Amplifier Research 75A250A Handbuch - Seite 15

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Model 75A250A

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.

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 Indic ate "Power On"

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

vacuum

when the POWER Switch is Depressed

If the Model 75A250A is operating in an otherwise normal fashion, the unit's front panel

fluorescent display (VFD) or the wiring to it could be defective.

CAUTION:

Manual Text

4-2

REV A