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5700A/5720A Series II Calibrator
Service Manual
5-80
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
PA OUT HI
OSC OUT HI
PACOM
22-220V
2.2 -22V
50K
-
+
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.