Ohms Troubleshooting; Troubleshooting - HP 3478A Technical Manual
Dilgital multimeter
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- Service manual (160 pages) ,
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Table of Contents
7-C-1. INTRODUCTION
7-C-2.
This Service Group has the Ohms trouble-
shooting information for the 3478A and is symptoms
oriented (i.e., what fails).
refer to Schematic 1 when using the troubleshooting
procedures.
7-C-3. Most ohms failures will show up in both the 2-
Wire and 4-Wire Ohms Function with the troubleshooting
procedure given in paragraph 7-C-4. If a failure shows
up in only the 4-Wire Ohms Function, go to paragraph 7-
C-22 for troubleshooting.
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-C-4.
2-WIRE AND 4-WIRE OHMS TROUBLE-
SHOOTING
7-C-5. An Ohms Failure can be Overload, Inaccurate,
Constant Zero, Floating, or Noisy Readings on some or
all ranges. A failure should show up with an appropriate
input applied to the 3478A. For example, an overload
OHMS TROUBLESHOOTING
Service Group C Contents
Title
Introduction ........................................................................... 7-C-1
2-Wire and 4-Wire Ohms Troubleshooting ........................... 7-C-4
Overload Readings on All Ranges ................................. 7-C-6
Inaccurate Readings on All
or Some Ranges...................................................... 7-C-8
Constant Zero Reading on All Ranges......................... 7-C-10
Noise on All Ranges..................................................... 7-C-12
Floating Readings on All
or Some Ranges.................................................... 7-C-14
Overload, Noise, or Constant Zero
Readings on Some Ranges................................... 7-C-16
Ohms Protection Circuitry Troubleshooting ........................ 7-C-18
Ohms Current Source Troubleshooting .............................. 7-C-20
4-Wire Ohms Troubleshooting ........................................... 7-C-22
Checking the A/D Controller ............................................... 7-C-24
Unless otherwise specified,
failures
due
to
static
SERVICE GROUP C
failure on the 3K ohm Range should show up with a 3K
ohm resistor applied to the input.
paragraphs
troubleshooting procedures (go to paragraph 7-32 for a
description of the failures).
7-C-6. Overload Readings on All Ranges
7-C-7. An Overload is normally caused by a high ohms
current (or an open between the INPUT Terminals and
the Input Circuitry). Make sure the DC Volts Function is
operating correctly, before troubleshooting for an ohms
failure. Do the following procedure.
a. Set the 3478A to the 2-Wire Ohms Function and
the 3K ohm Range.
b.
INPUT Terminals.
c. Using a high impedance Digital Voltmeter (like
the 3456A), measure the voltage between pin 17 and pin
19 of U102. Connect the low input of the voltmeter to pin
17 and the high input to pin 19.
d. If the reading on the test voltmeter is + 4V dc
and the 3478A displays an overload, the Range
Resistors in U102 may be too low. Replace U102.
e. If the reading is other than +4V dc, the Ohms
Current Source is defective. Go to paragraph 7-C-20 for
troubleshooting.
7-C-1
Paragraph
have
the
ohms
Connect a 3K ohm Resistor to the 3478A's
TM 11-6625-3071-14
The following
failures
and
the
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