Keysight 1143A, User'S And Service Manual

Page: 44 / 56

Theory of Operation
The 1143A Probe Offset Control and Power Module provides power and
offset control for two active probes, such as the 54701A. Use the schematics
at the end of this chapter to illustrate the following discussion. The
schematics cover the power and control assembly A1. The only electrical
parts not on this assembly are the two probe power cables.
Power Supplies
Use figure 7 below, and schematics 1 and 2 (figures 8 and 9). Two power
supplies provide
+
17.3 Vdc and
−
17.3 Vdc. The voltage of both supplies is set
by an adjustment on the
+
17.3 V supply. The negative supply follows the
positive supply so that a voltage increase in the positive supply causes a
voltage increase in the negative supply. Each supply has a current limit
adjustment which is adjusted at the factory for limiting at 330 mA.
The supplies use a linear regulator that allows a low differential between the
input voltage and the regulated output voltage. The output drives a pass
transistor which drives the load with it’s collector. A simplified diagram of
the voltage regulator IC is shown in the figure below.
Pin Function
1 + Input voltage
2 –2.0 V reference
3 +1.5 V reference
4 Current threshold adjust
5 – Input voltage
6 – Current sense
7 + Current sense
8 Non-inverting input
9 Inverting input
10 Fault alert (not used)
11 Fault delay
12 Driver sink
13 Driver source
14 Compensation/Shutdown
15 O. V. latch out/ Reset
16 Crowbar gate
Voltage Regulator IC Simplified Diagram
Figure 7
Service
Troubleshooting and Repair
44