Fluke 5720A service manual Troubleshooting the DAC Assembly A11

5700A/5720A Series II Calibrator

Service Manual

Note

The connection from the test points to the resistance values is through latching type relays for all resistances except 10M, 19M, and 100 MΩ values. These relays are put in the set or reset position and remain in that position even after power has been removed. To troubleshoot the path through the relays, simply set the Calibrator to the function in which a problem is detected and then turn off the Calibrator. The relays remain in either the set or reset position. Check individual relays by connecting an external power supply (a typical 9V battery works well) across the set coil to place the relay in the set position or the reset coil to place the relay in the reset position.

3.Repeat step 2 for the resistances in Table 5-2 and verify they are within the specified tolerance.

Table 5-2. Ohms Main Resistance Tolerance

Resistance

19Ω

100Ω

190Ω

1 kΩ

1.9 kΩ

Tolerance

+/-0.07% +/-0.03% +/-0.03% +/-0.03% +/-0.03%

4.Check the 100 MΩ resistance. Set up the ohmmeter for a two wire resistance

measurement and connect IN HI to TP4 and IN LO to TP3. Set the Calibrator to 100 MΩ, operate, INT SNS, and verify the ohmmeter reads 100 MΩ ±5.0%. If a failure is detected, check relay K5 and90.

5-13. Troubleshooting the DAC Assembly (A11)

Proceed as follows to troubleshoot the DAC assembly (A11):

1.Remove the front and back shields and place the DAC assembly on the extender card. Power up the Calibrator.

2.Check the 13V reference. Connect DMM to TP2 (common to TP3). The voltage should be between +13 and +14V and very stable. (Some drift may be seen while the assembly is warming up.) If the 13V reference is faulty, check the power supply voltages to the HR5 Reference Hybrid. If all the supplies are correct, the HR5 Reference Hybrid is probably at fault.

3.Check CH1 SHUNT. Connect an oscilloscope probe to TP6 (with common connected to TP3). Set the oscilloscope to 2V/div at 2 ms/div and verify it displays pulses as shown in Figure 5-17. If the displayed signal is incorrect, skip to Duty Cycle Control Circuit.

4.Check CH1 SERIES B. Connect an oscilloscope to TP4 (common to TP3).Set the oscilloscope to 10V/div at 2 ms/div and verify it displays pulses as shown in Figure 5-18. Next, set the Calibrator to 6.5V dc, operate. Verify the oscilloscope displays pulses with approximately 50% duty cycle as shown in Figure 5-19. If either of these displayed signals are incorrect, skip to Duty Cycle Control Circuit.

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