Fluke 5720A service manual Troubleshooting the Current/Hi-Res Assembly A7, Current Section

Troubleshooting 5

Component-level Troubleshooting

5-6. Troubleshooting the Current/Hi-Res Assembly (A7)

The Current/Hi-Res assembly generates output current and contains the high-resolution oscillator onto which the Oscillator Control assembly phase locks. Since these two functions are independent except for their digital control, troubleshooting procedures for each portion are presented separately.

5-7. Current Section

Proceed as follows to troubleshoot the current section of the Current/Hi-Res assembly (A7):

1.In order to troubleshoot the Current generating circuitry it is necessary to break the loop and use an external ac reference as the input source. Turn off the Calibrator and cut jumper E1. Connect an external variable ac reference to TP19 (common to TP20).

2.Place the Current/Hi-Res assembly on the analog reverse extender card and jumper the Calibrator OUTPUT HI binding post to the OUTPUT LO binding post. Turn on the Calibrator.

Note

All measurements are referenced to SCOM (TP20) unless otherwise specified.

3.Check the Complimentary Drive Circuit. Set the Calibrator to 200 μA, standby. Set the external ac reference to 7.0V at 1 kHz. Connect an oscilloscope to pin 11 of K2 and verify that it displays a signal similar to Figure 5-6. Connect the oscilloscope to pin 6 of K2 and verify that it displays a signal similar to Figure 5-7. If a failure is detected, check Q2, Q3, Q18, Q19, K2, and their associated components in the Complimentary Drive Circuit.

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