HP 3478A Technical Manual page 75

7-A-1. INTRODUCTION
7-A-2. This Service Group has the DC Volts and DC
Current troubleshooting information for the 3478A. The
Service Group is symptoms oriented (i.e., what fails) with
two different levels of troubleshooting.
determines the general area of the 3478A that causes
the failure and the
troubleshooting information for the area that fails.
Unless otherwise specified, refer to Schematic 1 when
using the troubleshooting procedures.
CAUTION
The instrument contains CMOS Integrated
Circuits (e.g. U102) which are extremely
susceptible to
discharge. It is especially important that
grounded tools and wrist straps be used
when handling or troubleshooting these
components.
7-A-3. PRE-TROUBLESHOOTING INFORMATION
7-A-4. Before doing any troubleshooting procedures,
perform the following:
a. Check the 3478A's Floating Common Power
Supplies and make sure they are stable, have the correct
value, and are not oscillating. The power supplies are as
follows (see Schematic 4):
DC VOLTS AND DC CURRENT TROUBLESHOOTING
Service Group A Contents
Title
Introduction ............................................................................7-A-1
Pre-Troubleshooting Information ...........................................7-A-3
DC Volts Troubleshooting......................................................7-A-5
Overload, Floating, Constant Zero (with input applied),
or Noisy Readings on All Ranges .............................7-A-7
Constant Zero Readings (with no input applied) on
All Ranges ................................................................7-A-9
Overload, Constant Zero, Floating, or Noisy Readings
on Some Ranges ....................................................7-A-11
Inaccurate Readings on All Ranges..............................7-A-17
Protection Circuitry Troubleshooting....................................7-A-19
Input Circuitry Troubleshooting............................................7-A-21
Overload Readings on All Ranges ................................7-A-23
Constant Zero Readings on All Ranges........................7-A-25
Floating Readings on All Ranges..................................7-A-27
Noisy Readings on All Ranges......................................7-A-29
DC Current Troubleshooting................................................7-A-31
Checking the A/D Controller ................................................7-A-33
The first level
second level has
failures due to static
SERVICE GROUP A
Power
Supply
specific b. Check and make sure the Reference Supplies
are at the correct level and quiet. The +10V supply can
be checked at U405 pin 6, the -10V supply at U404 pin 6,
and the buffered +10V supply at JM201 (see Schematic
3).
c. Make sure the Front/Rear Switch is making good
contact and not open.
7-A-5. DC VOLTS TROUBLESHOOTING
7-A-6.
Inaccurate, Constant Zero, Floating, or Noisy Readings
on all or some ranges. The following paragraphs have
the failures and the troubleshooting procedures (see
paragraph 7-30 for a description of these failures).
7-A-7. Overload, Floating, Constant Zero (with input
applied), or Noisy Readings on All Ranges
7-A-8. If a failure is noted on all ranges, the failure can
be caused by the Input Circuitry (Input Switching or
DC/Ohms Input Amplifier) or by the A/D Converter. To
determine the inoperative circuitry, do the procedure
which follows this paragraph (the same procedure is
7-A-1
Paragraph
Checked
at
+ 5V JM701
-15V JM702
+15V JM703
Typical DC Volts Failures are Overload,
TM 11-6625-3071-14
Voltage
Level
+ 4.9V to + 5 . 1V
-14.4V to -15.6V
+14.4V to +15.6V