Fluke 8842a instruction manual Circuitry Tested by the Analog Self-Tests

8842A

Instruction Manual

Table 6-17. Circuitry Tested by the Analog Self-Tests

t6-17.wmf

Some failures will cause many self-tests to fail. If this occurs, the fault is usually in the Track/Hold circuit, the A/D Converter, the Digital Controller circuit, or the Power Supply. Again, measure all of the power supply levels according to the limits specified in Table 6-23. The next step is to isolate the problem to a specific section.

If the self-tests display a large number of errors or if readings are noisy and/or in error, the problem is usually in the A/D Converter or Track/Hold circuit. (A large number of errors can also be caused by a problem in the Ohms Current Source.) To isolate the problem, connect a jumper between TP103 and Reference Low (TP306, or the L-shaped shield around U202). The display should typically read less than approximately 35 counts (i.e., +/-.000XX where XX is less than 35) on the 2V dc range. If a good reading can be obtained (less than approximately 35 counts), the A/D Converter and Precision Voltage Reference circuits are most likely good. A more conclusive test can be made by connecting a low-impedance dc source between Reference Low and TP103 with an output voltage between -2.0V and +2.0V. The reading on the display will be of opposite polarity to the voltage applied to TP103. (Disconnecting one end of R318 will usually make it possible to display readings within 0.1% to 0.5% of the actual input.) After it has been determined that the A/D Converter or the Track/Hold circuit is not functioning properly, proceed to the corresponding heading for detailed troubleshooting instructions and guidelines.

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