Fluke 76 Assembly instructions

PN 103926
Date September 1995
© 1995 Fluke Corporation, Inc. All rights reserved. Printed in U.S.A.
All product names are trademarks of their respective companies.
®
76
True RMS Multimeter
Service

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Service
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i
Table of Contents
Chapter Title Page
1 Introduction and Specifications............................................................ 1-1
1-1. Introduction............................................................................................. 1-3
1-2. Organization of the Service Manual ....................................................... 1-3
1-3. Conventions ............................................................................................ 1-3
1-4. Specifications.......................................................................................... 1-4
2 Theory of Operation ............................................................................... 2-1
2-1. Introduction............................................................................................. 2-3
2-2. Functional Block Description ................................................................. 2-3
2-3. Circuit Descriptions ................................................................................ 2-3
2-4. Input Overload Protection .................................................................. 2-4
2-5. Rotary Knob Switch ........................................................................... 2-4
2-6. Input Signal Conditioning Circuits..................................................... 2-4
2-7. Volts Functions .............................................................................. 2-4
2-8. Ohms Functions ............................................................................. 2-5
2-9. Current Functions........................................................................... 2-6
2-10. Continuity, Low Ohms and Diode Test Functions......................... 2-6
2-11. Analog Section of Integrated Multimeter IC (U4) ............................. 2-7
2-12. Capacitance Measurements............................................................ 2-7
2-13. Frequency Measurements............................................................... 2-7
2-14. Microcomputer Control...................................................................... 2-8
2-15. Peripherals to U4 ................................................................................ 2-9
2-16. AC Buffer....................................................................................... 2-10
2-17. AC Converter ................................................................................. 2-10
2-18. Active Filter ................................................................................... 2-10
2-19. A/D Converter................................................................................ 2-10
2-20. Beeper ............................................................................................ 2-10
2-21. Power Supply ................................................................................. 2-10
2-22. Display................................................................................................ 2-11
3 Maintenance............................................................................................ 3-1
3-1. Introduction............................................................................................. 3-3
3-2. Recommended Equipment ...................................................................... 3-3
3-3. Operator maintenance ............................................................................. 3-4

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3-4. Case Disassembly............................................................................... 3-4
3-5. Battery Replacement .......................................................................... 3-4
3-6. Fuse Test............................................................................................. 3-4
3-7. Fuse Replacement............................................................................... 3-6
3-8. PCA Removal..................................................................................... 3-6
3-9. Display Access ................................................................................... 3-7
3-10. Cleaning.................................................................................................. 3-8
3-11. Performance Tests................................................................................... 3-8
3-12. Setup................................................................................................... 3-8
3-13. Display Test........................................................................................ 3-9
3-14. DC Voltage Test ................................................................................. 3-9
3-15. mV DC Test........................................................................................ 3-9
3-16. AC Voltage Test ................................................................................. 3-10
3-17. Frequency Test ................................................................................... 3-10
3-18. Ohms Test........................................................................................... 3-11
3-19. Capacitance Test................................................................................. 3-12
3-20. Continuity Test ................................................................................... 3-12
3-21. DC and AC Current Test .................................................................... 3-12
3-22. Calibration .............................................................................................. 3-13
3-23. Troubleshooting...................................................................................... 3-14
3-24. Surface Mount Assemblies................................................................. 3-14
3-25. Power Supply Related Troubleshooting............................................. 3-15
3-26. Common (Shunt) Regulator Troubleshooting .................................... 3-15
3-27. Vdd (Series) Regulator Troubleshooting............................................ 3-16
4 List of Replaceble Parts......................................................................... 4-1
4-1. Introduction............................................................................................. 4-3
4-2. How to Obtain Parts................................................................................ 4-3
4-3. Manual Status Information ..................................................................... 4-3
4-4. Newer Instruments.................................................................................. 4-3
4-5. Service Centers ....................................................................................... 4-4
5 Schematic Diagrams .............................................................................. 5-1

iii
List of Tables
Table Title Page
1-1. Specifications ........................................................................................................... 1-5
2-1. Typical Voltage Levels and Tolerance (Refered to Common) ................................ 2-11
3-1. Recommended Test Equipment................................................................................ 3-3
3-2. DC Voltage Test....................................................................................................... 3-9
3-3. mV DC Voltage Test................................................................................................ 3-10
3-4. AC Voltage Test....................................................................................................... 3-10
3-5. Frequency Test ......................................................................................................... 3-11
3-6. Resistance Test......................................................................................................... 3-11
3-7. Capacitance Test ...................................................................................................... 3-12
3-8. Continuity Test......................................................................................................... 3-12
3-9. DC and AC mA Test ................................................................................................ 3-13
3-10. DC Amps Test.......................................................................................................... 3-13
3-11. Functional Description of Power Supply Components ............................................ 3-15
3-12. Voltage Levels ......................................................................................................... 3-16
4-1. 76 Final Assembly.................................................................................................... 4-5
4-2. A1 Main PCA........................................................................................................... 4-8

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v
List of Figures
Figure Title Page
2-1. Block Diagram ......................................................................................................... 2-3
2-2. 4V Range Simplified Schematic .............................................................................. 2-5
2-3. 400 Ohm Range Simplified Schematic .................................................................... 2-6
2-4. A/D Converter.......................................................................................................... 2-8
2-5. 100 nF Range Simplified Schematic........................................................................ 2-9
3-1. Assembly Details...................................................................................................... 3-5
3-3. Top Shield Insulator Replacement ........................................................................... 3-7
3-4. Display ..................................................................................................................... 3-9
3-5. Calibration Adjustment Points ................................................................................. 3-14
4-1. 76 Final Assembly.................................................................................................... 4-7
4-2. A1 Main PCA........................................................................................................... 4-10

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OPERATOR SAFETY INFORMATION
This meter has been designed and tested according to IEC Publication 1010-1.
Follow all safety and operating instructions to ensure that the meter is
used safely and is kept in good operating condition.
•Never use the meter if the meter or test leads look damaged.
•Always turn off power to the circuit before cutting, unsoldering, or
breaking the circuit. Small amounts of current can be dangerous.
•Never measure resistance in a circuit when power is applied to the circuit.
•Never touch the probes to a voltage source when the test leads are plugged into the 10A or
40 mA input jack.
•To avoid damage or injury, never use the meter on unprotected circuits that exceed 14000
volt-amps.
•Never apply more than 600V dc or ac rms (sine) between any input jack and earth ground.
•Always be careful when working with voltages above 60V dc or 30V ac rms. Such voltages
pose a shock hazard.
•Always keep your fingers behind the finger guards on the probe when making
measurements.
•Always use a high voltage probe to measure voltage if the peak voltage might exceed
600V.
SYMBOLS MARKED ON EQUIPMENT
SS DANGER - High voltage.
PAttention - refer to the manual. This symbol indicates that
information about usage of a feature is contained in the manual.
XFuse information.
USE THE PROPER FUSE
To avoid fire hazard, use only a fuse identical in type, voltage rating, and
current rating as specified on the case bottom fuse rating label.
DO NOT OPERATE DISASSEMBLED METER
Always operate the meter with case top and bottom properly assembled.
Access procedures and the warnings for such procedures are contained in this
Service Manual. Service procedures are for qualified service personnel only.
DO NOT ATTEMPT TO OPERATE IF PROTECTION MAY BE IMPAIRED
If the meter appears damaged or operates abnormally, protection may be
impaired. Do not attempt to operate it. When in doubt, have the meter
serviced.