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Fluke 76 Service

True rms multimeter
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76
True RMS Multimeter
Service
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.

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Summary of Contents for Fluke 76

  • Page 1 True RMS Multimeter Service 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.
  • Page 2 Service...

  • Page 3: Table Of Contents

    Table of Contents Chapter Title Page 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 Theory of Operation ................2-1 2-1. Introduction ..................... 2-3 2-2.
  • Page 4 Service 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.
  • Page 5 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...
  • Page 6 Service...
  • Page 7 100 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...
  • Page 8 Service...
  • Page 9 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.

  • Page 11: Introduction And Specifications

    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...
  • Page 12 Service...

  • Page 13: Introduction

    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.

  • Page 14: Specifications

    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.
  • Page 15 Introduction and Specifications Specifications 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 Vac <1.5s (for upscale only) Digital Display: Vdc <1s <1s to 40 k , <2s to 4 M , <10s to 40 M Operating Temperature: 0 C to 55 C Storage temperature:...
  • Page 16 Service Table 1-1. Specifications (cont) Burden Voltage Function Range Resolution Accuracy (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 (1.5%+2)†  4.000V 0.001V (0.3%+1) 40.00V 0.01V (0.3%+1)
  • Page 17 Introduction and Specifications Specifications Table 1-1. Specifications (cont) Frequency Counter Sensitivity and Trigger Level Minimum Sensitivity (RMS Sine Wave) Input Range* 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...
  • Page 18 Service...

  • Page 19: Theory Of Operation

    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.
  • Page 20 Service...

  • Page 21: Introduction

    Introduction Introduction 2-1. This section describes the theory of operation for the Fluke 76 multimeter. Functional block descriptions present an overview of circuit operation followed by circuit descriptions, which detail the major circuit functions. Schematic diagrams are provided in Section 5.

  • Page 22: Input Overload Protection

    Service Input Overload Protection 2-4. The V  input is protected from overload by a network consisting of three metal- oxide varistors (RV1, RV2 and RV3) and five current-limiting resistors (R1, R2, R8, R12 and RT1). The presence of an extremely high energy signal causes R1 (500 , 2W), a fusible resistor, to open.

  • Page 23: Ohms Functions

    Theory of Operation Circuit Descriptions DC mV R1, R8, R12, RTI 9.996M ACTIVE HIGH FILTER 9.996M CONVERTER (U2) l2f.eps Figure 2-2. 4V Range Simplified Schematic Ohms Functions 2-8. When the 400 Ohm range is selected, internal switches connect the resistor Z1-2 (9.996 M ) to resistor Z1-6 (1.0001 k ).

  • Page 24: Current Functions

    Service OHMS AND CAPACITANCE VOLTAGE SOURCE REFERENCE RESISTOR INTEGRATE REFERENCE 9.996M 1.0001k – HIGH R1, R8, R12, RTI ACTIVE HIGH FILTER INTEGRATE UNKNOWN UNKNOWN RESISTOR – •R •R l3f.eps Figure 2-3. 400 Ohm Range Simplified Schematic Current Functions 2-9. Input current through R5+R6 (for mA) or R6 (for amps) develops a voltage that is proportional to the input.

  • Page 25: Analog Section Of Integrated Multimeter Ic (U4)

    Theory of Operation Circuit Descriptions Analog Section of Integrated Multimeter IC (U4) 2-11. The a/d converter, autorange switching, frequency comparator, and most of the remaining analog circuitry are contained in the analog section of U4. Peripherals to this U4 analog section include the crystal clock, the system reference voltage, the ac converter, and the filter and amplifier resistors and capacitors.

  • Page 26: Microcomputer Control

    Service BUFFER INTEGRATOR COMPARATOR REFER TO WAVEFORM INPUT – VOLTAGE – – COMPARE CONTROL VOLTAGE REFERENCE 131.072 kHz CLOCK SWITCH AUTO INTE- READ READ HOLD ZERO GRATE +INPUT –INPUT MICROCOMPUTER INTEGRATE RAMP READ ZERO HOLD RAMP BASELINE 16.7 AUTO ZERO 8.33 REPEAT SAMPLE #1...

  • Page 27: Peripherals To U4

    Theory of Operation Circuit Descriptions The Touch Hold mode is a secondary software function. This means that the microcomputer performs a different control algorithm on the data. When the Touch Hold function is selected, the microcomputer does not allow a full resolution conversion to be completed unless the input signal is stable.

  • Page 28: Ac Buffer

    (R10 and R11). This provides an input signal range of zero to 200 mV ac. The converter's output range is zero to -200 mV DC. R34 is used to calibrate the ac gain of the Fluke Model 76. The converter output goes to the active filter in U4. The microprocessor in U4 multiplies the A-D reading by two to give a zero to 4000 count display.

  • Page 29: Display

    LCD. Both digital readings and an analog bar-graph display are presented in conjunction with annunciators and decimal points. Refer to the Fluke 76 User Manual for information about the display. Table 2-1. Typical Voltage Levels and Tolerance (Referred to Common)
  • Page 30 Service 2-12...

  • Page 31: Maintenance

    Chapter 3 Maintenance Title Page 3-1. Introduction ..................... 3-3 3-2. Recommended Equipment ..............3-3 3-3. Operator maintenance ................3-4 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 ..................
  • Page 32 Service...

  • Page 33: Introduction

    Introduction 3-1. This section contains maintenance information for the Fluke 76 meter and includes performance tests, calibration, general maintenance procedures, and troubleshooting. For operator maintenance, refer to the Users Manual (P/N 106275). The performance tests are recommended as a preventive maintenance tool to verify proper instrument operation.

  • Page 34: Operator Maintenance

    Service Operator maintenance 3-3. Warning To avoid electrical shock, remove the test leads before opening the case, and close the case before operating the meter. To prevent fire, install fuses with the rating shown on the back of the meter. Caution To avoid contamination with oil from the fingers, handle the pca by the edges or wear gloves.
  • Page 35 Maintenance Operator maintenance Decal Window Knob Assembly Decal, Top Case Case, Top Window, LCD Insulator Fuse,15A LCD, Assembly Shield, Top Screw 14-14, .250 Battery, 9V Screw #2-1/4x3/8 Fuse, 1A Main PCA Rotary Switch Screw #2-1/4x3/8 Shield Bottom Contact Annunciator Transducer Case Bottom 4 x Foot Non-Skid 4 x Screw 5/14x3/8...

  • Page 36: Fuse Replacement

    Service Fuse Replacement 3-7. Refer to Figure 3-1, and use the following procedure to examine or replace the meter's fuses: 1. Remove the top case. 2. Remove the defective fuse by gently prying one end of the fuse loose and sliding the fuse out of the fuse bracket.

  • Page 37: Display Access

    Maintenance Operator maintenance 3. The pca may now be removed from the bottom case. Note When reinstalling the pca, be sure to hold the bottom shield tab back, then insert the pca and re-adhere the bottom shiled tab to the top shield. If the insulator on the top shield is damaged during removal of the pca, order and replace the insulator as shown in Figure 3-3.

  • Page 38: Cleaning

    Users Manual. If the instrument fails any part of the test, calibration and/or repair is indicated. In the performance tests, the Fluke 76 meter is referred to as the unit under test (UUT). Setup 3-12.

  • Page 39: Display Test

    Maintenance Performance Tests Display Test 3-13. To test the display, turn the UUT on and check whether all display segments come on as indicated in Figure 3-4. mV AC mV DC Mk Hz 2001 3600 4100 l9f.eps Figure 3-4. Display DC Voltage Test 3-14.

  • Page 40: Ac Voltage Test

    Service Table 3-3. mV DC Voltage Test Input Step Display Reading Range Voltage 400 MV +350 MV 348.8 To 351.2 MV DC 40 MV SHORT 0 To .05 MV DC 40 MV +35 MV 34.84 To 35.16 MV DC 40 mV -35 mV -34.84 to -35.16 mV DC AC Voltage Test...

  • Page 41: Ohms Test

    Maintenance Frequency Test Table 3-5. Frequency Test Input Step Display Reading Range Voltage Freq. 300 mV 800 Hz 799.8 Hz to 800.2 Hz Ohms Test 3-18. 1. Select the Ohms function on the UUT. 2. Connect the Ohms Calibrator or Decade Resistor to the V  and COM input terminals of the UUT.

  • Page 42: Capacitance Test

    Service Capacitance Test 3-19. 1. Set the UUT function switch to  and connect the Decade Capacitor output to the V  and COM input terminals of the UUT. Note To enter capacitance, press and hold the push button for 2 seconds. 2.

  • Page 43: Calibration

    Calibrate the meter once a year to ensure that it performs according to specifications. Calibration adjustment points are identified in Figure 3-5. Use the following procedure to calibrate the Fluke 76: 1. Set the DC Voltage Calibrator to 0 volts.

  • Page 44: Troubleshooting

    The procedures provided in these paragraphs will help isolate problems with the meter. In these procedures, the meters are referred to as the unit under test (UUT). When troubleshooting the Fluke 76 meter, use the precautions listed on the "Static Awareness Sheet" to prevent damage from static discharge.

  • Page 45: Power Supply Related Troubleshooting

    Maintenance Troubleshooting Surface-mount components are removed and replaced by reflowing all the solder connections at the same time. Special considerations are required. The de-solder tool should use regulated hot air to melt the solder; there should be no direct contact between the tool and the component. Rework surface-mount assemblies with wire solder rather than with solder paste.

  • Page 46: Vdd (Series) Regulator Troubleshooting

    Service Table 3-12. Voltage Levels VBT- 6.1 (battery at full 9V charge) 3.2 (battery at low charge of 6.0V) REFH 1.23 0.04 1.23 0.15 0.15 2.2 to 1.7 (referenced from VSS) 1.07 to .85 (referenced from VSS) Make the following tests: 1.
  • Page 47 Maintenance Troubleshooting 1. If Vdd - Vss is low, a problem may exist with start-up device Q10. Check Q10 by momentarily connecting Vss to Vbt-. If both Vss and Vbt- now start up and operate correctly, check Q10 for an open. 2.
  • Page 48 Service 3-18...

  • Page 49: List Of Replaceble Parts

    Chapter 4 List of Replaceble Parts Title Page 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...
  • Page 50 Service...

  • Page 51: Introduction

    Introduction 4-1. This section contains an illustrated list of replaceable parts for the Fluke 76 True RMS Multimeter. Parts are listed by assembly; alphabetized by reference designator. Each assembly is accompanied by an illustration showing the location of each part and its reference designator.

  • Page 52: Service Centers

    Note This instrument may contain a Nickel-Cadmium battery. Do not mix with the solid waste stream. Spent batteries should be disposed of by a qualified recycler or hazardous materials handler. Contact your authorized Fluke service center for recycling information. Warning This instrument contains a fusible resistor (PN 107398).

  • Page 53: Final Assembly

    List of Replaceble Parts Table 4-1. 76 Final Assembly Reference Fluke Stock Description Tot Qty Notes Designator BATTERY,PRIMARY,ALKALINE,9V,0-200MA 614487 CAP,POLYES,0.01UF,+-10%,630V 107409 FUSE,.406X1.375,1A,600V,FAST 830828 FUSE,.406X1.5, 15A, 600V, FAST 820829 SCREW,PH,P,THD FORM,STL,4-24,.250 519116 SCREW,PH,P,THD FORM,STL,2-14,.375 821140 SCREW,PH,P,THD FORM,STL,4-14,.375 448456 SCREW,PH,P,THD FORM,STL,5-14,.750 733410...
  • Page 54 Service MP28 SEE VIEW MP27 MP17 MP57 MP18 SEE VIEW MP54 (2) MP15 MP14 H7 (4) Fluke 76 T & B MP10 (4) (1 of 2) l11f.eps Figure 4-1. 76 Final Assembly...
  • Page 55 List of Replaceble Parts MP20 MP23 MP25 MP19 MP21 MP59 MP24 View View Knob Sub-Assembly MP58 MP26 MP17 View Fluke 76 T & B (2 of 2) l12f.eps Figure 4-1. 76 Final Assembly (cont)

  • Page 56: A1 Main Pca

    Service Table 4-2. A1 Main PCA Reference Fluke Stock Description Tot Qty Notes Designator CAP,POLYES,0.01UF,+-10%,630V 107409 C2,C4,C12,C30 CAP,CER,270PF,+-1%,50V,C0G,0805 944301 CAP,CER,75PF,+-5%,COG,0805 107638 C5,C7 CAP,TA,15UF,+-20%,10V,3528 867593 CAP,TA,22UF,+-20%,6V,6032 876545 C8,C9 CAP,POLYCA,0.027UF,+-10%,63V 720979 CAP,TA,0.47UF,+-20%,25V,3216 876180 CAP,POLYPR,0.022UF,+-10%,63V 821579 C13,C19 CAP,CER,270PF,+-1%,50V,C0G,0805 944301 C14,C21 CAP,TA,47UF,+-20%,10V,7343 867580 C15,C16 CAP,CER,0.1UF,+-10%,25V,X7R,1206...
  • Page 57 IC,BPLR,TRUE RMS TO DC CONVERTER,SOIC 868000 IC N-WELL 76TRMS,ASSEMBLY TESTED 103728 LCD,4.5 DIGIT,BAR GRAPH,MULTIPLEXED 875534 IC, 1.23V,150 PPM T.C.,BANDGAP V. REF 634451 CRYSTAL,131.072KHZ,30PPM,SURFACE MT 912464 RNET,CERM,SIP,FLUKE 83 HI V DIVIDER 828152 NOTES 1. FUSIBLE RESISTOR. TO ENSURE SAFETY, USE EXACT REPLACMENT ONLY.
  • Page 58 Service FLUKE 76-1603 l13f.eps Figure 4-2. A1 Main PCA 4-10...
  • Page 59 List of Replaceble Parts FLUKE 76-1603 l14f.eps Figure 4-2. A1 Main PCA (cont) 4-11...
  • Page 60 Service 4-12...

  • Page 61: Schematic Diagrams

    Chapter 5 Schematic Diagrams Title Page A1 Main PCA ..................5-3...
  • Page 62 Service...

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