Fluke 5720A service manual AC elsewhere in the Calibrator

Troubleshooting 5

Component-level Troubleshooting

7.Check the output of the Output Stage. Set the Calibrator to 0V dc, standby. Connect an external 100Ω 1/4W carbon-film resistor between PACOM (TP10) and 1ST MID STAGE OUT, which is the left side of 1W resistor R41. Measure the dc voltage at the Mid-Stage output as is step 4. If the voltage is +0.8V, then the voltage at TP6 (PA OUT HI)should be +3.2±0.3V. If the voltage measured at the Mid-Stage output is -1.1V, then the voltage at TP6 should be +1.3±0.3V. If a failure is detected, check the components in the Output Stage Circuit.

Note

If switch SW201 on the Filter/PA Supply assembly was set to the low voltage mode, switch it back to the normal(±PA) mode before continuing.

8.Check the 220V RANGE AC ATTENUATOR. Connect a jumper from TP9 to TP10 on the DAC assembly and another jumper connecting TP1 on the Oscillator Output (A13) assembly to the bottom of R8 (node with R8, C42, C43, and pin 6 of U6) on the Oscillator Control (A12) assembly as done in the previous step 1. Set the Calibrator to 22V at 1 kHz, operate. Testing of this 220V Range AC Attenuator is divided into three steps. Measure the ac voltage with a DMM at TP6 (the Power Amplifier output), then measure the voltage at TP7 (Attenuator input) and verify they are the same ±10 mV. If a failure is detected, check relay K16B and its drive circuit.

Note

If relay K16B is not at fault, then PA OUT HI may be open from PA SENSE

AC elsewhere in the Calibrator.

9.Measure the ac voltage at TP8 with a DMM and verify it equals the voltage measured at TP7 divided by 100 ±1%. Also use an oscilloscope and check for any distortion at TP8. If a failure is detected, check Z1, U4, Q58, Q54, and their associated components. Next, using a DMM, measure the ac voltage at pin 8 of K11 and verify it is the same as measured at TP8. If a failure is detected, check relays K10B, K11, and their drive circuit.

10.Check the SC+ and SC- supplies. Set the Calibrator to 0V dc, standby. Using a DMM, measure the SC+ supply at U1 pin 7 and verify it is +18V ±10%. Next, measure the DC- supply at U1 pin 4 and verify it is -15V ±10%. If a failure is detected, check zener diodes VR15 and VR18.

Note

Remove any jumpers left in place from step 8 before continuing.

11.Check the SENSE CURRENT CANCELLATION circuit. Set the Calibrator to 100 Vdc, operate. Using a DMM, measure the voltage between pin 6 of U1 and TP1, and verify it is 2V ±25 mV. If a failure is detected, check U1 and its associated components.

Note

Step 12 should be done only when a problem with calibration of the 220V dc range exists.

12.Check the 220V DC INT. CAL. NETWORK. Set the Calibrator to 0V dc, standby. Connect a jumper from U5 pin 1 to SCOM (TP11). Using a DMM measure the 13V reference at TP2 of the DAC assembly (common to TP3)and Note the reading. Next, measure the buffered 13V reference at TP12 of the Power Amplifier and verify it is the same. If a failure is detected, check U5, K3, and their drive circuit. Next, using a

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