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TROUBLESHOOTING
6
6-37
If the display lights up, perform the self-test by pressing the SRQ button for 3 seconds.
(Remember, the input terminals must be disconnected from the test leads during the self-
tests. Otherwise, the 8842A may indicate errors are present.) The test numbers will
appear consecutively. "ERROR" will appear on the display if a test should fail. The
8842A can be held in each of the test configurations by momentarily pressing the SRQ
button. (Press any button to continue the tests.) With the description of the self-tests
given below, it may be possible to isolate the failure. For reference, the states of various
switches and logic lines are shown in Table 6-16 for each function, range, and reading
rate.
If only one or a few failures appear in the self-tests, the problem is usually in the DC
Scaling circuit. By carefully analyzing which failure(s) occurred, the fault can be located
to within a few components. (Table 6-17 shows which components are exercised by each
of the analog tests.) The heading DC Scaling Troubleshooting provides detailed
instructions on locating and repairing DC Scaling circuit problems. However, before
troubleshooting the DC Scaling circuit, all of the power supply levels should be measured
to verify they are within the limits specified in Table 6-23 under Power Supply
Troubleshooting, later in this section.