HP 3478A Technical Manual page 81

7-C-1. INTRODUCTION 7-C-1. INTRODUCTION
7-C-2. 7-C-2. This Se This Service rvice Group
shooting information for the 3478A and is symptoms shooting information for the 3478A and is symptoms
oriented oriented (i.e., (i.e., what what fails). fails).
refer to Schematic 1 when using the troubleshooting refer to Schematic 1 when using the troubleshooting
procedures. procedures.
7-C-3. Most ohms failure 7-C-3. Most ohms failures will show up
Wire and 4-W Wire and 4-W ire Ohms Function with the troubleshooting ire Ohms Function with the troubleshooting
procedure give procedure given in n in paragraph 7 paragraph 7-C-4.
up in only the 4-Wire Ohms Function, go to paragraph 7- up in only the 4-Wire Ohms Function, go to paragraph 7-
C-22 for troubleshooting. C-22 for troubleshooting.
The instrument contains CMOS Integrated The instrument contains CMOS Integrated
Circuits Circuits (e.g. (e.g. U102) U102) which
susceptible susceptible to to
discharge. discharge. It It is is especially
grounded tools and wrist straps be used grounded tools and wrist straps be used
when handling or troubleshooting these when handling or troubleshooting these
components. components.
7-C-4. 7-C-4. 2-WIRE AND 2-WIRE AND 4-WIRE
SHOOTING SHOOTING
7-C-5. 7-C-5. An Ohms An Ohms Failure can Failure can be Overloa
Constant Zero, Floating, or Noisy Readings on some or Constant Zero, Floating, or Noisy Readings on some or
all ranges. A failure should all ranges. A failure should show up with an a
input applied to the input applied to the 3478A. 3478A. For example, an
OHMS TROUBLESHOOTING OHMS TROUBLESHOOTING
T T i i t t l l e e
Introduction Introduction ........ ................. ..................
2-Wire and 4-Wire Ohms Troubleshooting 2-Wire and 4-Wire Ohms Troubleshooting .........
Overload Readings on Overload Readings on All Ranges
Inaccurate Readings on All Inaccurate Readings on All
or Some Ranges................................................... .. ... . 7-C or Some Ranges...................................................
Constant Zero Reading Constant Zero Reading on All
Noise Noise on on All All Ranges............ Ranges.....................
Floating Readings on All Floating Readings on All
or Some Ranges................................................. or Some Ranges................................................. ... ... 7-C
Overload, Noise, or Constant Zero Overload, Noise, or Constant Zero
Readings on Readings
Ohms Protection Circuitry Ohms Protection Circuitry Troubleshooting..................
Ohms Current Ohms Current Source Troubleshooting Source Troubleshooting ..........
4-Wire Ohms Troubleshooting 4-Wire Ohms Troubleshooting .........
Checking Checking the the A/D A/D Controller Controller ........
Group has has the the Ohms t Ohms trouble-
Unless otherwise Unless ot herwise specified,
s will show up in both the in both the 2- 2-
-C-4. If a failur If a failure shows
CAUTION CAUTION
which are are extremely extremely
failures failures due due to to static static
especially important important that
4-WIRE OHMS OHMS TROUBLE- TROUBLE-
be Overload, Ina d, Inaccurate,
show up with an appropriate
For example, an overload
SERVICE GROUP C SERVICE GROUP C
Service Group C Contents Service Group C Contents
.................. .................. .................. ..................
All Ranges ......... ...................
on All Ranges........... Ranges.....................
.................. ..................
on Some Some Ranges............. Ranges......................
Troubleshooting......................
....................
.................. ..................
................. .................. ..................
failure on the 3K ohm Range should show up with a 3K failure on the 3K ohm Range should show up with a 3K
ohm resistor ohm
rouble-
paragraphs paragraphs
troubleshooting procedures (go to paragraph 7-32 troubleshooting procedures (go to
specified,
description of the failures). description of the failures).
7-C-6. Overload Readings 7-C-6.
7-C-7. An Overload 7-C-7.
current (or an open between the INPUT Terminals and current (or an open between the INPUT Terminals and
e shows
the Input the Input Circuitry).
operating correctly, before troubleshooting for an ohms operating correctly, before troubleshooting for an ohms
failure. Do the failure.
a. a. Set the 34
the 3K ohm Range. the 3K ohm Range.
b. b.
INPUT Terminals. INPUT Terminals.
that
c. c. Using a
the 3456A), measure the voltage between pin 17 and pin the 3456A), measure the voltage between pin 17 and pin
19 of U102. Connect the low inpu 19 of U102.
17 and the high input to pin 19. 17 and the high input to pin
d. d. If the rea
and the 3478A displays an overload, the Range and the 3478A displays an overload, the Range
Resistors in U102 may be Resistors in U1
ccurate,
e. e. If the
ppropriate
Current Source Current Source is defective.
overload
troubleshooting. troubleshooting.
7-C-1 7-C-1
P P a a r r a a g g r r a a p p h h
.................. .......... . .. ... . 7-C 7-C-1 -1
................... ................ ...... .. .. 7- 7-C- C-4 4
.................... ........... . ... ... 7-C 7-C-6 -6
7-C-8 -8
........... . ... ... 7-C 7-C-10 -10
.................. ........... .. .. ... . 7-C 7-C-12 -12
7-C-14 -14
.................. .......... . .. ... . 7-C 7-C-16 -16
.... .. .. 7-C 7-C-18 -18
................. ....... ... ... 7-C 7-C-20 -20
.................. ............. .... ... ... 7-C 7-C-22 -22
.................. ......... ... ... 7-C 7-C-24 -24
resistor applied applied to to the the input.
have have the the ohms ohms
Overload Readings on All
An Overload is normally cau is normally caused by
Circuitry). Make sure the DC Make sure the DC Volts Function is
Do the following proc following procedure.
Set the 3478A to the 78A to the 2-Wire Ohms Function and
Connect a 3K Connect a 3K ohm Resistor ohm Resistor to the
Using a high impedance high impedance Digital Vo
Connect the low input of the voltmeter t
19.
If the reading on the ding on the test voltmeter
02 may be too low. too low. Replace U102.
If the reading is other reading is other than +4V dc,
is defective. Go to paragraph
TM 11-6625-3071-14 TM 11-6625-3071-14
input. The The following following
failures failures and and
paragraph 7-32 for a for a
on All Ranges Ranges
sed by a high ohms a high ohms
Volts Function is
edure.
2-Wire Ohms Function and
to the 3478A's 3478A's
Digital Voltmeter (like ltmeter (like
t of the voltmeter to pin o pin
test voltmeter is + 4V dc is + 4V dc
Replace U102.
than +4V dc, the Ohms the Ohms
Go to paragraph 7-C-20 for 7-C-20 for
the the