Fluke 5720A service manual 33. Power Amplifier with Calibrator Set to 22V at 1 kHz

5700A/5720A Series II Calibrator

Service Manual

1.Power up the Calibrator. In the following steps, reference all measurements to PACOM (TP10) unless otherwise Noted.

2.Set the Calibrator to 22V at 1 kHz, operate. This configures the Power Amplifier assembly as an amplifier with a gain of -10 as shown in Figure 5-33. Check the input for the correct voltage. Using a DMM, measure the ac voltage at TP3 and verify it is 2.2V ±5%. If this voltage is incorrect, check relay K10A and its drive circuit. Next, measure the output of the Power Amplifier. Using a DMM, measure the voltage at TP6 and verify it is 10 times that measured at TP3 ±5%. Also measure the dc offset at TP6 and verify it is less than 10 mV. If this voltage is incorrect, proceed with step 3. If the output voltage is correct skip to step 8.

		50K
OSC OUT HI	50K
	-
2.2 -22V		PA OUT HI
		22-220V
	+

PACOM

F5-33.EPS

Figure 5-33. Power Amplifier with Calibrator Set to 22V at 1 kHz

3.Before troubleshooting the main Power Amplifier loop, switch the ±PA supplies to the low voltage (±40SR Supplies) mode by switching switch SW201 on the Filter/PA Supply assembly. Using a DMM, measure the ±PA supplies and verify they are ±60V ±15V. If a failure is detected, the Filter/ PA Supply assembly may be at fault.

4.Check the Bias Current circuit. 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. Using a DMM, measure the mid stage output (1ST MID STAGE OUT),which is the left side of R41, and verify the dc voltage is either +0.8V ±0.2V or -1.1V ±0.2V. If this voltage is correct skip to step 6. If it is incorrect, remove Q12 from the assembly and recheck the Mid-Stage output. With Q12 removed, the Mid-Stage output should be 0.8V ±0.2V. If this voltage is incorrect, check CR53, Q31, Q32, and associated components in the Current Source Circuit. If the voltage is correct then the Input Stage or Mid-Stage circuitry is probably at fault.

5.Check the Input Stage and Mid-Stage. Set the Calibrator to 0V dc, standby. Using a DMM check measure the voltage at pin 10 of the HR8 hybrid and Note whether it is positive or negative. Next, measure the Input Stage output at TP4 and verify it is properly inverting. If a failure is detected, the Input Stage circuit is probably at fault. If the Input Stage is properly inverting, the Mid-Stage circuit is probably at fault.

6.Check the Output Stage Bias Current. 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. Using a DMM measure the Output Stage bias current by measuring the dc voltage between TP14 and TP15, verify it is 0.92V±20%. If a failure is detected, check Q7, Q4, Q5, Q10, Q11, and associated components.

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