Home

Service

1. Figure 4 2 A1 Main PCA 4 10 List of Replaceble Parts 4 ED PCB Y ASSEMB W FU DESIGNATION VI REFERENCE 1 CIRCUI FLUKE 76 1603 114f eps Figure 4 2 A1 Main PCA cont 76 Service 5 1 Al Main PCA Chapter 5 Schematic Diagrams 5 1 76 Service 5 2
2. 3 13 3 11 Functional Description of Power Supply 3 15 3 12 Voltage Level ie een een aan 3 16 4 1 76 Fma Assembly peer o I len 4 5 2 2 AL Man PEA ee Meere 4 8 iii 76 Service List of Figures Figure Title Page 2 1 Block Diagram ci aa 2 3 2 2 AV Range Simplified Schematic nennen 2 5 2 3 400 Ohm Range Simplified Schematic nennen 2 6 2 4 ASD ET ae 2 8 2 5 100 nF Range Simplified Schematic seen 2 9 321 Assembly Details niea ene eer ote eee t Reed 3 5 3 3 Top Shield Insulator 1 2 3 7 3 4 en A beth o Re 3 9 3 5 Calibration Adjustment Points nennen nennen nn 3 14 4 1 76 Fimal Assembly ect ode bi 4 7 4 2 AT M n POA T PHP 4 10 76 Service vi 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 e Never use the meter if the meter or test leads look damaged e Always turn off power to the circuit before cutting unsoldering or breaking the circuit Small amounts of current can be dangerous e Never measure resistance in a circuit when power is applied to the circuit e Neve
3. 400 2 3600 I9f eps Figure 3 4 Display DC Voltage Test 3 14 1 Set the UUT function switch to DC and connect the DC Voltage Calibrator output to the VA and COM input terminals of the UUT 2 Referring to Table 3 2 set the DC Voltage Calibrator for the output indicated in steps 1 through 4 only Verify that the UUT display reading is within the limits shown 3 Reset the source to OV Table 3 2 DC Voltage Test Display Reading 001 V DC 3 488 to 3 512V DC 3 488 to 3 512V DC and within 2 counts of 3 5V reading 34 88 to 35 12V DC 348 8 to 351 2V DC 597 to 603V DC mV DC Test 3 15 1 Set the UUT function switch to mV DC and connect the DC Voltage Calibrator output to the VQ and COM input terminals of the UUT Note For autorange press and hold the push button for 1 second 2 Referring to Table 3 3 set the DC Voltage Calibrator to the voltage indicated in the steps Verify that the UUT display reading is within the limits shown 3 Reset the source to OV 3 9 76 Service Table 3 3 mV DC Voltage Test A Display Reading 350 MV 348 8 To 351 2 MV DC SHORT 0 05 MV DC 35 MV 34 84 35 16 MV DC 35 mV 34 84 to 35 16 mV DC AC Voltage Test 3 16 Warning Injury hazard Connect the ground common low side of the AC calibrator to common on the UUT 1 Set the UUT function switch to V AC and connect the AC Calibrator to the VO and COM input termina
4. 76 Service 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 1 4 Display Response Time of Digital Display Operating Temperature Storage temperature Temperature Coefficient Relative Humidity Altitude Storage Battery type Battery Life Continuity Beeper Vibration Drop Enclosure Size HxWxL Weight EMC Surge Protection Safety Certifications Introduction and Specifications Specifications Table 1 1 Specifications Digital 4000 counts updates 4 sec Analog 63 segments updates 40 sec Frequency 9 999 counts Capacitance 9 999 counts Vac lt 1 5s for upscale only 1s Q 1s to 40 2s to 4 MQ 10s to 40 MQ 0 C to 55 C 40 C to 60 C 0 1 x specified accuracy lt 18 C or 28 C 90 0 C to 3
5. 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 76 Service 113f eps 10241 FLUKE 76 1603 27
6. 5 125W 200PPM 1206 RES MF 10K 0 1 125W 25PPM 1206 RES MF 90K 0 1 125W 25PPM 1206 RES CERM 510 5 125W 200PPM 1206 RES CERM 56 2K 1 125W 100PPM 1206 RES CERM 205K 1 125W 100PPM 1206 RES VAR CERM 100K 25 RES CERM 536K 1 125W 100PPM 1206 RES CERM 280K 1 125W 100PPM 1206 RES CERM 68 1K 1 125W 100PPM 1206 RES VAR CERM 100 25 0 2W RES CERM 620K 5 125W 200PPM 1206 RES CERM 82K 5 125W 200PPM 1206 RES CERM 33K 5 125W 200PPM 1206 107409 944301 107638 867593 876545 720979 876180 821579 944301 867580 747287 740563 904636 747261 867262 912451 944645 821637 742676 912469 820910 832477 742684 887179 107398 876177 810630 836387 836387 876321 877076 745992 107375 746388 106382 740548 740548 106366 106374 746388 831305 769836 912493 845420 886833 851845 106937 811919 811794 746669 Be 2 24 2 2 2 2 20 NN A DY BH WAND ABA DY A DY BH BR List of Replaceble Parts Table 4 2 A1 Main PCA cont Reference Description Tot Qty Designator RES CERM 107K 1 125W 100PPM 1206 875224 RES CERM 2 2K 5 125W 200PPM 1206 746479 THERMISTOR POS 1 1K 20 25 C 867192 VARISTOR 910 10 1 0MA 876193 VARISTOR 910 10 1 0MA 876193 VARISTOR 910 10 1 0MA 876193 DIODE RECT BRIDGE BV 50V 10 1A 912456 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
7. 1 99 4 to 100 60 plus O reading 995 to 1 005 9 95 to 10 05 kQ 99 5 to 100 5 kQ 995 to 1 005 MQ 9 87 to 10 13 MQ Decades of 1 9 189 0 to 191 00 plus O reading 1 891 to 1 909 kQ 18 91 to 19 09 kQ 189 1 to 190 9 kQ 1 891 to 1 909 MQ 18 78 to 19 22 MQ Decades of 3 5 348 4 to 351 60 plus O reading 3 485 to 3 515 kQ 34 85 to 35 15 kQ 348 5 to 351 5 kQ 3 485 to 3 515 MQ 34 62 to 35 38 MQ 76 Service Capacitance Test 3 19 1 Set the UUT function switch to Q 1 and connect the Decade Capacitor output to the and COM input terminals of the UUT Note To enter capacitance press and hold the push button for 2 seconds 2 Referring to Table 3 7 set the Decade Capacitor for the output indicated in the steps Verify that the UUT display reading is within the limits shown Table 3 7 Capacitance Test t e 100 nF open no test leads 0 to 0 50 nF Display Readings 1000 nF open no test leads 0to0 5n 1000 nF 800 nF 784 6 to 815 4 nF 10 uF 1 1 uF 1 077 to 1 123 uF Conducting performance tests of the 400Q 4kQ 40kQ and 10uF ranges verifies that the discrete and integrated circuitry needed to support the other capacitance ranges are working within specifications Therefore these tests indirectly verify that the meter will meet specification in the 10uF 100uF 1000uF and 10 000uF ranges Continuity Test 3 20 1 Set the UUT function switch to 400 1 2 Referring to Table 3 8 ap
8. A voltage level proportional to the unknown input signal charges integrator capacitor C11 for an exact amount of time This capacitor is then discharged by a reference voltage of opposite polarity The discharge time which is proportional to the level of the unknown input signal is measured by the digital circuits in U4 and sent to the display Basic timing for the a d converter is defined as a series of eight integrate and read de integrate cycles followed by a 40 ms autozero phase However the 40 MO capacitance overload recovery autoranging and Touch Hold modes all require variations from the basic timing Capacitance Measurements 2 12 Refer to Figure 2 5 100 nF Range Simplified Schematic Capacitance measurements to 10 000 uF are made by measuring the charge required to change the voltage across the unknown capacitor from zero to the system reference voltage This technique is referred to as a ballistic type of measurement the configuration of which is the same as for Ohms The unknown capacitor is discharged through the 1 0001k Ohm resistor of Z1 then charged during the a d converter integrate cycle through the appropriate Z1 resistor The voltage drop across the Z1 resistor is integrated by the a d converter During the a d read cycle the charge is held on the capacitor and a count is accumulated The microcomputer calculates a display value from the latched count the capacitor is discharged and the cycle repeats Frequency
9. C10 C11 C13 C19 C14 C21 C15 C16 C17 C18 C20 C31 C33 C34 C35 CR1 CR4 L1 01 92 Q3 Q9 Q12 Q4 Q5 R1 R2 R3 R4 R17 R18 R23 R5 R6 R7 R8 R12 R9 R59 R10 R11 R13 R29 R60 R61 R64 R14 R15 R16 R19 R20 R25 R37 R21 R22 R38 R24 R28 R34 R35 R44 R57 Table 4 2 A1 Main PCA Description CAP POLYES 0 01UF 10 630V CAP CER 270PF 1 50V C0G 0805 CAP CER 75PF 5 COG 0805 CAP TA 15UF 20 10V 3528 CAP TA 22UF 20 6V 6032 CAP POLYCA 0 027UF 10 63V CAP TA 0 47UF 20 25V 3216 CAP POLYPR 0 022UF 10 63V CAP CER 270PF 1 50V COG 0805 CAP TA 47UF 20 10V 7343 CAP CER 0 1UF 10 25V X7R 1206 CAP CER 22PF 10 50V COG 1206 CAP CER 3 3PF 0 25PF 1500V C0J CAP CER 0 01UF 10 50V X7R 1206 CAP TA 4 7UF 20 10V 3528 DIODE SI 100 PIV 1 AMP SURFACE MOUNT FERRITE CHIP 60 OHM 100 MHZ 1806 TRANSISTOR SI NPN SELECT IEBO SOT 23 TRANSISTOR SI NPN SMALL SIGNAL SOT 23 TRANSISTOR SI NPN SMALL SIGNAL SOT 23 TRANSISTOR SI PNP 50V 0 2W SOT 23 TRANSISTOR SI P CHAN SOT 23 TRANSISTOR SI PNP SMALL SIGNAL SOT 23 TRANSISTOR SI PNP SELECT ICER SOT 23 RES MF 450 10 100PPM FLMPRF FUS RES CERM 1M 1 2W 100PPM RES CERM 1 5K 1 125W 100PPM 1206 RES CERM 1M 1 125W 100PPM 1206 RES WW 9 99 0 25 1W 50PPM RES WW 0 010 0 25 1W 100PPM RES CERM 1K 5 125W 200PPM 1206 RES CERM 750 2 2W 100PPM MELF RES CERM 510 5 125W 200PPM 1206 RES MF 51 1K 0 1 0 1W 25PPM 0805 RES CERM 100K
10. Manual P N 106275 The performance tests are recommended as a preventive maintenance tool to verify proper instrument operation A one year calibration cycle is recommended to maintain the specifications given in Section 1 of this manual Recommended Equipment 3 2 Test equipment recommended for the performance tests and calibration is listed in Table 3 1 Ifthe recommended equipment is not available instruments with equivalent specifications may be used Table 3 1 Recommended Test Equipment Equipment Minimum Specifications Recommended Model DMM Calibrator DC Voltage 0 600V Fluke Models 5100B 5101B plus Transconductance Accuracy 05 5102B 5700A 5220A or or Power Amplifier AC Voltage 0 600V Fluke Models 5500A 5700A Accuracy 0 2 5725A Frequency 45 Hz 20 kHz DC mA 0 35 mA DCA 0 10A Accuracy 0 1 AC mA 0 35 mA ACA 0 10A Accuracy 0 3 Frequency 45 Hz 1 kHz Function Generator Sinewave voltage 0 1V rms Philips 5190X Frequency 1 Hz 20 kHz Frequency Accuracy 002 Decade Resistor Resistance 0 35 MQ General Resistance Accuracy 05 RDS 77B Decade Capacitor Capacitance 100 pF 1 1 uF GenRad 1412 BC Accuracy 0 5 76 Service 3 4 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 c
11. Measurements 2 13 A voltage comparator is used for both signal detection in frequency mode and threshold detection in continuity mode In frequency mode digital pulses from the voltage comparator are routed to the counter Pressing the range push button while in frequency mode causes a range change in the primary function ac volts that may change the sensitivity 2 7 76 Service BUFFER 7 INTEGRATOR COMPARATOR 7 y REFER TO WAVEFORM INPUT __1 VOLTAGE u COMPARE A D e 9 4 CONTROL 4 5 VOLTAGE 4 REFERENCE 131 072 kHz 2 v CLOCK SWITCH AUTO INTE READ READ NO ZERO GRATE INPUT INPUT HO MICROCOMPUTER 1 ON 2 ON ON ON 3 ON 4 ON 5 ON ON ON INTEGRATE ZERO RAMP READ HOLD BASELINE Ze 167 7 zu AUTO ZERO 167 gt 8 33 lt REPEAT y mS l SAMPLE 1 SAMPLE 2 SAMPLE 8 8 MINOR CONVERSIONS 200 mS 1 FULL RESOLUTION CONVERSION AS SEEN AT INTEGRATOR OUTPUT l4f eps Figure 2 4 A D Converter Microcomputer Control 2 14 A microcomputer integrated within U4 controls the various instrument functions and drives the display The position of the rotary switch S1 is decoded by the microcomputer from the three inputs F0 F1 and F2 All function modes input ranging signal routing active filter enable a d timing and mode are cont
12. Replacement iei da 3 4 Test tee m orn et s eee eges 3 4 Fuse Replacement ioi oue bes 3 6 PCAREMOVAL en es en Beer nen nn 3 6 Display Access ei neh 3 7 Cleaning ei tace dt reete 3 8 Performance Tests cte dte is 3 8 ehe eave 3 8 Display Testa 3 9 DE Voltage Tit AA id 3 9 np OMIT 3 9 AG Voltage Test za sera epe rt eter S 3 10 Frequency Test dz 3 10 OMS Test aset a Meo eiie eee 3 11 Capacitance Fest order ie Dee det grietas el 3 12 Continuity IC dia 3 12 DE and AG Current Testine parene 3 12 Calibration 3 13 Troubleshooting 2 1 3 14 Surface Mount 3 14 Power Supply Related Troubleshooting sss 3 15 Common Shunt Regulator Troubleshooting sss 3 15 Vdd Series Regulator Troubleshooting esses 3 16 76 Service 3 2 Maintenance 3 Introduction Warning These service instructions are for use by qualified personnel only To avoid electric shock do not perform any servicing other than that contained in the Users or Service manual unless you are qualified to do so 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
13. SPRING DETENT MP26 SHOCK ABSORBER MP27 DECAL CASE TOP MP28 LABEL WINDOW MP31 HOLSTER amp FLEXSTAND ASSY YELLOW MP32 TEST LEADS MP54 SPACER SNAP NYL 125 MP57 INSULATOR MP58 SPACER FOAM MP 59 1 C SHIELD 1 SWITCH ROTARY 2 SWITCH MOMENTARY YELLOW S4 CONTACT ANNUNCIATOR USERS MANUAL INTERNATIONAL PRINTED MATL WARRANT CARD FLUKE 70 20 SERIES QUICK REFERENCE CARD NOTES 1 TO ENSURE SAFETY THESE PARTS MUST BE INSTALLED 2 NON PROCURABLE ORDER NEXT HIGHER ASSEMBLY PN 104067 3 SEE FIGURE 3 3 TOP SHIELD INSULATOR REPLACEMENT 614487 107409 830828 820829 519116 821140 448456 733410 642959 659524 707190 642967 654228 104106 640565 103887 103892 646653 885848 649632 885843 646661 646679 428441 103884 844340 890298 855742 106416 104059 104098 104163 885876 890280 106275 897806 106278 List of Replaceble Parts 76 Service en ob 9 or gz 3 LL MP57 A1 SI iM A Y Sp al PLT Y IZA 9 Za KAS 2 ERU A MEN T fe POSE 03 W 0 PEO Z SEE VIEW B 4 a PB RAS SEE VIEW 111 Figure 4 1 76 Final Assembly 4 6 View A gt S 17 ESP A ID View C C List of Replaceble Parts Fluke 76 T 8 B 2 of 2 Figure 4 1 76 Final Assembly cont 112f eps 4 4 7 76 Service Reference Designator C1 C2 C4 C12 C30 C3 C5 C7 C6 C8 C9
14. 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 e 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 e 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 e 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 e Printed Circuit Assembly The term pca is used to represent a printed circuit board and its attached parts e 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 e User Notation
15. input terminal to the 40 mA input terminal The display should read between 10 Ohms and 12 Ohms This procedure tests F1 1A 600V If the display reads a high resistance or OL overload replace the fuse and test again If the display reads any other value further servicing is required Mask Bracket j m 2 p LCD my LCD Bracket u Conn LCD PCA ILC Shield LA LCD Assembly Maintenance Operator maintenance Case Top Insulator Fuse 15A Shield Top Screw 14 14 250 Battery 9V Knob Assembly Figure 3 1 Assembly Details lef eps 3 76 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 3 Install a new fuse of the same size and rating Make sure the new fuse is centered in the fuse holder 4 Ensure that the case top rotary switch and circuit board switch are in the OFF position 5 Reinstall the top case and the four Phillips screws into the bottom case PCA Removal 3 8 1 The 15A fuse F2 must be removed to access the screw that holds the pca to the case bottom refer to Fuse Replacement above 2 Carefully pull back the bottom shield tab on the top shield as shown in Figure 3 2 Be 3 6 sure n
16. 0 C 75 30 C to 40 C 45 40 C to 50 C 35 50 C to 55 C Operating 2 000 meters 12 000 meters 9v NEDA 1604A or IEC 6LR61 500 hrs typical with alkaline 4096 Hz Per MIL T28800E Class III Sinusoidal Non Operating ANSI ISA S82 01 1994 and EN61010 1 1993 1 meter drop to hardwood on concrete Conforms to IP 40 Per IEC 529 2 8 cm x 7 5 cm x 16 6 cm 1 12 in x 2 95 in x 6 55 in 12 oz 340g EN 50081 1 EN 50082 1 6 KV peak per IEC 1010 1 1990 09 and ANSI IEEE C62 41 1991 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 locations lt 600V UL3111 CSA CAN C22 2 No 1010 1 92 and EN61010 part 1 1993 C and CSA UL and TUV pending 1 76 Service Table 1 1 Specifications cont Burden Voltage Range Resolution Accuracy Typical 400 0 mV is Hz to 1 kHz 4 000V 40 00V 400 0V 600V Not Applicable 3 1 Not Applicable 3 1 3 1 40 00 mV 0 01 mV 0 3 5 Not Applicable 400 0 mV 0 1 0 3 1 400 00 0 10 0 4 2 4 000 kQ 0 001 kQ 0 4 1 40 00 kQ 0 01 kQ 0 4 1 Not Applicable 400 0 kQ 0 1 KQ 0 4 1 4 000 MQ 0 001 MQ 0 4 1 40 00 MQ 196 3 Capacitance 99 99 nF 999 9 nF 9 999 uF Not Applicable 99 99 uF 999 9 uF 1 9 2 9999 uF 10 Typical f f setya YT 400 5 Typical Duden 4000 5 Typica
17. 1 1111 MQ Z1 3 and 9 996 MQ provide 1 MQ reference resistor The 4 MQ and 40 MQ ranges use the 9 996 MQ resistor alone 2 5 76 Service OHMS AND CAPACITANCE VOLTAGE SOURCE lx y e LOW me REFERENCE A D RESISTOR INTEGRATE 2 3 REFERENCE zi 2 71 Z VAner 9 996M 1 0001k I_ R13 e ANN gt HIGH R1 R8 R12 RTI ACTIVE 5 ANN FILTER HIGH A D UNKNOWN VR UNKNOWN RESISTOR R e gt LOW VRx IxeRx Rx VRREF IxeRREF RREF I3f 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 The dc voltage is routed to the active filter and a d converter inside U4 Ac voltage is routed to the ac buffer ac converter U2 active filter and a d converter The 4 mA and 4A dc ranges use the a d converter is 40 mV range The 4 mA ac and 4A ac ranges use the x10 ac buffer range Continuity Low Ohms and Diode Test Functions 2 10 Q13 provides the source current for the continuity low Ohms and diode test functions Inputs are sensed through R2 R2 and R62 form a 10 1 divider for measuring voltages in diode test and the 8 kQ range of low Ohms The continuity function is the 400 Ohm range of low Ohms and uses a comparator to turn on the beeper when the input is lt 20 Ohms Low Ohms makes voltage measurem
18. 2 2 4V Range Simplified Schematic This input is connected to the high end of the 9 996 MQ resistor Z1 1 through R1 R8 R12 and RTI If the AC volts function is selected dc blocking capacitor C1 is also connected in series In AC the signal is routed to U2 through the AC buffer in U4 If the DC volts function is selected C1 is shorted by S1 contacts 6 and 7 Internal switches connect the 9 996 MQ and 1 1111 MQ resistors Z1 2 and 3 The low end of the 1 1111 MQ resistor Z1 7 is connected to the COM input through S1 contacts 11 and 12 This produces the divide by 10 ratio needed for the 400 mV ac 4V ac and 4V dc ranges The 400 mVac and 4Vac ranges require frequency compensation which is supplied by C20 not shown in Figure 2 2 For the 40V range internal switches connect the Z1 4 101 01 kQ resistor to provide a divide by 100 ratio In the 400V range Z1 5 10 01 kQ produces a divide by 1 000 ratio And in the 600V range the 71 6 1 0001 resistor provides a divide by 10 000 ratio Theory of Operation 2 Circuit Descriptions S1 DC mV R2 3 2 Z 1 vw B 34 c 7 1 6 ANN R1 R8 1 R12 RTI 21 9 996M 2 4v 59 V DC 7 5 ACTIVE R19 FILTER HIGH 713 9 996M_ A D 11 46 81 CONVERTER U2 12 COM 4 I LOW l2f eps Figure 2 2 4V Range Simplified Schematic Ohms Functions 2 8 When the 400 Ohm range is selected internal switch
19. 7 of Power Supply Components Q10 Power supply startup device G provides Q8 base startup current Q10 is always off during meter operations VR1 provides the system reference voltage It is used for the a d converter reference and as a reference for both supply regulators Vdd Vdd regulator compensation and bypass compensation and bypass Common regulator compensation and Vss bypass an em HN 3 15 76 Service Table 3 12 Voltage Levels 3 0 0 2 3 2 0 2 6 1 battery at full 9V charge 3 2 battery at low charge of 6 0V 1 23 0 04 1 23 0 15 0 X 0 15 2 2 to 1 7 referenced from VSS 1 07 to 85 referenced from VSS Make the following tests 1 Check for 1 23V 40 mV Vrefh at the cathode of VR1 If Vrefh is not correct check VR1 R19 R20 R21 and R44 carefully If Vrefh is still incorrect U4 is bad 2 If Vrefh is correct measure the voltage at U4 pin 6 PS1 If Vpsl is not equal to 0 15V check R37 and R38 If Vpsl is still at an incorrect voltage U4 is bad 3 Check the bias generator circuit With the exception of resistor R35 620 kQ the bias generator which sets the bias level for all U4 analog circuitry is internal to U4 A problem with this circuit could cause the on chip power supply op amps to fail Measure the dc voltage between U4 pin 8 Vbias and DGND If 0 2V Vbias lt 0 2V the bias generator is okay If Vbias is not correct check R35 If Vbias i
20. 7 2 14 Microcomputer Control enne 2 8 2 15 Peripherals to U4 sea en e t E peut 2 9 2 16 AG Buffer er eR Ae epe qd 2 10 2 17 AG Converter uec p ee a Pet id ote eid 2 10 2 18 Active Filter iud eae aa 2 10 2 19 A D Converter ns an eerte isa 2 10 2 20 Beeper rese EE ER EE ERR E ERE ER e NS 2 10 2 21 Power Supply pee ttt eere re eene 2 10 2 22 Display enge lie 2 11 3 EEE ER 3 1 32 3 3 3 2 Recommended Equipment esses 3 3 3 3 Operator maintenance eensenseensenseensnnenneennnnnnnnenneensennssns ns 3 4 76 Service 3 4 Case Disassembly 3 4 3 5 Battery Replacement sess 3 4 3 6 deis 3 4 3 7 Fuse Replace tii dsd 3 6 3 8 era Nd 3 6 3 9 Display Access cooooonocononononoconoconoconoconncn 3 7 O ON 3 8 3 11 Performance Tot tn cete Hee eet ee e id 3 8 3 12 3 8 3 13 Display Lest iue nee panel 3 9 3 14 DC Voltage TeSst eon eite cer i eere er erre ede ee DE eh ee 3 9 3 15 MVDC arat aet Ae e ae ias 3 9 3 16 AC Voltage Test I ai s 3 10 3 17 Erequency Test etui ee endete 3 10 3 18 Ohms Test Em 3 11 3 19 Capacitance ad 3 12 3 20 Continuity Tes
21. Any special notes 1 e factory selected part Caution A symbol indicates a device that may be damaged by static discharge How to Obtain Parts 4 2 Electrical components may be ordered directly from the Fluke Corporation and its authorized representatives by using the part number under the heading FLUKE STOCK NO In the U S order directly from the Fluke Parts Dept by calling 1 800 526 4731 Parts price information is available from the Fluke Corporation or its representatives Prices are also available in a Fluke Replacement Parts Catalog which is available upon request In the event that the part ordered have been replace by a new or improved part the replacement will be accompanied by an explanatory note and installation instructions if necessary To ensure prompt delivery of the correct part include the following information when you place an order Instrument model and serial number Part number and revision level of the pca containing the part Reference designator Fluke stock number Description as given under the DESCRIPTION heading Quantity Manual Status Information 4 3 The Manual Status Information table that precedes the parts list defines the assembly revision levels that are documented in the manual Revision levels are printed on the component side of each pca Newer Instruments 4 4 Changes and improvements made to the instrument are identified by incrementing the revision letter marked on the affec
22. Hz 5 counts Input Range Normal Mode Rejection Common Mode Rejection Ratio 1 Unbalance Overload Protection Function Input Impedance Nominal 600V dc 600V ac rms sine 600V dc 600V ac rms sine gt 10 lt 100 pF 10 MQ lt 100 pF gt 120 dB at dc 50 Hz or 60 Hz gt 120 dB at dc 50 Hz or 60 Hz gt 60 dB at 50 Hz or 60 Hz gt 60 dB at 50 Hz or 60 Hz gt 60 dB dc to 60 Hz 600V dc 600V ac rms sine gt 10 lt 100 pF ac coupled Short Circuit Current Full Scale Voltage Open Circuit Test Voltage lt 4 0 MQ lt 450 mV dc lt 40 MQ 600V dc lt 1 3V de lt 1 3V dc lt 500 uA 600V rms sine lt 3 1V de 2 45V de Diode Test 600V dc 600V rms sine 800 UA typical 107 V Hz max 76 Service 1 8 Chapter 2 Theory of Operation 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 2 10 2 11 2 12 2 13 2 14 2 15 2 16 2 17 2 18 2 19 2 20 2 21 2 22 Title Page Iritroduction ET 2 3 Functional Block nn 2 3 Circuit Descriptions Banana 2 3 Input Overload 2 4 Rotary Knob Switch tiun eerta er eee p reete esos 2 4 Input Signal Conditioning Circuits essere 2 4 Volts Functions iii err are a Bst 2 4 Ohms FUNCHONS ueteri nee ge 2 5 Current Fu
23. True RMS Multimeter Service PN 103926 Date September 1995 O 1995 Fluke Corporation Inc All rights reserved Printed in U S A K E LJ All product names are trademarks of their respective companies 76 Service Table of Contents Chapter Title Page 1 Introduction and Specifications eee 1 1 A O ER p E te OR THEE PR CURAR 1 3 1 2 Organization of the Service Manual see 1 3 1535 sates a sa 1 3 1 4 Specifications miaa tet E et eet teri Eee eis 1 4 2 Theory of Operation icon tada 2 1 A eld 2 3 2 2 Functional Block Description esses 2 3 2 3 Circuit Descriptions iii aei eie eq dice 2 3 2 4 Input Overload Protection sss 2 4 2 5 Rotary Knob Switch iiie cere repe te eren 2 4 2 6 Input Signal Conditioning Circuits sss 2 4 2 7 Volts FUNCHONS 2 4 2 8 Ohms Functions er dosis 2 5 2 9 Current F nctions elite neri pe eee dis 2 6 2 10 Continuity Low Ohms and Diode Test Functions 2 6 2 11 Analog Section of Integrated Multimeter IC UA 2 7 2 12 Capacitance 0 4 2 7 2 13 Frequency 2
24. a fault to the component level before replacing a part Surface mount assemblies are probed from the component side The probes should contact only the pads in front of the component leads With the close spacing involved ordinary test probes can easily short two adjacent pins on an SMT IC Due to the limited space on the surface of the circuit board component locations are not labeled Therefore this service manual is a vital source for component locations and values Figures provided in Section 5 of this manual provide component location information Also remember that SMT components are not individually labeled keep any new or removed component in a labeled package Maintenance 3 Troubleshooting Surface mount components are removed and replaced by reflowing all the solder connections at the same time Special considerations are required e 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 e Rework surface mount assemblies with wire solder rather than with solder paste Multicore X38B no clean wire solder is recommended e A good connection with SMT requires only enough solder to make a positive metallic contact Too much solder causes bridging while too little solder can cause weak or open solder joints With SMT the anchoring effect of the through holes is missing solder provides the only means of mechanical fastening Therefore the pca must b
25. and top cover and adjust R21 to obtain the proper display 6 Set the UUT to the V function and set the source for an output of 3 500V ac at 100 Hz 7 The UUT should display 3 500V ac 0 002 If necessary remove the four case screws and top cover and adjust R34 to obtain the proper display 3 13 76 Service R21 ADJUST FOR 3 500V DC 0 001 R34 ADJUST FOR 3 500V AC 0 002 110f eps Figure 3 5 Calibration Adjustment Points Troubleshooting 3 23 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 Surface Mount Assemblies 3 24 The 76 multimeter incorporates surface mount technology SMT on the printed circuit assembly pca Surface mount components are much smaller than their predecessors with leads soldered directly to the surface of a circuit board no plated through holes are used Unique servicing troubleshooting and repair techniques are required to support this technology The information offered in the following paragraphs serves only as an introduction to SMT We do not recommend that you attempt a repair based only on the information presented here Since sockets are seldom used with SMT shotgun troubleshooting cannot be used isolate
26. d current to U4 at 10 mA during Ohms and continuity overloads Power supply regulation and system operation are maintained during any of these overloads The mA input is protected from overloads by F1 1A 600V while F2 15A 600V protects the A input Milliamp shunt resistor R5 is protected from overload currents below the F1 fusing level by the U1 and diode clamp network Rotary Knob Switch 2 5 Input signals are routed from the overload protection circuits to a double sided switch wafer This switch wafer provides the necessary connections to implement signal conditioning and function encoding for U4 Input Signal Conditioning Circuits 2 6 Each input signal is routed through signal conditioning circuitry before reaching multimeter chip U4 Incoming signals received through the input are routed to precision resistor network Z1 This divider network precisely scales the input for the various voltage ranges and provides precision reference resistors that are used for the Ohms and capacitance functions Input divider Z1 is used in two modes series and parallel In volts functions a series mode provides four divider ratios In the Ohms function a parallel mode provides five reference resistors During the following discussion refer to schematic and signal flow diagrams in Section 5 Volts Functions 2 7 In volts functions signal flow for input divider Z1 begins with a voltage that appears at the input See Figure
27. e display drives the display and controls most analog and digital logic functions The push button initiates various operating modes for the meter Output from the digital section can be viewed on the liquid crystal display LCD and is audible through the beeper INPUTS v INPUT PROTECTION Y INPUT SIGNAL CONDITIONING 1 U1 INTEGRATED MULTIMETER IC ANALOG i POWER 45 SECTION SUPPLY 4 gt MICROPROCESSOR 4 a pt 1 LCD Hf eps Figure 2 1 Block Diagram Circuit Descriptions 2 3 The following paragraphs discuss the functional blocks in Figure 2 1 Consult the schematic diagrams located in Section 5 for additional circuit details not provided in this section 2 3 76 Service 2 4 Input Overload Protection 2 4 The 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 RTI The presence of an extremely high energy signal causes R1 5000 2W a fusible resistor to open Thermistor rises to a high impedance during a sustained voltage overload in the Ohms or continuity mode Transistors Q1 and Q2 form a voltage clamp network This clamp limits the overloa
28. e especially clean to ensure a strong connection An oxidized pca pad causes the solder to wick up the component lead leaving little solder on the pad itself Power Supply Related Troubleshooting 3 25 The two regulator circuits are interrelated a malfunction in either the common regulator or the Vdd regulator may cause a problem in the other Refer to Tables 3 11 and 3 12 for descriptions of power supply components and voltage levels To isolate the problem regulator circuit disconnect the battery and drive Vdd Vss 6 2V with a power supply This procedure tests the common regulator independently of the Vdd regulator Now check for Vdgnd Vss 3 2V 0 2V If this test is successful the problem lies with the Vdd regulator refer to Vdd Regulator Troubleshooting later in this section If this test is not successful the problem lies with the common regulator continue with the Common Regulator Troubleshooting below Note that if the common regulator works or has been repaired check both supplies with the 9V battery supply Common Shunt Regulator Troubleshooting 3 26 To troubleshoot the common regulator connect the power supply so that Vdd Vss and DGND digital ground are supplied from an external power supply This procedure overdrives the large on chip shunt transistors the bias current from the power supply ranges from 10 mA to 100 mA Refer to the schematic for a diagram of the common regulator Table 3 11 Functional a
29. ents of the input to provide the higher resolution readings of the 40 Ohm range Diode test also makes voltage measurements of the input A single beep sounds when the input is lt 0 75V a continuous tone sounds for inputs lt 60 mV 2 6 Theory of Operation 2 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 U4 uses the dual rate dual slope a d converter circuit shown in Figure 2 4 A D Converter For most measurements the basic a d conversion cycle is 25 ms for a rate of 40 measurements per second A single conversion at this rate is called a minor cycle sample Each minor cycle sample is used to provide updates at a rate of 40 per second for the fast response bar graph display and fast autoranging Eight minor cycle samples are necessary to accumulate data for displaying a full resolution 4000 count full scale measurement on the digital display A 40 ms autozero phase occurs following every eight sample sequence Therefore each digital display update requires 240 ms approximating four updates per second Basic a d conversion elements and waveforms are illustrated in Figure 2 4 A D Converter
30. er 1 Introduction and Specifications Title Page Introduction ai er E namen 1 3 Organization of the Service Manual sse 1 3 1 3 Specifications keen 1 4 76 Service 1 2 Introduction and Specifications 1 Introduction 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 e 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 e Section 2 Theory of Operation This section first categorizes instrument
31. erter readings The microcomputer can disable the filter completely or enable the filter fast response mode by shorting R17 and R18 with internal IC switches A D Converter 2 19 Precision resistor network Z1 8 9 and 10 connects to the three a d buffer integrator range resistors Z1 8 connects to 190 kQ for the 1V range de integrate Z1 9 connects to 166 for the 400 mV range Z1 10 connects to 16 for the 40 mV range Z1 11 is the summing node of the integrator circuit The autozero capacitor C10 stores op amp and comparator offsets The integrator capacitor is C11 The system reference voltage 1 23V is generated by VR1 and R44 The 1 000V reference voltage for the a d converter is supplied through U4 1 REFD This voltage is adjusted by R21 the dc calibration potentiometer in conjunction with R19 and R20 In addition to generating the a d reference the VR1 voltage is used for power supply reference voltage comparator offset generation and the Ohms and capacitance source voltage Beeper 2 20 Devices Q9 Q11 R57 R63 and R64 make up the beeper drive circuit Power Supply 2 21 The power supply consists of two regulators one shunt and one series which set Vdd at 3 0V and Vss at 3 2V for all battery voltages down to 6 5V The shunt common regulator sets Vdgnd Vss Vdgnd COM 0V and consists of an op amp and current shunt devices integrated on U4 Resistors R37 and R38 provide voltage division The se
32. es connect the resistor Z1 2 9 996 MQ to resistor Z1 6 1 0001 kQ See Figure 2 3 400 Ohm Range Simplified Schematic Then through switch contacts S1 6 7 and 9 these resistors form a reference resistor of 1 The source voltage is connected internally at both VO and V4 of U4 The current is routed through two parallel resistor s Z1 6 and Z1 2 1 0001 kQ and 9 996 into Sl at contacts 6 and 9 The signal then travels out of S1 at contact 7 through R1 R8 R12 and and to the input The signal then goes through the unknown resistance and back to the COM input The same current flows through the unknown resistance and the reference resistor The voltage dropped across the unknown resistance is sensed from the input jack through R2 and S1 contacts 2 and 3 to OVS of U4 The a d converter senses the voltage drop across the 1 kQ reference resistor through the low RRS of U4 through R13 and high VO and V4 points These two voltages are used by the a d converter to perform a ratiometric measurement Since the same current flows through the reference and unknown resistors the ratio of the resistance values is the same as the ratio of the voltage drops across them For the 4 range the 10 010 resistor Z1 5 used in parallel with the 9 996 MQ resistor Z1 2 forms a 10 kQ reference resistor For the 40 range 101 01 kQ Z1 4 and 9 996 MQ form a 100 reference resistor And for the 400 range
33. l Not Applicable 8kQ 10 Typical 1 1 1 E 1 1 0 3 1 El El 0 4 000 mA 0 001 mA H1 5 4 11 mV mA us Hz to 1 kHz 40 00 mA 0 01 mA 5 2 11 mV mA 4 0 001 1 5 4 t 0 03 V A 10 00A 0 01A 1 5 2 t 0 03 V A 4 000 mA 0 001 mA 5 5 11 mV mA 40 00 0 01 5 2 11 mV mA 4 0 001A 5 5 0 03 V A 10 00A 0 01A 5 2 0 03 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 400 and 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 lt duty cycle lt 75 lt 1 kHz 1 zi 0 0 0 0 1 6 Introduction and Specifications 1 Specifications Table 1 1 Specifications cont Frequency Counter Sensitivity and Trigger Level 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
34. ls 2 Set the AC Calibrator for the output given in Table 3 4 and verify that the UUT display reading is within the limits shown Note When the input is open in the V AC function it is normal for the meters to read some counts on the display This is due to ac pickup in the ac amplifier when the ac amplifier is unterminated Table 3 4 AC Voltage Test AAA Display Reading 19 2 to 20 8 mV AC 343 0 to 357 0 mV AC 343 0 to 357 0 mV AC 194 to 206V AC 3 432 to 3 569V AC 3 432 to 3 569V AC 34 46 to 35 55V AC 344 6 to 355 5V 589 to 611V AC 2 3 4 5 6 7 8 9 Frequency Test 3 17 1 Set the UUT function switch to Hz and connect the Function Generator output to the and COM input terminals of the UUT 2 Referring to Table 3 5 set the Function Generator for the output indicated in the steps Verify that the UUT display reading is within the limits shown Maintenance Frequency Test Table 3 5 Frequency Test tae ae Display Reading 709 8 Hz to 8002 Fe Ohms Test 3 18 1 Select the Ohms function on the UUT 2 Connect the Ohms Calibrator or Decade Resistor to the VQ gt and COM input terminals of the UUT 3 Referring to Table 3 6 set the Decade Resistor or Ohms Calibrator to the resistance value indicated in steps through 7 Verify that the display reading is within the limits shown Table 3 6 Resistance Test Input Range Display Readin Decades of
35. mbly before washing the pca and do not reinstall it until the pca is completely dry Do not use detergent of any kind for cleaning the pca Do not remove lubricants from the switch when cleaning the pca Clean the instrument case with a mild detergent and water The pca may be washed with isopropyl alcohol or deionized water and a soft brush Remove the display assembly and fuses before washing and avoid washing the switch if possible Dry the pca with clean dry air at low pressure then bake it at 50 C for 24 hours Performance Tests 3 11 Performance tests are recommended for incoming inspection periodic maintenance and for verifying the specifications in the 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 1 Allow the UUT to stabilize to room temperature 23 C 5 C 73 F 9 F 2 Check the fuses and battery and replace them if necessary Refer to the battery and fuse replacement procedures in this section Warning To prevent fire install fuses in accordance with the rating shown on the back of the meter Warning Connect the ground common low side of the DC calibrator to common on the UUT Maintenance 3 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
36. nctions d et teer ais 2 6 Continuity Low Ohms and Diode Test Functions 2 6 Analog Section of Integrated Multimeter IC U4 2 7 Capacitance 2 7 Frequency 2 7 Microcomputer Control eere 2 8 Peripherals to UA reU retis ann 2 9 ACB it see Rs eie intet gene 2 10 AC Converter iu iia 2 10 Active Flat ee eee eh 2 10 A D Gonvertet iie iii 2 10 hide H 2 10 Power Supply ti ee ete e p n tes Benefit 2 10 Display et et e eet et ree e deed 2 11 2 1 76 Service 2 2 Theory of Operation 2 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 Functional Block Description 2 2 The instrument is partitioned into analog and digital sections See Figure 2 1 Block Diagram The integrated multimeter chip U4 performs both analog and digital functions which are explained in more detail below The analog section of U4 contains the a d converter active filter frequency comparator analog signal routing range switching and power supply functions The digital section of U4 executes software functions formats data for th
37. ontamination with oil from the fingers handle the pca by the edges or wear gloves PCA contamination may not cause immediate instrument failure in controlled environments Failures typically show up when contaminated units are operated in humid areas Case Disassembly 3 4 Use the following procedure to disassemble the case 1 Set the function switch to OFF and disconnect the test leads if they are installed 2 Remove the four Phillips screws from the bottom case 3 Turn the meter face up grasp the top case and pull the top case from the meter Battery Replacement 3 5 The meter is powered by a single 9V battery NEDA 1604 6F22 or 006P Refer to Figure 3 1 and use the following procedure to replace the battery 1 Remove the top case as described under Case Disassembly 2 Lift the battery from the case bottom and install the new battery 3 Reinstall the top case and the four Phillips screws into the bottom case Fuse Test 3 6 Use the following procedure to test the internal fuses of the meter l 2 Turn the rotary selector switch to the E position Plug a test lead into VO 9L input terminal and touch the probe to the 10A input terminal The display should indicate between 0 1 and 0 5 Ohms This tests F2 15A 600V If the display reads OL overload replace the fuse and test again If the display reads any other value further servicing is required Move the probe from the 10A
38. ot to crease the tab Bottom Shield Tab Insulator Top Shield I7f eps Figure 3 2 PCA Removal Maintenance 3 Operator maintenance 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 Insulator Top View gt Side View Top Shield 3 I8f eps Figure 3 3 Top Shield Instulator Replacement Display Access 3 9 Caution To prevent contamination do not handle the conductive edges of the LCD interconnects If they are contaminated clean them with alcohol Refer to Figure 3 1 Nr A NS Remove the four Phillips screws from the back side of the pca Remove the LCD mounting bracket Insert a small screwdriver under the edges of the display holding Bracket and gently pry the bracket loose from the snaps Turn the bracket upside down to remove the LCD Before installing a new LCD make sure that all connector contact Points are clean 3 7 76 Service 3 8 Cleaning 3 10 Caution To avoid damaging the meter do not use aromatic hydrocarbons or chlorinated solvents for cleaning These solutions will react with the plastics used in the instruments Do not allow the LCD to get wet Remove the display asse
39. overload protection and input signal conditioning circuits other devices peripheral to U4 are needed to support the meter s features The active filter and a d converter circuits require off chip resistors and capacitors Digital drive and level 76 Service shifting circuits are needed for the beeper drive A voltage reference is generated separate from U4 and some discrete resistors and transistors support the power supply AC Buffer 2 16 The ac buffer can be configured for an ac gain of 1 or 10 R14 and R15 provide for the X10 gain C5 blocks any dc gain and provides a driven guard voltage The ac buffer drives the frequency comparator and the ac converter U2 AC Converter 2 17 The ac converter U2 is a monolithic true rms to dc converter The signal to the converter comes from the ac buffer The signal is first attenuated by the voltage divider 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 Active Filter 2 18 The active filter uses components R17 R18 C8 and C9 The active filter is a second order low pass filter with two poles at 5 9 Hz in normal mode It filters input signal noise and ac ripple from the ac converter yielding stable a d conv
40. ply a short as indicated Verify that the UUT display and beeper indicate as shown Table 3 8 Continuity Test 1 400Q short tone 2 4002 open OL DC and AC Current Test 3 21 1 Set the UUT Function switch to amps 2 Set the output of the Current Calibrator to standby and connect it to the 40 mA and Common input terminals of the UUT 3 Set the Current Calibrator to the output shown in Table 3 9 Verify that the UUT display reading is within the limits shown Maintenance 3 Calibration Table 3 9 DC and AC mA Test NOTE To enter AC mA press and hold button for 2 seconds 4 Set the output of the Current Calibrator to standby and connect it to the 10A and Common input terminals of the UUT Return the UUT to A DC 6 Set the Current Calibrator to the output shown in Table 3 10 and verify that the UUT display reading is within the limits shown Table 3 10 DC Amps Test LL Calibration 3 22 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 Setthe DC Voltage Calibrator to 0 volts 2 Select the V function on the meter 3 Connect the DC Voltage Calibrator to the VQ gt and COM input terminals of the UUT 4 Set the DC Voltage Calibrator for an output of 3 5V dc 5 The UUT should display 3 500V dc 0 001 If necessary remove the four case screws
41. r touch the probes to a voltage source when the test leads are plugged into the 10A or 40 mA input jack e avoid damage or injury never use the meter on unprotected circuits that exceed 14000 volt amps e Never apply more than 600V dc or ac rms sine between any input jack and earth ground e Always be careful when working with voltages above 60V dc or 30V ac rms Such voltages pose a shock hazard e Always keep your fingers behind the finger guards on the probe when making measurements e Always use a high voltage probe to measure voltage if the peak voltage might exceed 600V SYMBOLS MARKED ON EQUIPMENT h DANGER High voltage A Attention refer to the manual This symbol indicates that information about usage of a feature is contained in the manual Fuse 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 Chapt
42. ries Vdd regulator which sets Vdd Vdgnd is made up of another on chip op amp along with devices Q3 R24 and R25 Q3 is the series regulator element and R24 and R25 are for voltage sensing Capacitors C14 and C21 provide circuit compensation and Theory of Operation 2 Circuit Descriptions power supply decoupling for the shunt and series regulators respectively Q10 provides base drive for Q3 at turn on Voltage level information is presented in Table 2 1 Display 2 22 The liquid crystal display LCD operates under control of the microcomputer Segments are driven by the computer and displayed on the 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 3 0 0 2 3 2 0 2 6 1 battery at full 9V charge 3 2 battery at low charge of 6 5V 1 23 0 04 1 23 0 15 0 0 15 2 2 to 1 7 referenced from VSS 1 07 to 85 referenced from VSS 76 Service UY UY UY UY UY UY UY UY UY UY UY UY 59 UY UY UY UY UY UY UY U9 Uy UY Uy W Uy SED GER NDR oo Chapter 3 Maintenance Title Page Introductio series Bern ORE ones 3 3 Recommended Equipment esses 3 3 Operator maintenance sss enne 3 4 Case Disassembly i icc erri ttr edere 3 4 Battery
43. rolled by the microcomputer Theory of Operation 2 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 When a stable reading occurs the conversion is completed and the microcomputer generates and freezes the corresponding display The microcomputer now waits for a change in the signal to exceed a certain threshold then begins watching for a stable reading again Note that a reading 1s forced when the Touch Hold function is first selected Also open test lead signals generally do not update the display The SMOOTHING mode is another secondary software function In the Smoothing function the equivalent of eight readings are averaged resulting in a more stable display of noisy input signals OHMS AND CAPACITANCE VOLTAGE SOURCE IGHARGE CHARGE e gt LOW REFERENCE AID 5 RESISTOR INTEGRATE zi Z1 sty 9 996M 1 1111M CHARGE R13 ANN HIGH 6 1 2 71 7 1 0001k X DISCHARGE R1 R8 R12 RTI VW R2 ANN gt HIGH A UNKNOWN L vc r da CAPACITOR x gt LOW V I5f eps Figure 2 5 100 nF Range Simplified Schematic Peripherals to U4 2 15 In addition to input
44. s still wrong replace U4 4 Measure the ac voltage between Vdgnd and Vss If it is greater than 10 mV ac check C21 An open C21 causes common regulator instability The dc level may also be incorrect 5 Ifthe common regulator still does not work circuitry internal to U4 is bad Replace UA Vdd Series Regulator Troubleshooting 3 27 If a problem still exists after the common regulator troubleshooting continue with the following Vdd regulator troubleshooting Often a short or sneak current path causes power supply problems Refer to the schematic for a diagram of the Vdd Series Regulator Make the following tests 1 Measure the dc operating current from the 9V battery If the current is greater than 1 2 mA a sneak current path exists A sneak current path can be very difficult to find 2 First visually check for both solder bridges on U4 pins and other circuit board shorts Isolate the current path at the negative battery terminal Vbt The components connected to Vbt are C35 CR4 and R63 Remove these parts one at a time Measure I bat after each removal to isolate the problem 1 Ifthe excess battery current stops after removing R63 either R63 R57 Q9 or Q11 may be bad 2 Ifthe extra current is still present with all parts removed remove Q10 and check for excess battery current If I bat is now correct Q10 is bad If I bat 1s still excessive U4 is probably at fault If the power supply is not working bu
45. t ed ette ee diete esta ei Ee Et Yd 3 12 3 21 DC and AC Current Test eene nennen 3 12 3 22 Calibrationzis cocta edited e denne 3 13 3 23 Tro bleshoothg aonais doge ani da 3 14 3 24 Surface Mount Assemblies 3 14 3 25 Power Supply Related Troubleshooting sss 3 15 3 26 Common Shunt Regulator Troubleshooting 3 15 3 27 Vdd Series Regulator Troubleshooting 3 16 List of Replaceble Parts una 4 1 4 1 aer tee Rer t re nee na 4 3 4 2 How to Obtain 4 3 4 3 Manual Status Information ennn nen 4 3 4 4 Newer Instr ments irre a a IT 4 3 4 5 Service Centers aid ee e iii 4 4 Schematic DidGrams oli 5 1 List of Tables Table Title Page 1 15 Specifications it ii 1 5 2 1 Typical Voltage Levels and Tolerance Refered to Common 2 11 3 1 Recommended Test 2 2 1 21 2 nennen nnn 3 3 3 2 DG Voltage Testa een etie eer ie 3 9 3 3 MV DE Voltage Lestat 3 10 3245 AG Voltage Testi ec Re den Dee Pas engere 3 10 3 5 Frequency Tdi A IHRES ST en 3 11 3 6 Resistance Test u a ad it 3 11 3 7 Capacitance T st aint oa 3 12 3 8 COMUNI Vestir ae e A per eid 3 12 39 AC mA TE Ai 3 13 3 10 DG Amps Test antun
46. t battery current is normal perform the following tests 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 Measure the ac voltage between Vdd and Vss An unstable Vdd regulator can be caused by an open 14 If the voltage is greater than 10 mV ac check C14 The de level may also be incorrect For a final check of U4 remove the battery and supply Vdd 3 0V Vdgnd 0 and Vss 3 2V from an external power supply Measure the voltage at U4 pin 7 Vps0 If it does not equal 1 23V 0 15V check R24 and R25 carefully If Vps0 is still incorrect U4 is bad 3 76 Service How to Obtain Parts Manual Status Information esse 4 3 Newer Instruments Chapter 4 List of Replaceble Parts 4 1 76 Service 4 2 List of Replaceble Parts A 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 The parts lists give the following information Reference designator An indication ifthe part is subject to damage by static discharge Description Fluke stock number Total quantity
47. ted pca These changes are documented in a manual supplement which when applicable is included with the manual 4 3 76 Service Service Centers 4 5 A list of service centers is located at the end of this section ON Note ee Ni Cd 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 To ensure safety use exact replacement only Manual Status Information REF OR FLUKE REVISION OPTION ASSEMBLY NAME PART NO LEVEL NO 4 4 Table 4 1 76 Final Assembly Reference Designator Description BATTERY PRIMARY ALKALINE 9V 0 200MA C1 CAP POLYES 0 01UF 10 630V F1 FUSE 406X1 375 1A 600V FAST F2 FUSE 406X1 5 15A 600V FAST H1 SCREW PH P THD FORM STL 4 24 250 H2 SCREW PH P THD FORM STL 2 14 375 H3 SCREW PH P THD FORM STL 4 14 375 H7 SCREW PH P THD FORM STL 5 14 750 J1 4 RECEPTACLE INPUT LS1 AF TRANSD PIEZO 20 MM MP1 MP2 CONTACT 600V FUSE MP3 MP4 600 VOLT FUSE CONTACT MP6 CONTACT BATTERY MP7 CONTACT BATTERY MP9 SHIELD TOP PLATED MP10 FOOT NON SKID MP14 CASE BOTTOM MP15 SHIELD BOTTOM MP17 CASE TOP MP18 WINDOW LCD FLUKE 76 MP19 BRACKET LCD MP20 MASK BRACKET MP21 CONN ELASTOMERIC LCD TO PWB 1 900 L MP23 KNOB SWITCH MP24 SHAFT KNOB MP25

Service

Contents

Download Pdf Manuals

Related Search

Related Contents