Filter
Top Products
Troubleshooting
Component-level Troubleshooting
5
5-63
14. Check the DAC 20V Range. Set the Calibrator to 20V dc, operate. Using a DMM
measure the DAC output voltage at TP8 (common to TP3). It should be a nominal
20V dc (±0.5V for an uncalibrated instrument). If a failure is detected, check Q14,
Q15, Q18, Q19, Q20, Q21, and their associated components.
15. Check the SHUNT LINEARITY CONTROL CIRCUIT. Set the Calibrator to 10V
dc, operate. Measure the voltage with a DMM at U2 pin 7 (common to TP3).It
should be a nominal -10V dc (±0.35V for an uncalibrated instrument and ±0.1V for a
calibrated instrument). Set the Calibrator to 20V dc, operate, and again measure the
voltage at U2B pin 7. It should still be the nominal -10V dc within the same
tolerance. If either of these voltages are incorrect, check U2B, Q22, and the
associated 80 kΩ resistors in the HR6 Hybrid assembly.
16. Check the SENSE HI CURRENT CANCELLATION CIRCUIT. Set the Calibrator
to 10V dc, operate. Measure the voltage with a DMM at TP8 (common to TP3) and
Note the reading. Next, measure the output high binding post of the Calibrator
(common to TP3). It should be the same as the Noted reading ±10 ppm. If a failure is
detected, check U1A and associated components in the Sense Hi Current
Cancellation Circuit.
5-14. Duty-cycle Control Circuit
Proceed as follows to troubleshoot the duty-cycle control circuit:
1. Check the 8 MHz clock for U6. Connect an oscilloscope to U6 pin 10(common to
TP1). Set the oscilloscope for 2V/div at 100 ns/div and verify the display shows an 8
MHz clock similar to that shown in Figure 5-22. If a failure is detected, check U7
and its associated components. The input signal (CLK and CLK*) to U7 is a low-
level(400 mV p-p) 8 MHz clock generated by the Regulator/Guard Crossing
assembly.
2. Check OUT1 from U6. Connect an oscilloscope to U6 pin 16 (common to TP1). Set
the Calibrator to 6.5V dc, operate, and set the oscilloscope to 2V/div at 2 ms/div.
The oscilloscope should display a TTL-level square wave with approximately a 50%
duty cycle. If a failure is detected, check U6 and all its control lines.
3. Check OUT2 from U6. Connect an oscilloscope to U6 pin 20 (common to TP1). Set
the Calibrator to 6.5V dc, operate, and the oscilloscope to 2V/div at 2 ms/div. The
oscilloscope should display a TTL level square wave. If a failure is detected, U6 is
probably at fault.
4. Check CH1 FLOATING. Connect an oscilloscope to U13 pin 6 (common to TP1).
Set the Calibrator to 6.5V dc, operate, and the oscilloscope to 2V/div at 2 ms/div.
The oscilloscope should display a TTL level signal similar to Figure 5-23. If a
failure is detected, check U13 and its associated components.
5. Check the Clock to U14. Connect an oscilloscope to U10 pin 4 (common= TP3) and
set it for 2V/div at 100 ns/div. Verify it displays a 8 MHz clock similar to Figure 5-
24. If a failure is detected, check U10 and its associated components.
6. Check the Watchdog Clock. Connect an oscilloscope to U14 pin 8(common to TP3)
and verify it displays a TTL level 4 MHz clock. If a failure is detected, check U14.
7. Check the Watchdog Timer. Connect an oscilloscope to U15 pin 13(common to
TP3) and verify it displays a constant logic high. If a failure is detected, check U15
and associated components.
8. Check the Q1 and Q1* output of U14. Connect an oscilloscope to U14 pin 5
(common to TP3). Set the Calibrator to 6.5V dc, operate, and set the oscilloscope to
2V/div at 2 ms/div. The oscilloscope should display a TTL-level square wave with