22, R17, CR5, R23, R27
and the gain pot, R25. U1 is a 90°, 2-way
splitter. The signal at pin 1 of U1 is split
and applied to pins 3 and 4. The signal
reflects off CR4 and CR5 and is passed to
pin 2. The gain between pins 1 and 2
changes with the voltage applied across
CR4 and CR5. This voltage is controlled by
the gain-adjust pot R25. The more
positive the voltage, the more the diodes
CR4 and CR5 conduct therefore the less
gain through the circuit. The gain
controlled output is coupled through C14
and the pi-type divider circuit consisting of
R8, R5 and R9 that drops the level before it
applied to the phase-shifter circuit.
The level controlled signal connects to the
phase-shifter circuit that consists of U2,
C20, C21, CR2, and CR3. U2 is a 90°, 2-
way splitter. The signal at pin 1 of U2 is
split and applied to pins 3 and 4. The signal
reflects off CR2 and CR3 and is passed to
pin 2. The phase shift between pins 1 and
2 changes with the voltage applied across
