AR 10043414, Operating And Service Manual, Page: 38 / 44

Model 25A250B
20 Rev A
4.3 TROUBLESHOOTING
CAUTION:
The transistors used in the Model 25A250B amplifier are GaN HEMT
transistor. 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 GaN HEMTs. The GaN HEMT packaging
should only be opened at Electrostatic Discharge (ESD)-approved
workstations, by individuals who are familiar with the handling of
microwave GaN HEMTs and other ESD-sensitive devices.
Troubleshooting the Model 25A250B 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 25A250B 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—Amplifier Fault

4.3.6—Low or No Power Output (DC Tests)

4.3.7—Low or No Power Output (RF Test)