AR 10046246, Operating And Service Manual

Page: 40 / 46

Model 125A250
22 Rev B
4.3 TROUBLESHOOTING
CAUTION:
The transistors used in the Model 125A250 amplifier are GaN HEMT and
LDMOS transistors. These devices are very reliable when installed in a
suitable circuit, but they can be easily damaged by improper
troubleshooting or handling techniques.
The gate junctions of the GaN HEMTs have a high input impedance and
are susceptible to static damage or damage due to the use of an
ungrounded soldering iron. Do not try to check the GaN HEMTs with an
ohmmeter.
Use caution when troubleshooting the GaN HEMTs; do not short the gate
to the ground or to the drain.
CAUTION:
Use care when unpacking new transistors. The transistors’ packaging
should only be opened at Electrostatic Discharge (ESD)-approved
workstations, by individuals who are familiar with the handling of
microwave GaN HEMTs, LDMOS and other ESD-sensitive devices.
Troubleshooting the Model 125A250 in a logical manner can speed the solution to a problem. The settings of
potentiometers (pots), capacitors (caps), or other variables should not be disturbed until other problems have
been eliminated. Comparing the measured DC voltages to those shown on the schematics can solve many
problems. Before measuring circuit voltages, first verify that the voltages to the circuits are correct.
Model 125A250 troubleshooting symptoms and remedies are described in the sections that follow

4.3.1—Power On Indication Doesn’t Display on Front Panel Vacuum Fluorescent Display (VFD) when
POWER Switch is Depressed

4.3.2—The Unit Cannot be Operated Remotely

4.3.3—Thermal Fault

4.3.4—Interlock Fault

4.3.5—PS2 (Power Supply) Fault

4.3.6—Amplifier Faults

4.3.7—Low or No Power Output (DC Tests)

4.3.8—Low or No Power Output (RF Test)