Fluke 5720A service manual Troubleshooting the Oscillator Control Assembly A12

Troubleshooting 5

Component-level Troubleshooting

5-17. Troubleshooting the Oscillator Control Assembly (A12)

The first part of this troubleshooting procedure (steps 1 through 7) checks the averaging converter. The second part (steps 8 through 10) checks the AC/DC transfer circuit.

Note

During normal operation of the Calibrator, 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 Oscillator Control assembly. Defeat the monitoring by connecting a jumper from TP9 to TP10 on the DAC assembly and another jumper connecting SDL (P501 pin 11A/11C) to SCOM (P502pin 32A/32C) on the Oscillator Control assembly.

After defeating the monitoring circuit, remove the rear shield and place the Oscillator Control assembly on the extender card. Power up the Calibrator and proceed as follows to troubleshoot the Oscillator Control assembly (A12):

1.Check inputs to the Oscillator Control assembly. Set the Calibrator to 1.0V at 1 kHz, operate. On this Oscillator Control assembly measure the DAC assembly output voltage at TP3 (common to TP2) for 3.16V dc ±50 mV. Also measure OSC SENSE HI at TP1 for a nominal 1.0V ac ±50 mV. If these voltages are incorrect, check the input switching relay K1 and its drive circuitry.

2.Check the CURRENT CANCELLATION CIRCUITS. Set the Calibrator to 1.0V at 1 kHz, operate. Measure the ac voltage at TP1 (common to TP2) and Note the reading. Move the DMM test leads to the output of the Calibrator. The measured ac voltage should be the same as Noted ±10 ppm. If a failure is detected, check the CURRENT CANCELLATION CIRCUITS as outlined on the schematic.

3.Check the BUFFER AMP circuit. Set the Calibrator to 1.0V at 1 kHz, operate, and measure the BUFFER AMP output at TP4 (common to TP2). It should be the same voltage as on TP1 ±1%, with <<50 mV of offset. Change the Calibrator output to 10.0V and again measure the BUFFER AMP output. The voltage should remain the same ±1%. If these voltages are incorrect, check the input attenuator (Z2 and K3)and the BUFFER AMP (U3 and associated components).

4.Check the RECTIFYING AMP circuit. Connect an oscilloscope to the anode of CR4 (common to TP2). Set the oscilloscope to 2V/div. and 200 us/div. The oscilloscope should display a signal as in Figure 5-27. If the signal is incorrect, check U5, U7, Q3, Q4, CR4, CR5, and associated components in the RECTIFYING AMP circuit.

5.Check the 15 BIT DAC circuit. Set the Calibrator to 1.0V at 1 kHz, operate. Connect a DMM to U26 pin 6 (common to TP2) which is 14 BITDAC OUT. It should be stable dc voltage between 0 and -3.16V. If this voltage is incorrect, check U10, U26, Q19, and associated components in the 15 BIT DAC circuit.

6.Check the ERROR INTEGRATOR output. Set the Calibrator to 1.0V at 1 kHz, operate. Connect an oscilloscope to U11 pin 6 (common to TP2) which is ERROR INT. OUT. Set the oscilloscope to 500 mV/div and 200us/div. The oscilloscope should display a ripple similar to Figure 5-28. The dc level of this signal can be between -5V and +5V. If an error is detected, check U11 and associated components.

7.Check the OSC CONT signal. Set the Calibrator to 1.0V at 1 kHz, operate. Connect the oscilloscope to TP5 (common to TP2). OSC CONT should be a dc voltage between +5 and -5V, free of distortion. If this voltage is incorrect, check U8, U9B, and associated components in the 3 POLE FILTER circuit and buffer amp U9A.

5-69