Fluke 5720A service manual Magnitude Control Circuit

5700A/5720A Series II Calibrator

Service Manual

CR4, and associated components. If the measure voltage is much greater than 227V skip to the Magnitude Control Circuit troubleshooting starting at step 10.

7.Check HV OUT from the High Voltage Control assembly. Set the Calibrator to +220V dc, operate. Using a DMM measure the dc voltage at pin 1A or 1C of P611 and verify it reads +220V ±0.2V. If a failure is detected, check K4, K11, K5, K12, K3, and associated components.

Note

The following steps 8 and 9 check the operation of the High Voltage Control assembly in the 2.2A Current Range. For these checks it will be necessary to install a jumper connecting the front panel OUTPUT HI binding post to the OUTPUT LO binding post. If may be useful to refer to Figure 2-27 in the Theory of Operation Section.

8.Check the output from the Magnitude control circuit. Set the Calibrator to 220 mA dc at 1 kHz, operate. Connect an oscilloscope to TP3 and verify it displays a signal similar the previously shown Figure 5-31, except the amplitude is typically 40V p-p. The measured amplitude may not be 40V p-p, for now just verify that the 1 kHz signal is present. If a failure is detected, skip to the Magnitude Control circuit troubleshooting in this section.

9.Check the High Voltage Control assembly in the 2.2A Range. Set the Calibrator to 220 mA at 1 kHz, operate. Connect the DMM low to VICOM (pins 16A/C of P612) and the DMM high to VI+ (pins 15A/C of P612)and verify it is +5.3V ±10%. Next, move the DMM high to VI- (pins 14A/C of P612) and verify it is -5.3 ±10%. If a failure is detected, check T1, K14, K2, CR5, and associated components in the 2A Range Power Supply Filter circuit.

Note

Remove the front panel jumper connecting OUTPUT HI to OUTPUT LO before continuing.

5-20. Magnitude Control Circuit

Proceed as follows to troubleshoot the magnitude control circuit:

1.Check the Square Wave Generator. Power down the Calibrator and connect the oscilloscope, set for 2V/div and 500 us/div, to TP2. When the Calibrator is turned on, the oscilloscope should display a TTL-level 1 kHz square wave for approximately 1 second. If a failure is detected, check U4, U7A, and associated components in the Square Wave Generator.

2.Check the Reference and Error Amplifier. Power down the Calibrator and connect an oscilloscope, set for 5V/div, to the cathode of CR10. At power up of the Calibrator the oscilloscope should display a dc voltage which ramps up to approximately +14V and remains there for approximately 1 second before returning to 0V. If a failure is detected, check U2D, U7B, Q2, Q4, VR1, and associated components in the Reference and Error Amplifier circuit.

3.Check the Square Wave Amplifier. Power down the Calibrator and connect an oscilloscope, set for 5V/div, to pin 1 of U1. At power up of the Calibrator the oscilloscope should display a dc voltage which ramps down to approximately -14V and remains there for approximately 1 second before returning to 0V. If a failure is detected, check U1A and associated components. If this voltage is correct and at

5-76