Giga-tronics GT 9000, Operation & Maintenance Manual

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Corrective Action
1. Be sure that the correct leveling mode is activated (external ALC should not be enabled
unless an external detector is in use).
2. A failure of the internal detector is unlikely, but it is important to verify that the
detector signal is in fact being received by the level control board (A10). When there is
an apparent failure of the leveling system, disconnecting the detector cable and
observing the resulting changes is often a revealing experiment.
3. If the problem occurs above 500 MHz, set the output frequency to 5 GHz, and
disconnect the coax from J2 on the Detector Buffer board (A200). The front panel
output should be approxi18 dBm.
4. Measure the center conductor of the coax with a voltmeter; it should be at approximately
-300 mV.
5. If the problem occurs below 500 MHz, set the output frequency to 400 MHz, and
disconnect the coax from J3 on the Detector Buffer board (A200). The front panel
output should be approxi18 dBm.
6. Measure the center conductor of the coax with a voltmeter; it should be at approximately
-500 mV.
5.3.7 Level Control Circuit
The level control circuit board (A10) is basically a loop amplifier which must balance a variable
input (feedback from the level detector) against a reference signal. The detector can be either
the internal detector mounted outside the RF module, or a remote detector (in the external ALC
mode). The reference signal is programmed by the system computer, and incorporates level
characterization data unique to the instrument in which it is installed. The level control circuit,
using its output to drive the leveler in the RF module, adjusts output power in whichever
direction will equalize the reference and variable inputs to the loop amplifier. So long as the
output is between the extremes of its control range, the output is leveled and the indicator on
the front panel will remain lit.
Corrective Action
1. If the LEVEL indicator on the front panel is dark, the leveler output should be at one
extreme or the other of its control range, and comparing the control signal with the
result will indicate the nature of the problem (see Section 5.3.6). If the indicator is lit,
the control signal should be within its control range, and if the output power is
nevertheless incorrect, it is likely that the level control circuit is receiving a bad input, is
defective, or has been incorrectly calibrated.
2. If possible, replace the level control circuit with a spare. Make sure that the
potentiometers on the board have not been improperly adjusted (see the calibration
procedure in Chapter 4). Check all inputs and control signals to the board, the data lines
to the digital/analog converter, the AM input, the AM control line, and the detector
input. Be sure that the instrument has not been set to the wrong leveling mode (if an
external detector is not being used, external ALC should be off). An excellent way to
investigate a problem in the leveling system is to open the loop, by disconnecting the
detector or the leveler control line, and observe the resulting changes.
Maintenance
Manual No. 120AM00250, Rev C, September 1998
5-11