Keysight 1143A, User'S And Service Manual

Page: 33 / 56

The
−
17.3 supply is out of tolerance
•
Make sure the +17.3 V supply is in tolerance.
•
Make sure the supply is not loaded into current limit.
•
Check the values of parts in the voltage divider sticks that feed the
error amplifier of the
−
17.3 V regulator (R47–R51).
There is a current limit problem
•
Check that the supply is not loaded into current limit.
•
Check the values of parts in the current sense circuit, particularly the
current sense resistors (R42 and R53).
To troubleshoot the offset circuitry
The offset circuitry is essentially a string of op-amps and you can use
conventional signal tracing techniques. A complete parts list, component
locator, and schematics are provided later in this chapter.
1
Load the appropriate offset output circuit with a 511-
Ω
resistor.
For probe 1 connect the resistor between TP5 and TP3, and for probe 2
connect it between TP5 and TP4.
2
Measure the offset output voltage (TP3 for probe 1 or TP4 for
probe 2).
•
With the controls set to Local and Zero, the voltage should be adjustable
(R9 probe 1, R29 probe 2) to 0.0 Vdc
±
100
µ
V.
•
With the controls set to Local and Variable, the voltage should be variable
by the front panel controls greater than
±
2.58 Vdc.
The offset output is a
±
5 mA current. Into a 511-
Ω
resistor the
nominal voltage swing should be
±
2.58 Vdc but it depends on the
value of the resistor. For the most accurate test, measure the offset
current swing directly.
Service
Troubleshooting and Repair
33