Keithley 2750, Service Manual

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Model 2750 Multimeter/Switch System Service Manual Troubleshooting 4-3
Tr
oubleshooting
Power-on self-test
During the power-on sequence, the Model 2750 will perform a checksum test on its
EPROM (U156 and U157) and test its RAM (U151, U152, U164 and U193). If one of
these tests fails, the instrument will lock up.
Front panel tests
There are two front panel tests: one to test the functionality of the front panel keys and one
to test the display. In the event of a test failure, refer to “
Display board checks ” for details
on troubleshooting the display board.
KEY test
The KEY test allows you to check the functionality of each front panel key. Perform the
following steps to run the KEY test:
1. Press SHIFT and then TEST to access the self-test options.
2. Use the up or down RANGE key to display “TEST: KEY.”
3. Press ENTER to start the test. When a key is pressed, the label name for that key is
displayed to indicate that it is functioning properly. When the key is released, the
message “NO KEY PRESS” is displayed.
4. Pressing EXIT tests the EXIT key. However, the second consecutive press of EXIT
aborts the test and returns the instrument to normal operation.
DISP test
The display test allows you to verify that each segment and annunciator in the vacuum flu-
orescent display is working properly. Perform the following steps to run the display test:
1. Press SHIFT and then TEST to access the self-test options.
2. Use the up or down RANGE key to display “TEST: DISP.”
3. Press ENTER to start the test. There are five parts to the display test. Each time
ENTER is pressed, the next part of the test sequence is selected. The five parts of
the test sequence are as follows:
a. All annunciators are displayed.
b. The segments of each digit are sequentially displayed.
c. The 12 digits (and annunciators) are sequentially displayed.
d. The annunciators located at either end of the display are sequentially
displayed.
e. The LEDs are displayed.
4. When finished, abort the display test by pressing EXIT. The instrument returns to
normal operation.