6-83
NOTE
After a COLD START, the 2430 will need partial recalibration after it
is
repaired to return it
to correct adjustment.
1. If SELF CAL fails, the calibration constants will most likely not be close enough to an operationally
good value for the portions of the Preamp that work to function properly. Do a COLD START to
replace the stored calibration constants with nominal values.
Troubleshooting Procedure:
The Preamplifier has constants for Position Offset, Position Gain, Balance, Normal and Invert Gain,
and Max Variable Gain. There is some interaction between adjustments. This effect is addressed by
always using the previously stored constants in any setup and executing SELF CAL twice from a
COLD START to assure an iterative solution of the calibration constants.
IF A SELF DIAG OR EXTENDED DIAG TEST FAILS, ONE CANNOT ASSUME THE HARDWARE IS
DEFECTIVE UNLESS THE SAME TEST FAILS A SELF CAL. The reason that SELF CAL must be run
to assure a hardware failure is that SELF CAL computes new values of the constants for each test
and uses them in the subsequent tests; whereas, diagnostic tests use previously stored constants for
making the tests. If those stored values are not valid for the present operating temperature of the
scope, the test may not be able to converge to a solution.
The PA tests, if passed, indicate that the analog acquisition circuitry is functional.
Testing Method:
Preamplifiers A1OU320(CH 2) and A1OU420(CH 1) (schematic diagram 9):8000
PA
NOTE
If any CCD or Peak Detector is changed, do not run a SELF CAL until the CCD OUTPUT
Gain has been set using the EXT CAL ADJUSTS, test pattern number
6.
Adjust the
±
2
division gain for the changed channel, Side
1
and Side
2,
according to the directions
given in the display.
2. If the DAC system is functioning normally, the most probable cause of a failure is a faulty Peak
Detector. Replace the Peak Detector of the failing channel.
1. Do a COLD START and use the FORCE DAC test to determine if the DAC system can control the
PO-OFFSET voltages (PD11, PD13, PD21, and PD23) correctly. If not, troubleshoot the DAC
system.
Troubleshooting Procedure:
The SPECIAL menu choices under Extended Functions in Version 2.0 firmware provide a diagnostic
switch to divide the signal acquisition path. CAL PATH ON:OFF turns on or off the calibration signal
path to the Peak Detectors. It is a useful diagnostics device in the event that large offset errors have
driven the display off-screen. Switching CAL PATH ON eliminates the Attenuators and Preamplifiers
from the input signal path and places the calibration reference level on the display. If that brings the
display back on screen, then the offset problem may be isolated to the Attenuators or Preamplifiers; if
not, then the problem may be in the Peak Detectors or CCDs. With CAL PATH ON, the FORCE DAC
test may be used to check the operation of the Peak Detectors and CCDs using the CURS (CAL)
adjustment.
Table 6-6 (cont)
Maintenance-2430 Service
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