Fluke 5720A service manual Component-level Troubleshooting

Troubleshooting 5

Component-level Troubleshooting

voltage at TP2 and verify it is 0.317 times the voltage at TP1 ±10%. If a failure is detected, check U5 and associated components.

Note

During Wideband operation, internal software monitors the output and makes corrections or trips the instrument to standby. This internal monitoring can cause problems when attempting to troubleshoot the Wideband Output assembly. If during the following step the Calibrator trips to standby, defeat the monitoring by connecting a jumper from TP9 to TP10 on the DAC assembly.

5.Check the RMS Sensor and Amplitude Control circuit during Wideband operation from 12 kHz to 1.1 MHz. Set the Calibrator to 3.5V at 12 kHz operate. Using a DMM measure and Note the ac voltage at TP7 and verify it is 7V ±10%. Measure the dc voltage at TP1 and verify it is the same as measured at TP7 ±10%. If a failure is detected, check U1, U2, U3, and associated components. Next measure the dc voltage at TP2 and verify it is 0.317 times the voltage at TP1 ±10%. If a failure is detected, check U5 and associated components. Verify that FETs Q2 and Q3 are turned off by measuring the dc voltage at TP5. (It should be -11.4V ±20%.) If the voltage at TP5 is incorrect skip to step 8.

6.Check the WB Amplitude Control line. Remove jumper E3 from the J3 pins and connect an external variable dc reference voltage to TP18 (low to TP19). Set the Calibrator to 3.5V at 12 kHz operate and set the external dc reference voltage to 2V. Using a DMM measure TP3 and verify it is a negative voltage. Next set the external dc reference voltage to 3V and verify TP3 changes to a positive voltage. If a failure is detected, check U12B and associated components.

Note

Remove the external dc reference and replace jumper E3 before continuing.

7.Check the X10 Amplifier circuit. Set the Calibrator to 3.5V at 1.2 Mhz, operate. Connect an Oscilloscope (set for ac coupling) to the input of the X10 Amplifier which is the junction with R46, R47, and R49. Verify the Oscilloscope displays a distortion-free 1.2 MHz sinewave. If a failure is detected, check the cable at J1 and the Wideband Output assembly. Note the amplitude of the input sine wave and move the Oscilloscope probe to TP13 and verify is displays a distortion-free 1.2 MHz sine wave which is ten times larger in amplitude. If a failure is detected check Q4-Q10 and associated components in the X10 Amplifier circuit.

8.Check the Overload Control circuit. Remove jumper E3 from the J3 pins and connect an external variable dc reference voltage to TP18 (low to TP19). Set the Calibrator to 3.5V at 1.2 MHz operate and set the external dc reference voltage to 2V. Connect an Oscilloscope to TP5 and verify it is a negative voltage. Connect a DMM to TP1 and slowly increase the external dc reference in 10 mV steps until the voltage at TP5 goes positive. Verify the voltage at TP1 is 7.7V ±5%. If a failure is detected, check U4A, U4B, VR2, and associated components.

Note

Remove the external dc reference and replace jumper E3 before continuing.

9.Check the Attenuators. Connect a DMM to the 50Ω termination at the Wideband Output type "N" connector on the Calibrator front panel. Set the Calibrator for output

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