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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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.

1-1
Chapter 1
Introduction and Specifications
Title Page
1-1. Introduction............................................................................................. 1-3
1-2. Organization of the Service Manual ....................................................... 1-3
1-3. Conventions ............................................................................................ 1-3
1-4. Specifications.......................................................................................... 1-4

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Introduction and Specifications
Introduction 1
1-3
Introduction 1-1.
This service manual provides information on maintaining, troubleshooting, and repairing
the Fluke 76 Multimeter. The manual also provides specifications, theory of operation,
calibration routines, testing and troubleshooting procedures, parts replacement
information, and schematic diagrams.
A meter under warranty will be promptly repaired or replaced (at Fluke's option) and
returned at no charge. See the registration card for warranty terms. If the warranty has
expired, the meter will be repaired and returned for a fixed fee. Contact the nearest
Service Center for information and prices. A list of U.S. and International Service
Centers is included at the end of Section 4 of this manual.
Organization of the Service Manual 1-2.
The following descriptions for the various sections serve to introduce the manual:
•Section 1. Introduction and Specifications
This section describes both use of the Service Manual and application of special
terminology (conventions) to describe the meter's circuitry. A complete set of
specifications appears at the end of this section.
•Section 2. Theory of Operation
This section first categorizes instrument circuitry into functional blocks, with a
description of each block's role in overall operation. A detailed circuit description is
then given for each block. These descriptions explain operation to the component
level and support the troubleshooting and repair procedures defined in Section 3.
•Section 3. Maintenance
This section provides complete maintenance information ranging from general
maintenance, cleaning instructions, and detailed troubleshooting to repair procedures
involving component-level adjustments. Troubleshooting and repair procedures rely
heavily on both the Theory of Operation presented in Section 2 and the Schematic
Diagrams shown in Section 5.
•Section 4. List of Replaceable Parts
The section includes parts lists for all standard assemblies. Information on how and
where to order parts is also provided.
•Section 5. Schematic Diagrams
This section provides schematic diagrams for all assemblies. A list of mnemonic
definitions is also included to aid in identifying signal name abbreviations.
Conventions 1-3.
Throughout the manual, certain notational conventions are used. A summary of these
conventions follows:
•Printed Circuit Assembly
The term "pca" is used to represent a printed circuit board and its attached parts.
•Circuit Nodes
Individual pins or connections on a component are specified by a dash (-) following
the component reference designator. For example, pin 19 of U30 would be U30-19.
•User Notation

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Generally, function switch positions to select, input terminals to use, and display
notation to be read are presented in this manual as they are seen on the multimeter.
Special terms (mnemonics) used in text descriptions of multimeter circuitry
correspond to terms used on the schematic diagrams in Section 5.
Specifications 1-4.
Specifications for Fluke Model 76 are presented in Table 1-1 on the following page.
Accuracy is specified for a period of one year after calibration, at 18°C to 28°C (64°F to
82°F) with relative humidity to 90%. AC conversions are ac-coupled and true rms
responding.
Accuracy Specifications are given as:
±([% of reading] + number of least significant digits)

Introduction and Specifications
Specifications 1
1-5
Table 1-1. Specifications
Display: Digital: 4000 counts, updates 4/sec
Analog: 63 segments, updates 40/sec
Frequency: 9,999 counts
Capacitance: 9,999 counts
Response Time of
Digital Display:
Vac <1.5s (for upscale only)
Vdc <1s
Ω<1s to 40 kΩ, <2s to 4 MΩ, <10s to 40 MΩ
Operating Temperature: 0°C to 55°C
Storage temperature: -40°C to 60°C
Temperature Coefficient: 0.1 x (specified accuracy)/ °C (<18°C or> 28°C)
Relative Humidity: 90% (0°C to 30°C)
75% (30°C to 40°C)
45% (40°C to 50°C)
35% (50°C to 55°C)
Altitude: Operating: 2,000 meters
Storage: 12,000 meters
Battery type: 9v, NEDA 1604A or IEC 6LR61
Battery Life: 500 hrs typical with alkaline
Continuity Beeper: 4096 Hz
Vibration: Per MIL-T28800E, Class III
Sinusoidal, Non Operating
Drop: ANSI/ISA-S82.01-1994 and EN61010-1, 1993
1 meter drop to hardwood on concrete
Enclosure: Conforms to IP-40 Per IEC-529
Size (HxWxL): 2.8 cm x 7.5 cm x 16.6 cm
(1.12 in x 2.95 in x 6.55 in)
Weight: 12 oz (340g)
EMC: EN 50081-1, EN 50082-1
Surge Protection: 6 KV peak per IEC 1010-1, 1990-09 and ANSI/IEEE C62.41-1991
Safety: 600V AC, 600V DC maximum voltage between any terminal and earth ground.
Complies with IEC-1010-1, 1990-09 and ANSI/ISA-S82.01-94 for use in
overvoltage category III locations, ≤600V, UL3111, CSA/CAN C22.2 No.
1010.1-92 and EN61010 part 1-1993.
Certifications: and CSA
UL and TUV pending

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Table 1-1. Specifications (cont)
Function Range Resolution Accuracy
Burden Voltage
(Typical)
400.0 mV 0.1 mV ±(1.9%+4)†
(45 Hz to 1 kHz) 4.000V 0.001V ±(1.9%+2)†
40.00V 0.01V ±(1.5%+2)† Not Applicable
400.0V 0.1V ±(1.5%+2)†
600V 1V ±(1.5%+2)†
4.000V 0.001V ±(0.3%+1)
40.00V 0.01V ±(0.3%+1) Not Applicable
400.0V 0.1V ±(0.3%+1)
600V 1V ±(0.3%+1)
40.00 mV* 0.01 mV ±(0.3%+5) Not Applicable
400.0 mV 0.1 mV ±(0.3%+1)
Ω400.0Ω0.1Ω±(0.4%+2)
4.000 kΩ0.001 kΩ±(0.4%+1)
40.00 kΩ0.01 kΩ±(0.4%+1) Not Applicable
400.0 kΩ0.1 kΩ±(0.4%+1)
4.000 MΩ0.001 MΩ±(0.4%+1)
40.00 MΩ0.01 MΩ±(1%+3)
Capacitance 99.99 nF 0.01 nF ±(1.9%+2)**
999.9 nF 0.1 nF ±(1.9%+2)**
9.999 μF 0.001 μF ±(1.9%+2)** Not Applicable
99.99 μF 0.01 μF ±(1.9%+2)**
999.9 μF 0.1 μF ±(1.9%+2)**
9999 μF 1 μF ±10% Typical
400Ω0.1Ω5% Typical Not Applicable
40Ω40Ω* 0.01Ω5% Typical
(Lo-Ohms) 400Ω0.1Ω5% Typical Not Applicable
8 kΩ1Ω10% Typical
Diode Test 2.450V 0.001V ±2% Typical Not Applicable
4.000 mA 0.001 mA ±(1.5%+4)† 11 mV/mA
(45 Hz to 1 kHz) 40.00 mA 0.01 mA ±(1.5%+2)† 11 mV/mA
4A 0.001A ±(1.5%+4)† 0.03 V/A
10.00A*** 0.01A ±(1.5%+2)† 0.03 V/A
4.000 mA 0.001 mA ±(0.5%+5) 11 mV/mA
40.00 mA 0.01 mA ±(0.5%+2) 11 mV/mA
4A 0.001A ±(0.5%+5) 0.03 V/A
10.00A*** 0.01A ±(0.5%+2) 0.03 V/A
Frequency‡ 99.99 0.01 Hz ±(0.01%+1)
(1 Hz to 20 kHz) 999.9 0.1 Hz ±(0.01%+1) Not Applicable
9.999 kHz 0.001 kHz ±(0.01%+1)
20.00 kHz 0.01 kHz ±(0.01%+1)
* In 40Ωand 40 mV ranges, thermals may introduce additional errors. Maximum accuracy is obtained
when both probe tips are maintained at the same temperature.
** With film capacitor or better and residual (open lead reading) subtracted from measurement. This meter
uses a dc-type measurement technique.
*** 10A continuous, 20A for 30 seconds.
† From 5% to 100% of specified range.
‡ For rectangular waveforms 25% ≤duty cycle ≤75% . VAC ≤1 kHz.

Introduction and Specifications
Specifications 1
1-7
Table 1-1. Specifications (cont)
Frequency Counter Sensitivity and Trigger Level
Input Range* Minimum Sensitivity (RMS Sine Wave)
500 Hz to 20 kHz 1.0 Hz to 500 Hz**
400 mV ac Not Applicable Not Applicable
4V ac 0.3V 0.7V
40V ac 3V 7V
400V ac 30V 70V
600V ac 300V Not Applicable
* Maximum input for specified accuracy = 10 x Range or 600V.
** Display rattle for sine waves below 500 Hz = 5 counts.
Function Overload
Protection*
Input
Impedance
(Nominal )
Common Mode
Rejection Ratio (1
KΩUnbalance)
Normal Mode
Rejection
600V dc
600V ac rms (sine)
>10 MΩ,
<100 pF
>120 dB at dc,
50 Hz, or 60 Hz
>60 dB at 50
Hz or 60 Hz
600V dc
600V ac rms (sine)
10 MΩ,
<100 pF
>120 dB at dc,
50 Hz, or 60 Hz
>60 dB at 50
Hz or 60 Hz
600V dc
600V ac rms (sine)
>10 MΩ,
<100 pF
>60 dB, dc to
60 Hz
(ac-coupled)
ΩOpen Circuit
Test Voltage
Full Scale Voltage Short Circuit
Current
< 4.0 MΩ<40 MΩ
600V dc,
600V rms (sine)
<1.3V dc <450 mV dc <1.3V dc <500 uA
Diode Test

600V dc,
600V rms (sine)
<3.1V dc 2.45V dc 800 uA typical
*107 V-Hz max.

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2-1
Chapter 2
Theory of Operation
Title Page
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

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