Appendix Service Information
Troubleshooting Hints
161
Service
Information
Troubleshooting Hints
This section provides a brief check list of common failures. Before
troubleshooting or repairing the power supply, make sure that the failure is in
the power supply rather than any external connections. Also make sure that
the power supply is accurately calibrated. The power supply’s circuits allow
troubleshooting and repair with basic equipment such as a 6½-digital
multimeter.
Unit Reports Errors 740 to 750
These errors may be produced if you accidentally turn off power of the unit
during a calibration or while changing a non-volatile state of the instrument.
Recalibration or resetting the state should clear the error. If the error persists,
a hardware failure may have occurred.
Unit Fails Self-Test
Verify that the correct power-line voltage setting is selected. Also, ensure that
all terminal connections are removed while the self-test is performed. Failure
of the DAC U131 on the PC board will cause many self-test failures.
Bias Supplies Problems
Check that the input to the voltage regulators of the bias supplies is at least
1 V greater than their output. Circuit failures can cause heavy loads of the bias
supplies which may pull down the regulator output voltages. Check the
voltages of bias supplies as tabulated below.
Table A-1 Bias Supplies Voltages
Some circuits produce their own local bias supplies from the main bias
supplies. Be sure to check that these local bias supplies are active. In particular,
the ADC (analog-to-digital converter), ac input, and front panel sections have
local bias supplies. Always check that the power supplies are free of ac
oscillations using an oscilloscope. Failure of bias supplies will cause many self-
test failures.
Bias Supply Minimum Maximum Check At
+5V Floating +4.75 V +5.25 V U110 pin 2
-5.1V Floating -4.75 V -5.25 V Anode of CR114
+15V Floating +14.25 V +15.75 V Anode of CR104
-15V Floating -14.25 V -15.75 V Cathode of CR105
