Fluke 5720A service manual 15. Waveform at TP16, F5-15.EPS

Troubleshooting 5

Component-level Troubleshooting

These relays are latched in the set or reset position and remain in that position even after power has been removed. To troubleshoot the signal path through the Switch Matrix assembly simply set the Calibrator to the function in which a problem is detected and then turn off the Calibrator. Now remove the Switch Matrix from the instrument and, using an ohmmeter, check the signal path by using the relay chart in the schematic section. 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.

1 V

10 ms

F5-15.EPS

Figure 5-15. Waveform at TP16

2.Check the Motherboard relay RLY+V supply voltage. Set the Calibrator to 10V dc, operate. Connect an oscilloscope to RLY+V (common to TP9),which is at the cathode of CR2, and verify it is 4.5V ±0.5V. Now verify that each time the EX SNS key is pressed the voltage at RLY+V momentarily jumps to 7.5V ±1.2V. If a failure is detected, check the control line PC5, U17, Q1, Q2, and associated components.

3.Check the Motherboard relay control lines. Motherboard relays K1-K10 are controlled by U13 on the Switch Matrix assembly. If aMotherboard relay problem is suspected, verify U13 by using the relay chart in the Switch Matrix schematic section and checking for correct logic levels at its output.

4.Check the 2.2V Range Attenuator. Set the Calibrator to 2V dc, operate. Using a DMM measure the dc voltage at TP2 (common to TP8) and verify it is 2V ±6 mV.

Note

This 2V can be as much as ±0.12V if the DAC assembly is uncalibrated. If a failure is detected, check K1, K2, and the 5:1 divider part of the HR1 resistor network.

5.Check the 2.2V Range Amplifier. Set the Calibrator to 2V dc, operate. Using a DMM measure the dc voltage at TP4 (common to TP8) and verify it is 2V ±6 mV.

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