Tektronix 2440, User Manual

Page: 298 / 531

Maintenance— 2440 Service
8100
POSITION
OFFSET
8200
POSITION
GAIN
Table 6-6 (cont)
Acquisition System Position Offset:
Testing Method:
Position Offset is calculated at 50 mV per division only. Position offset must be performed for all four
acquisition modes to compensate for the common-mode offsets in the CCD arrays that are not
corrected by CCD centering. After the hardware is set up and the CCD constants set for the particular
mode of operation, the Preamplifier is balanced by executing the balance routine, but only changing the
DAC settings— not the cal constants. This assures an accurate position measurement.
The Position Offset is then calculated by acquiring a ground level signal and comparing the Acquisition
Memory value to what a ground acquisition should be (center screen is OOh). If the value is not within
1/2 DL for calibration or 1 DL for diagnostics, the position DAC outputs (CH1-PA-POS and/or CH2-
PA-POS) are adjusted to compensate. When the acquisition is within limits, the test result is set to
PASS. If the position offset cannot be adjusted to within specification, the acquisition count for an
abort is taken, and the test result is set to FAIL.
Troubleshooting Procedure (refer to test 8000 for more information):
1. Check that the decoupling network, R 420/C423 or R 222/C222 is functional.
2. Use the FORCE DAC test to determine if the CH1-PA-POS and/or CH2-PA-POS voltages are
being controlled by the DAC System. If not, troubleshoot the DAC System.
3. Use the SPECIAL menu choice of CAL PATH ON to determine whether the offset error is prior to
the Peak Detectors or after. Troubleshoot in the appropriate direction to locate the source of the
offset error.
4. Check that the bypassing components in series with the power supplies are not open. Also check
that the dc bias voltages at the Preamp are approximately the levels indicated on the schematic
diagram in the service manual.
5. If the Preamp hybrid itself is suspected of being defective and the other channel is fully functional,
swap Preamp hybrids between channels to see if the problem moves to the other channel. If so,
replace the defective Preamp. If not, check the hybrid mount connections of the defective channel
for corrosion or contamination that may be causing a poor contact.
6. Swap Peak Detector hybrids between channels to see if the problem channel reverses. If so,
replace the faulty Peak Detector.
Preamplifier Position Gain A10U420 and A10U320 (schematic diagram 9):
Testing Method:
Position Gain is calculated at 50 mV per division only, using the stored Position Offset calibration con­
stant. The DAC counts corresponding to + 4 divisions of Position Offset are added to the Position
Offset constant and an acquisition is made. After storing the results, a corresponding —4 division
acquisition is made, and the two values of acquisition memory are checked for eight divisions of
change in the calibration limits. The Position Gain constant is then calculated as a result of the data
taken and stored as a Position Gain calibration constant. A Position Gain constant more than 20%
different from the nominally expected value will cause the test to fail.
NOTE
POSITION GAIN is not an iterative calibration as the gain is directly calculated.
6-90