Keithley 2500, Service Manual

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4-10 Troubleshooting Model 2500 Service Manual
Display board circuits
The display board includes a microcontroller that controls the Vacuum Fluorescent
Display (VFD) and interprets key data. The microcontroller has four peripheral I/O ports
that are used for the various control and read functions.
The VFD module can display up to 52 characters. Each character is organized as a 5
×
7
matrix of dots or pixels and includes a long under-bar segment to act as a cursor. The
display uses a common multiplexing scheme with each character refreshed in sequence.
Circuitry includes the grid drivers and dot drivers.
Troubleshooting
Troubleshooting information for the various circuits is summarized below. Refer to the
component layout drawings at the end of Section 6 for component locations.
Display board checks
If the front panel display tests indicate that there is a problem on the display board, use
Table 4-1 . See “Principles of operation” for display circuit theory.
Table 4-1
Display board checks
Step
Item/
component
Required condition Remarks
1
2
3
4
5
6 Front panel test
J1033
U902, pin 1
U902, pin 43
U902, pin 32
U902, pin 33 Verify that all segments operate.
+5V, ±5%
Goes low briefly on power up, and then goes high.
4MHz square wave.
Pulse train every 1ms.
Brief pulse train when front panel key is pressed. Use front panel display test.
D5V supply.
Microcontroller RESET.
Controller 4MHz clock.
Control from main processor.
Key down data sent to main
processor.