Specifications

Model 75A250A Manual Text
4-2
REV A
4.3 TROUBLESHOOTING
The techniques used in troubleshooting solid state instruments are similar to those used in vacuum tube
instruments. For instance, a good way to start troubleshooting is to check the supply voltage at the amplifier supply
voltage terminal. If it is low or nonexistent, check the power supply components starting with the AC fuse.
The power supply for the driver should be a nominal 28 volts, and the power supply for the output stage should be
a nominal 28 volts. Incorrect voltage can result in over-dissipation in the transistors or severe distortion and non-
linearity of the amplifier. The power supplies may be disconnected from the RF board to enable troubleshooting
without danger of damaging the RF circuitry.
Finally, determine if the individual amplifier stages are operational by injecting a signal into the transistor gate and
looking for an indication of output.
CAUTION:
BEWARE OF VOLTAGES APPLIED TO THE GATE OF A MOSFET TRANSISTOR IN EXCESS
OF ±±20 VOLTS, THIS WILL RESULT IN TRANSISTOR GAT FAILURE.
4.3 Troubleshooting
Troubleshooting the Model 75A250A 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 75A250A Troubleshooting Categories:
Subsection 4.3.1—Front Panel Vacuum Fluorescent Display (VFD) Doesn’t Indicate “Power On”
when the POWER Switch is Depressed
Subsection 4.3.2—The Unit Cannot be Operated Remotely
Subsection 4.3.3—Thermal Fault
Subsection 4.3.4—Interlock Fault
Subsection 4.3.5—Voltage Faults
Subsection 4.3.6—Low or No Power Output (DC Tests)
4.3.1 Front Panel Vacuum Fluorescent Display (VFD) Doesn’t Indicate “Power On” when the
POWER Switch is Depressed (Schematic Diagram No. 1011806)
4.3.1.1 If the Model 75A250A is operating in an otherwise normal fashion, the unit’s front panel
vacuum fluorescent display (VFD) or the wiring to it could be defective.