Keithley SourceMeter 3A 2420, Service Manual

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4-4 Troubleshooting
CHAR SET test
The character set test lets you display all characters. Perform the following steps to run the
character set test:
1. Display the MAIN MENU by pressing the MENU key.
2. Select TEST, and press ENTER to display the SELF-TEST MENU.
3. Select DISPLAY-TESTS, and press ENTER to display the following menu:
FRONT PANEL TESTS
KEYS DISPLAY-PATTERNS CHAR-SET
4. Select CHAR-SET, and press ENTER to start the character set test. Press any key except
EXIT to cycle through all displayable characters.
5. When finished, abort the character set test by pressing EXIT. The instrument returns to
the FRONT PANEL TESTS MENU. Continue pressing EXIT to back out of the menu
structure.
Principles of operation
The following information is provided to support the troubleshooting tests and procedures
covered in this section of the manual. Refer to the following drawings:
Figure 4-1 — Overall block diagram
Figure 4-2 — Analog circuitry block diagram
Figure 4-3 — Power supply block diagram
Figure 4-4 — Output stage simplified schematic
Figure 4-5 — Digital circuitry block diagram
Overall block diagram
Figure 4-1 shows an overall block diagram of the Model 2420. Circuitry may be divided into
three general areas:
• Analog circuits — includes sourcing circuits such as the DACs, clamps, output stage,
and feedback circuits, as well as measurement circuits such as the A/D converter.
• Digital circuits — includes the microcomputer that controls the analog section, front
panel, and GPIB and RS-232 ports, as well as associated interfacing circuits.
• Power supplies — converts the AC line voltage into DC voltages that supply the power
for the digital and analog circuits, and the output stage.
Analog circuits
Figure 4-2 shows the analog circuitry block.