Fluke 5720A service manual Troubleshooting the Ohms Main Assembly A10

Troubleshooting 5

Component-level Troubleshooting

less than 4 μV. If a failure is detected check U7, U8, and associated components. Using a DMM measure the dc voltage across the 500Ω resistor in Z2 by connecting the DMM HI to TP18 and the DMM LO to TP17. Note this voltage which should be around 5.0V. Next, measure the voltage across the 1 kΩ resistor in Z2 by connecting the DMM HI to TP16 and the DMM LO to TP6. The voltage should be double and opposite polarity (gain = -2)of that previously measured (about -10V). If a failure is detected check U9, U10, and associated components.

1 V

10 ms

F5-16.EPS

Figure 5-16. Waveform at Pin 13 of U6

5-12. Troubleshooting the Ohms Main Assembly (A10)

Proceed as follows to troubleshoot the Ohms Main assembly (A10):

1.MΩ resistor in R1. All of the fixed values of resistance, available from the Calibrator, reside on the Ohms Main assembly except the 1.9Ω, 1.0Ω, and short. The fixed values of resistance from 10Ω to 100 MΩ are connected to the Calibrator output binding posts through relays on the Ohms Main, Ohms Cal, and Motherboard assemblies. The Ohms Main assembly is checked by verifying each fixed value of resistance and the relay path to INT OUT HI, INT SENSE HI, OHMS OUT LO, and OHMS SENSE LO. Place the Ohms Main assembly on the extender card, power up the Calibrator, and continue as follows.

2.Connect a four-terminal ohmmeter (such as the Fluke Model 8505A) to the Ohms

Main assembly as follows. Connect 8505A IN HI to TP4, 8505A IN LO to TP3, 8505A SENSE HI to TP5, and 8505A SENSE LO to TP2. Set the Calibrator to 10Ω, operate, EXT SNS and verify the ohmmeter reads 10Ω ±0.07%. If a failure is detected, turn off the Calibrator and remove the Ohms Main assembly from the

extender card. Use the schematic and the relay chart to check the connection from the test points to the 10Ω resistance.

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