Fluke 5720A service manual Troubleshooting the High Voltage Control Assembly A14

5700A/5720A Series II Calibrator

Service Manual

11.Check the Phase Lock circuit. Connect a frequency counter to the Calibrator output. Set the Calibrator to 2V at 1 kHz, operate, and check the frequency accuracy to be within 0.01% of the nominal. If the frequency is out of tolerance, proceed as follows:

a.Check the ZERO CROSSING DETECTORS by connecting an oscilloscope to U23 pins 7 and 12 (common = TP1). Ensure they convert the sine wave inputs to square wave outputs.

b.Check the PHASE DETECTOR outputs by connecting an oscilloscope to U22 pins 5 and 7. With the circuit not phased locked one of these points should be pulsing. If not, check U22 and associated components.

c.Check the LOOP FILTER & CHARGE PUMP circuit by connecting an oscilloscope to TP7. With the circuit not phase-locked, TP7 should be either +13V or -13V. If this voltage is incorrect, check U17, U31A, and associated components. If this voltage is correct, the problem is probably U19 and/or U15 in the MULTIPLIER circuit.

12.Check the PHASE SHIFTER circuit. Using a dual-channel oscilloscope connect one channel to the TP2 (oscillator output), and the other channel to TP8 (P SHIFT HI). Set the Calibrator to 2V at 1 kHz, operate. On the front panel select the "Phase Ctrls Menu" softkey, followed by the "Phase Shift" softkey to turn the phase shift function on. The oscilloscope should show two sine waves that are in phase. By pressing the "Adjust Phase Shift" softkey and then either the softkeys or rotating the edit knob the phase between these two signal should shift over the entire 360 degree range. If a failure is detected, first check the control lines for this circuit. On U27 check pin 6 (INHPH) for a logic low, pin 9 (BPH) and pin 10 (APH) should toggle between a logic high and low depending on the degrees of phase shift selected. If these control lines are incorrect, check U33. If the control lines are correct check U27, U28, and the associated components in the PHASE SHIFTER circuit.

5-19. Troubleshooting the High Voltage Control Assembly (A14)

1.Remove the rear shield from the High Voltage Control assembly and place it on the extender card. All measurements will be referenced to PACOM (TP6) unless Noted otherwise. Power up the Calibrator and proceed as follows.

Warning

High voltages are exposed when troubleshooting the high voltage control assembly.

2.Measure the voltage at TP4 with a DMM and verify it is between +0.1 and +0.3V. If a failure is detected, check U2C, U6, and associated components. Next, power down the Calibrator and connect an oscilloscope, set for 20V/div and 500 us/div, to TP3.

At power up of the Calibrator the oscilloscope should displays a 1 kHz square wave which ramps up to the level shown in Figure 5-31, remains there for approximately 1 second, and goes to a 0V. If a failure is detected, skip to the Magnitude Control circuit troubleshooting in this section.

Note

The following step 3 checks the operation of the High Voltage Control assembly in the 1100V AC Range. If may be useful to refer to Figure 2-25 in the Theory of Operation Section.

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