Fluke 87 V/AN User manual

®
87 V/AN
Digital Multimeter
Service Manual
PN 2153596
July 2004
© 2004 Fluke Corporation, All rights reserved. Printed in USA
All product names are trademarks of their respective companies.
Limited permission for the U.S. Government to reproduce Fluke
Item #2153596 is provided pursuant to FAR 52.227.14 Alt. II.

LIMITED WARRANTY AND LIMITATION OF LIABILITY
Each Fluke product is warranted to be free from defects in material and workmanship under normal use and
service. The warranty period is one year and begins on the date of shipment. Parts, product repairs, and
services are warranted for 90 days. This warranty extends only to the original buyer or end-user customer of
a Fluke authorized reseller, and does not apply to fuses, disposable batteries, or to any product which, in
Fluke's opinion, has been misused, altered, neglected, contaminated, or damaged by accident or abnormal
conditions of operation or handling. Fluke warrants that software will operate substantially in accordance
with its functional specifications for 90 days and that it has been properly recorded on non-defective media.
Fluke does not warrant that software will be error free or operate without interruption.
Fluke authorized resellers shall extend this warranty on new and unused products to end-user customers
only but have no authority to extend a greater or different warranty on behalf of Fluke. Warranty support is
available only if product is purchased through a Fluke authorized sales outlet or Buyer has paid the
applicable international price. Fluke reserves the right to invoice Buyer for importation costs of
repair/replacement parts when product purchased in one country is submitted for repair in another country.
Fluke's warranty obligation is limited, at Fluke's option, to refund of the purchase price, free of charge repair,
or replacement of a defective product which is returned to a Fluke authorized service center within the
warranty period.
To obtain warranty service, contact your nearest Fluke authorized service center to obtain return
authorization information, then send the product to that service center, with a description of the difficulty,
postage and insurance prepaid (FOB Destination). Fluke assumes no risk for damage in transit. Following
warranty repair, the product will be returned to Buyer, transportation prepaid (FOB Destination). If Fluke
determines that failure was caused by neglect, misuse, contamination, alteration, accident, or abnormal
condition of operation or handling, including overvoltage failures caused by use outside the product’s
specified rating, or normal wear and tear of mechanical components, Fluke will provide an estimate of repair
costs and obtain authorization before commencing the work. Following repair, the product will be returned to
the Buyer transportation prepaid and the Buyer will be billed for the repair and return transportation charges
(FOB Shipping Point).
THIS WARRANTY IS BUYER'S SOLE AND EXCLUSIVE REMEDY AND IS IN LIEU OF ALL OTHER
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY IMPLIED WARRANTY
OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. FLUKE SHALL NOT BE LIABLE
FOR ANY SPECIAL, INDIRECT, INCIDENTAL, OR CONSEQUENTIAL DAMAGES OR LOSSES,
INCLUDING LOSS OF DATA, ARISING FROM ANY CAUSE OR THEORY.
Since some countries or states do not allow limitation of the term of an implied warranty, or exclusion or
limitation of incidental or consequential damages, the limitations and exclusions of this warranty may not
apply to every buyer. If any provision of this Warranty is held invalid or unenforceable by a court or other
decision-maker of competent jurisdiction, such holding will not affect the validity or enforceability of any other
provision.
Fluke Corporation
P.O. Box 9090
Everett, WA 98206-9090
U.S.A.
Fluke Europe B.V.
P.O. Box 1186
5602 BD Eindhoven
The Netherlands
11/99
To register your product online,visit register.fluke.com

i
Table of Contents
Title Page
Introduction....................................................................................................... 1
Contacting Fluke............................................................................................... 1
Unpacking the Meter......................................................................................... 2
Inspection...................................................................................................... 2
Inspection Interval ........................................................................................ 2
Preparations for Use.......................................................................................... 2
Precautions and Safety Information.................................................................. 2
Electrical Symbols ............................................................................................ 4
Specifications.................................................................................................... 5
General Specifications.................................................................................. 5
Detailed Specifications................................................................................. 5
Theory of Operation.......................................................................................... 9
Functional Block Diagram............................................................................ 9
Power Supply and Voltage Reference .......................................................... 10
Function Selection and Overload Protection ................................................ 11
Analog Signal Conditioning ......................................................................... 13
VAC.......................................................................................................... 13
VDC.......................................................................................................... 14
mVDC ...................................................................................................... 14
Temperature.............................................................................................. 14
Ohms Below the 6 MΩRange.................................................................. 14
Ohms in the 6 MΩ& 50 MΩRange and Siemens in the 60 nS Range.... 15
Continuity in all Ohms and the Siemens Ranges...................................... 15
Capacitance............................................................................................... 15
Diode Test ................................................................................................ 15
mA, µA and AAC..................................................................................... 15
mA, µA and ADC..................................................................................... 16
Active Filter.................................................................................................. 16
800 Hz Low Pass Filter................................................................................. 16
RMS to DC Converter.................................................................................. 16
Analog to Digital Converter ......................................................................... 17
Secondary Analog Circuits........................................................................... 17
Keypad.......................................................................................................... 17
Microprocessor & Support Circuits.............................................................. 18
LCD .............................................................................................................. 18

87 V/AN
Service Manual
ii
Backlight....................................................................................................... 18
Beeper........................................................................................................... 18
Troubleshooting ................................................................................................ 19
Restoration Actions....................................................................................... 19
Basic Maintenance............................................................................................ 19
Cleaning the Meter........................................................................................ 19
Opening the Meter Case ............................................................................... 20
Accessing the PCA and Replacing the LCD................................................. 21
Reassembling the Meter Case....................................................................... 22
Replacing the Battery.................................................................................... 22
Testing Fuses and Current Circuitry............................................................. 22
Replacing the Fuses...................................................................................... 23
Required Equipment.......................................................................................... 24
Performance Tests............................................................................................. 24
Basic Operability Tests................................................................................. 25
Testing the Fuses .......................................................................................... 25
Testing the Display....................................................................................... 25
Testing the Pushbuttons................................................................................ 27
Testing Meter Accuracy................................................................................ 27
Calibration Adjustment..................................................................................... 29
Calibration Adjustment Counter................................................................... 29
Calibration Adjustment Password................................................................. 29
Changing the Password ............................................................................ 29
Restoring the Default Password ............................................................... 30
Meter Buttons Used in the Calibration Steps................................................ 31
Calibration Adjustment Procedure................................................................ 31
Service and Parts............................................................................................... 33
Schematic Diagrams.......................................................................................... 38

iii
List of Tables
Table Title Page
1. Electrical Symbols................................................................................................. 4
2. AC Voltage Function Specifications..................................................................... 6
3. DC Voltage, Resistance, and Conductance Function Specifications..................... 6
4. Temperature Specifications................................................................................... 6
5. Current Function Specifications............................................................................ 7
6. Capacitance and Diode Function Specifications ................................................... 7
7. Frequency Counter Specifications......................................................................... 7
8. Frequency Counter Sensitivity and Trigger Levels............................................... 8
9. Electrical Characteristics of the Terminals............................................................ 8
10. MIN MAX Recording Specifications.................................................................... 8
11. Input Path Components ......................................................................................... 11
12. Overload Protection Components.......................................................................... 12
13. Required Tools and Equipment............................................................................. 24
14. Display Features.................................................................................................... 25
15. Accuracy Tests ...................................................................................................... 27
16. Calibration Adjustment Steps................................................................................ 32
17. 87 V/AN Final Assembly...................................................................................... 33
18. A1 Main PCA........................................................................................................ 35

87 V/AN
Service Manual
iv

v
List of Figures
Figure Title Page
1. 87 V/AN Block Diagram....................................................................................... 10
2. Opening the Meter, Battery and Fuse Replacement.............................................. 20
3. Removing LCD Mask to Access LCD .................................................................. 21
4. Testing the Current Input Fuses............................................................................. 23
5. Display Features.................................................................................................... 25
6. Restoring the Default Password ........................................................................... 30
7. 87 V/AN Final Assembly...................................................................................... 34
8. A1 LED PCA......................................................................................................... 39

87 V/AN
Service Manual
vi

1
Introduction
XW Warning
To avoid shock or injury:
•Read “Precautions and Safety Information” before
performing the verification tests or calibration adjustment
procedures documented in this manual.
•Do not perform the verification tests or calibration
adjustment procedures described in this manual unless you
are qualified to do so.
•The information provided in this manual is for the use of
qualified personnel only.
WCaution
•The 87 V/AN Digital Multimeter contains parts that can be
damaged by static discharge.
•Follow the standard practices for handling static sensitive
devices.
The 87 V/AN Service Manual provides the following information:
•Safety information
•Specifications
•Theory of operation
•Basic maintenance (cleaning, replacing the battery and fuses)
•Performance test procedures
•Calibration adjustment procedures
•Replaceable parts and schematics
For complete operating instructions, refer to the 87 V/AN Users Manual.
Contacting Fluke
To contact Fluke or locate the nearest Service Center, call one of the following telephone
numbers:
USA: 1-888-44-FLUKE (1-888-443-5853)
Canada: 1-800-36-FLUKE (1-800-363-5853)
Europe: [phone]
Japan: [phone]
Singapore: [phone]
Anywhere in the world: [phone]
Or, visit Fluke's Web site at www.fluke.com.
Toregister your product, visit register.fluke.com

87 V/AN
Service Manual
2
Unpackingthe Meter
Open the Multimeter box. Inside you will find the 87 V/AN Digital Multimeter
(hereafter referred to as “the Meter”) the test leads, the Product Manuals CD, the printed
87 V/AN User Manual, and the printed 87 V/AN Service Manual (this manual). Remove
the Meter from its plastic wrapping.
Inspection
Inspect all contents for any visible shipping damage. Look for scratches or any other
damage. If the unit is damaged, contact Fluke immediately using the contact information
stated previously.
Inspection Interval
Inspect the Meter and test leads before each use.
XWWarning
To avoid possible electric shock or personal injury, inspect the
test leads for damaged insulation or exposed metal. Check the
test leads for continuity. Replace damaged test leads before
using the Meter.
Preparations for Use
Before using or servicing the Meter, read all associated safety information. Make sure
you have complete understanding of all safety issues.
PrecautionsandSafety Information
In this manual, a Warning identifies conditions and actions that pose hazard(s) to the
user; a Caution identifies conditions and actions that may damage the Meter or the test
instruments.
XWWarning
To avoid possible electric shock or personal injury, follow
these guidelines:
•Use this Meter only as specified in this manual or the
protection provided by the Meter might be impaired.
•Do not use the Meter if it is damaged. Before using the
Meter, inspect the case. Look for cracks or missing plastic.
Pay particular attention to the insulation surrounding the
connectors.
•Make sure the battery door is closed and latched before
operating the Meter.
•Replace the battery as soon as the battery indicator (M)
appears.
•Remove test leads from the Meter before opening the
battery door.
•Inspect the test leads for damaged insulation or exposed
metal. Check the test leads for continuity. Replace damaged
test leads before using the Meter.

Digital Multimeter
Precautions and Safety Information
3
•Do not apply more than the rated voltage, as marked on the
Meter, between the terminals or between any terminal and
earth ground.
•Never operate the Meter with the cover removed or the case
open.
•Use caution when working with voltages above 30 V ac rms,
42 V ac peak, or 60 V dc. These voltages pose a shock
hazard.
•Use only the replacement fuses specified in this manual.
•Use the proper terminals, function, and range for
measurements.
•Avoid working alone.
•When measuring current, turn off circuit power before
connecting the Meter in the circuit. Remember to place the
Meter in series with the circuit.
•When making electrical connections, connect the common
test lead before connecting the live test lead; when
disconnecting, disconnect the live test lead before
disconnecting the common test lead.
•Do not use the Meter if it operates abnormally. Protection
may be impaired. When in doubt, have the Meter serviced.
•Do not operate the Meter around explosive gas, vapor, or
dust.
•Use only a single 9 V battery, properly installed in the Meter
case, to power the Meter.
•When servicing the Meter, use only specified replacement
parts.
•When using probes, keep fingers behind the finger guards
on the probes.
•Do not use the Low Pass Filter option to verify the presence
of hazardous voltages. Voltages greater than what is
indicated may be present. Make a voltage measurement
without the filter to detect the possible presence of
hazardous voltage, then select the filter function.
XCaution
To avoid possible damage to the Meter or to the equipment
under test, follow these guidelines:
•Disconnect circuit power and discharge all high-voltage
capacitors before testing resistance, continuity, diodes, or
capacitance.
•Before measuring current, check the Meter's fuses.
See " Testing Fuses (F1 and F2) ”.

87 V/AN
Service Manual
4
ElectricalSymbols
Electrical symbols used on the Meter and in this manual are explained in Table 1.
Table 1. Electrical Symbols
BAC (Alternating Current) JEarth ground
FDC (Direct Current) IFuse
XHazardous voltage. PConforms to European Union
directives
WRisk of Danger. Important information.
See Manual. $ Conforms to relevant Canadian Standards Association directives
MBattery TDouble insulated
RContinuity test or continuity beeper tone. ECapacitance
tUnderwriters Laboratories GDiode
CAT III
IEC overvoltage category III
CAT III equipment is designed to protect
against transients in equipment in fixed-
equipment installations, such as
distribution panels, feeders and short
branch circuits, and lighting systems in
large buildings.
CAT IV
IEC overvoltage category IV
CAT IV equipment is designed to
protect against transients from the
primary supply level, such as an
electricity meter or an overhead or
underground utility service.
s
Inspected and licensed by TÜV Product Services.

Digital Multimeter
Specifications
5
Specifications
General Specifications
Maximum Voltage between any Terminal and Earth Ground: 1000 V rms
WFuse Protection for mA or µA inputs: 44/100 A, 1000 V FAST Fuse
WFuse Protection for A input: 11 A, 1000 V FAST Fuse
Display: Digital: 6000 counts updates 4/sec. The Meter also has 19,999 counts in high-resolution mode.
Analog Bargraph: 33 segments, updates 40/sec. Frequency: 19,999 counts, updates 3/sec at > 10 Hz.
Temperature: Operating: -20 °C to +55 °C; Storage: -40 °C to +60 °C
Altitude: Operating: 2000 m; Storage: 10,000 m
Temperature Coefficient: 0.05 x (specified accuracy)/ °C (< 18 °C or > 28 °C)
Electromagnetic Compatibility: All ranges unless otherwise noted: In an RF field of 3 V/m total
accuracy = specified accuracy + 20 counts
Except: Temperature not specified.
Relative Humidity: 0 % to 90 % (0 °C to 35 °C); 0 % to 70% (35 °C to 55 °C)
Battery Type: 9 V zinc, NEDA 1604 or 6F22 or 006P
Battery Life: 400 hrs typical with alkaline (with backlight off)
Vibration: Per MIL-PRF-28800 for a Class 2 instrument
Shock: 1 Meter drop per IEC 61010-1:2001
Size (HxWxL): 1.25 in x 3.41 in x 7.35 in (3.1 cm x 8.6 cm x 18.6 cm)
Size with Holster and Flex-Stand: 2.06 in x 3.86 in x 7.93 in (5.2 cm x 9.8 cm x 20.1 cm)
Weight: 12.5 oz (355 g)
Weight with Holster and Flex-Stand: 22.0 oz (624 g)
Safety: Complies with ANSI/ISA S82.01-2004, CSA 22.2 No. 1010.1:2004 to 1000 V Overvoltage
Category III, IEC 664 to 600 V Overvoltage Category IV. UL listed to UL61010-1. Licensed by TÜV to
EN61010-1.
Detailed Specifications
For all detailed specifications:
Accuracy is given as ±([% of reading] + [number of least significant digits]) at 18 °C to
28 °C, with relative humidity up to 90 %, for a period of one year after calibration
adjustment. In the 4 ½-digit mode, multiply the number of least significant digits
(counts) by 10. AC conversions are ac-coupled and valid from 3 % to 100 % of range.
The Meter is true rms responding. AC crest factor can be up to 3 at full scale, 6 at half
scale. For non-sinusoidal wave forms add -(2 % Rdg + 2 % full scale) typical, for a crest
factor up to 3.
Tables 2 through 10 list the Meter’s detailed specifications.

87 V/AN
Service Manual
6
Table 2. AC Voltage Function Specifications
Function Range Resolution Accuracy
45 - 65 Hz 30 - 200 Hz
200 - 440 Hz
440 Hz - 1
kHz 1 - 5 kHz 5 - 20 kHz1
±(0.7 % + 4)
±(0.7 % + 2)
±(2.0 % + 4)
±(2.0 % + 20)
600.0 mV
6.000 V
60.00 V
600.0 V
0.1 mV
0.001 V
0.01 V
0.1 V ±(2.0 % + 4)3
unspecified
1000 V 1 V
±(1.0 % + 4)
unspecified unspecified
K2,4
Low pass filter ±(0.7 % + 2)
±(1.0 % + 4)
+1 % + 4
-6 % - 45unspecified
unspecified unspecified
1. Below 10 % of range, add 12 counts.
2. The Meter
is a true rms responding meter. When the input leads are shorted together in the ac functions, the Meter
may display a residual reading between 1 and 30 counts. A 30 count residual reading will cause only a 2-digit
change for readings over 3 % of range. Using REL to offset this reading may produce a much larger constant error
in later measurements.
3. Frequency range: 1 kHz to 2.5 kHz.
4. A residual reading of up to 13 digits with leads shorted, will not affect stated accuracy above 3 % of range.
5. Specification increases from -1% at 200 Hz to -6% at 440 Hz when filter is in use.
Table 3. DC Voltage, Resistance, and Conductance Function Specifications
Function Range Resolution Accuracy
L6.000 V
60.00 V
600.0 V
1000 V
0.001 V
0.01 V
0.1 V
1 V
±(0.05 % + 1)
±(0.05 % + 1)
±(0.05 % + 1)
±(0.05 % + 1)
F
mV 600.0 mV 0.1 mV ±(0.1 % + 1)
e
nS
600.0 Ω
6.000 kΩ
60.00 kΩ
600.0 kΩ
6.000 MΩ
50.00 MΩ
60.00 nS
0.1 Ω
0.001 kΩ
0.01 kΩ
0.1 kΩ
0.001 MΩ
0.01 MΩ
0.01 nS
±(0.2 % + 2)1
±(0.2 % + 1)
±(0.2 % + 1)
±(0.6 % + 1)
±(0.6 % + 1)
±(1.0 % + 3) 2
±(1.0 % + 10) 1
1. When using the REL ∆function to compensate for offsets.
2. Add 0.5 % of reading when measuring above 30 MΩin the 50 MΩrange.
Table 4. Temperature Specifications
Temperature Resolution Accuracy1,2
-200 °C to +1090 °C
-328 °F to +1994 °F
0.1 °C
0.1 °F
1 % + 10
1 % + 18
1. Does not include error of the thermocouple probe.
2. Accuracy specification assumes ambient temperature stable to ±1 °C. For ambient temperature changes of ±5 °C,
rated accuracy applies after 1 hour.

Digital Multimeter
Specifications
7
Table 5. Current Function Specifications
Function Range Resolution Accuracy1, 2 Burden Voltage (typical)
mA
\
(45 Hz to 2 kHz)
mA
[
60.00 mA
400.0 mA4
6.000 A
10.00 A3
60.00 mA
400.0 mA4
6.000 A
10.00 A3
0.01 mA
0.1 mA
0.001 A
0.01 A
0.01 mA
0.1 mA
0.001 A
0.01 A
±(1.0 % + 2)
±(1.0 % + 2)
±(1.0 % + 2)
±(1.0 % + 2)
±(0.2 % + 4)
±(0.2 % + 2)
±(0.2 % + 4)
±(0.2 % + 2)
1.8 mV/mA
1.8 mV/mA
0.03 V/A
0.03 V/A
1.8 mV/mA
1.8 mV/mA
0.03 V/A
0.03 V/A
µA B
(45 Hz to 2 kHz)
µAF
600.0 µA
6000 µA
600.0 µA
6000 µA
0.1 µA
1 µA
0.1 µA
1 µA
±(1.0 % + 2)
±(1.0 % + 2)
±(0.2 % + 4)
±(0.2 % + 2)
100 µV/µA
100 µV/µA
100 µV/µA
100 µV/µA
1. AC conversions are ac coupled, true rms responding, and valid from 3 % to 100 % of range, except 400 mA range
(5 % to 100 % of range) and 10 A range (15 % to 100 % or range).
2. The Meter is a true rms responding meter. When the input leads are shorted together in the ac functions, the
Meter may display a residual reading between 1 and 30 counts. A 30 count residual reading will cause only a 2
digit change for readings over 3 % of range. Using REL to offset this reading may produce a much larger constant
error in later measurements
3. W10 A continuous up to 35 °C; < 20 minutes on, 5 minutes off at 35 °C to 55 °C. 20 A for 30 seconds maximum;
> 10 A unspecified.
4. 400 mA continuous; 600 mA for 18 hrs maximum.
Table 6. Capacitance and Diode Function Specifications
Function Range Resolution Accuracy
E10.00 nF
100.0 nF
1.000 µF
10.00 µF
100.0 µF
9999 µF
0.01 nF
0. 1 nF
0.001 µF
0.01 µF
0.1 µF
1 µF
±(1 % + 2)1
±(1 % + 2)1
±(1 % + 2)
±(1 % + 2)
±(1 % + 2)
±(1 % + 2)
G3.000 V 0.001 V ±(2 % + 1)
1. With a film capacitor or better, using Relative mode to zero residual.
Table 7. Frequency Counter Specifications
Function Range Resolution Accuracy
Frequency
(0.5 Hz to 200 kHz,
pulse width > 2 µs)
199.99
1999.9
19.999 kHz
199.99 kHz
> 200 kHz
0.01 Hz
0.1 Hz
0.001 kHz
0.01 kHz
0.1 kHz
±(0.005 % + 1)
±(0.005 % + 1)
±(0.005 % + 1)
±(0.005 % + 1)
unspecified

87 V/AN
Service Manual
8
Table 8. Frequency Counter Sensitivity and Trigger Levels
Minimum Sensitivity (RMS Sine wave)
Input Range1
5 Hz - 20 kHz 0.5 Hz - 200 kHz
Approximate Trigger
Level
(DC Voltage Function)
600 mV dc
600 mV ac
6 V
60 V
600 V
1000 V
70 mV (to 400 Hz)
150 mV
0.3 V
3 V
30 V
100 V
70 mV (to 400 Hz)
150 mV
0.7 V
7 V (≤140 kHz)
70 V (≤14.0 kHz)
200 V (≤1.4 kHz)
40 mV

1.7 V
4 V
40 V
100 V
Duty Cycle Range Accuracy
0.0 to 99.9 % Within ±(0.2% per kHz + 0.1 %) for rise times < 1 µs.
1. Maximum input for specified accuracy = 10X Range or 1000 V.
Table 9. Electrical Characteristics of the Terminals
Function
Overload
Protection1
Input Impedance
(nominal)
Common Mode
Rejection Ratio
(1 kΩunbalance)
Normal Mode Rejection
L1000 V rms 10 MΩ< 100 pF > 120 dB at dc,
50 Hz or 60 Hz
> 60 dB at 50 Hz or 60 Hz
F
mV 1000 V rms 10 MΩ< 100 pF > 120 dB at dc,
50 Hz or 60 Hz
> 60 dB at 50 Hz or 60 Hz
K1000 V rms 10 MΩ< 100 pF (ac-
coupled)
> 60 dB, dc to 60 Hz
Full Scale Voltage Typical Short Circuit Current
Open Circuit
Test Voltage To 6.0 MΩ50 MΩor
60 nS 600 Ω
6 k 60 k 600 k 6 M 50 M
e1000 V rms < 7.5 V dc < 4.1
V dc
< 4.5
V dc
1 mA 100 µA 10 µA 1µA 1 µA 0.5 µA
G1000 V rms < 3.9 V dc 3.000 V dc 0.6 mA typical
1. 106V Hz max
Table 10. MIN MAX Recording Specifications
Nominal Response Accuracy
100 ms to 80 %
(dc functions)
120 ms to 80 %
(ac functions)
250 µs (peak)1
Specified accuracy ±12 counts for changes > 200 ms in duration
Specified accuracy ±40 counts for changes > 350 ms and inputs > 25 % of range
Specified accuracy ±100 counts for changes > 250 µs in duration
(add ±100 counts for readings over 6000 counts)
(add ±100 counts for readings in Low Pass mode)
1. For repetitive peaks: 1 ms for single events.

Digital Multimeter
Theory of Operation
9
Theoryof Operation
This section provides the theory of operation for the 87 V/AN Digital Multimeter to a
depth that is required for troubleshooting to the component level. The functional block
diagram provides an overview for the description. The schematic diagrams included in
the manual are referred to during the following detailed circuit descriptions.
Functional Block Diagram
Figure 1 shows the top-level function block diagram for the Meter. Each of the blocks in
this diagram is discussed in detail in the following paragraphs.
The parameter to be measured is connected with test leads to the appropriate two input
terminals shown at the left of the block diagram. After the Meter is set to the desired
function, the signal is routed to the signal conditioning circuit. Either automatically or
manually, a range is selected that puts the signal to be measured within the dynamic
range of the analog-to-digital converter (ADC) or other signal conditioning circuits like
the RMS-to-DC converter discussed in “RMS to DC Converter”. A scaled AC signal
voltage is routed directly, or via an 800-Hz low-pass filter, to the RMS-to-DC converter
circuit. A DC input signal or DC output of the RMS-to-DC converter (for AC functions)
is routed to a low-pass, 6-Hz, 2-pole active filter to prepare it for ADC measurement.
The conditioned analog input signal voltage is converted to a digital value by the ADC
and sent to the microprocessor. The microprocessor converts this digital value for
display on the LCD based on the function, range and keypad entered options. The output
of the signal-conditioning block is also routed to the secondary analog circuits block to
be further conditioned for input to the fast ADC contained within the microprocessor
block. Other analog circuits that control the behavior of the Meter are located in the
secondary analog circuits block and are discussed later.

87 V/AN
Service Manual
10
Low
Pass
Filter
Function
Selection
&
Overload
Protection
V/Ohms
mA/µA
AMPS
Common
RMS
Converter
Back-
light
Beeper
Keypad
Reference
Power
Supply
Analog
Signal
Conditioning
Analog
to
Digital
Converter
Microprocessor
&
Support
Circuits
Active
Filter Secondary
Analog
Circuits
LCD
ama101f.eps
Figure 1. 87 V/AN Block Diagram
Power Supply and Voltage Reference
A 9-V alkaline battery supplies power to the Meter and is connected via a cable assembly
to J3 on the A1 printed circuit assembly (PCA). Diode CR3 protects the Meter from
damage due to accidental polarity reversal at J3.
Q13 is used as a remote power switch that connects the battery to the power supply
regulators when the Meter is turned on. When S1 is in the OFF position, Q14 is off to
allow the battery voltage to turn off Q13 via R73. When the Meter is turned on, S1
makes a momentary contact to the battery voltage via R80, turning on Q14, which turns
on Q13, which connects the battery voltage to R74, which holds Q14 until S1 is returned
to the OFF position.
When Q13 is on, battery voltage is applied to regulators U6 and U10, bypass capacitors
C12 and C14 and the low battery detect divider resistors R15 & R16. Note that since the
power supply levels are referenced to the COMMON input terminal of the Meter, the
negative end of the battery is the -2.5 V power supply.
U6 is a 3.3-V regulator that generates the +0.8-V (-2.5 V + 3.3 V) power supply. U10 is
a 5.0-V regulator that generates the +2.5-V (-2.5 V + 5.0 V) power supply. U10 is

Digital Multimeter
Theory of Operation
11
enabled and disabled via control line V5* from microprocessor U2. C15 bypasses the
output of U10.
When U10 is enabled, 5 V is supplied to the voltage reference U8 input. U8 is a 2.5-V
reference with a buffered output and accurately holds the COMMON input 2.5 V above
the -2.5 V power supply or at 0 V. The triangular ground symbol represents this
reference level throughout the Meter schematic. C16, C57, C50 and R109 bypass the
output of U8 keeping the voltage between COMMON and the -2.5 V power supply noise
free and stable. C50 and R109 form a low Q bypass that is directed at electromagnetic
interference (EMI).
Function Selection and Overload Protection
Sections of rotary switch S1 connect the V/Ohms and mA/µA input jacks to the signal
conditioning circuit as required for the respective Meter functions. The Meter schematic
symbols for each of the sections of S1 have the functions that correspond to the closed
positions listed next to them. Table 11 indicates the components that complete normal
operation signal path/paths from input jacks to the required signal conditioning
circuit/circuits for each Meter function.
Table 11. Input Path Components
Function Components
K
VAC RT1, R1, R2, C1, Z1(9.997 M), C43, R94
V
VDC RT1, R1, S1(1,3), Z1(9.997 M)
mL
mV dc RT1, R1, S1(1,3), Z1(9.997 M), S1(29,5), R85
mL- HiZ
mV dc – HiZ RT1, R1, S1(29,5), R85
d
Temperature RT1, R1, S1(1,2), R3, S1(29,5), R85, S1(1,3)
Z1(9.997M)
T
Ohms (Below 6 MΩ)RT1, R1, S1(1,2), R3, Z2, S1(4,5), R85
T
Ohms (6 MΩ, 50 MΩ, nS) RT1, R1, S1(1,2), R3, S1(1,3), Z1(9.99 7M)
X
Continuity (below 6 MΩ)RT1, R1, S1(1,2), R3, Z2, S1(4,5) R85
X
Continuity (6 MΩ, 50 MΩ) RT1, R1, S1(1,2), R3, S1(1,3), Z1(9.997 M)
E
Capacitance RT1, R1, S1(1,2), R3, Z2, S1(4,5) R85
G
Diode Test RT1, R1, S1(1,2), R3, Z2, S1(4,5), R85

87 V/AN
Service Manual
12
The Meter uses diode clamps, a positive temperature coefficient thermistor, metal oxide
varistors (MOVs) and fuses for protection when inadvertent overload conditions are
applied across the input terminals. Table 12 shows the components that limit and direct
overload currents to prevent damage to the Meter.
Table 12. Overload Protection Components
Function Components Protected Protection Components
U1 pin 3 Z1(9.997 M), U1-3 clampsK
VAC S1(1,2)(1,3)(29,5)(4,5), Z1(9.997 M) RT1, R1, RV1, RV2, Z2, RV3
U1 pin 3 Z1(9.997 M), U1-3 clampsV
VDC S1(1,2)(29,5)(4,5), Z1(9.997 M) RT1, R1, RV1, RV2, Z2, RV3
U1 pin 36 RT1, R1, CR10, VR1, CR8, CR9, R3, U1-36
clamps
U1 pin 3 RT1, R1, CR10, VR1, CR8, CR9, Z1(9.997 M),
U1-3 clamps
U1 pin 1 RT1, R1, CR10, VR1, CR8, CR9, R85, U1-1
clamps
mL, d
mV dc, Temperature
S1(4,5) Z2, RV3, RV2, RT1, R1, CR10, VR1, CR8,
CR9
U1 pin 36 RT1, R1, CR10, VR1, CR8, CR9, R3, U1-36
clamps
U1 pin 3 RT1, R1, CR10, VR1, CR8, CR9, Z1(9.997 M),
U1-3 clamps
U1 pin 1 Z2, R85, U1-1 clamps
T, E,X
Ohms, Capacitance,
Continuity
S1(29,5) RT1, R1, CR10, VR1, CR8, CR9
U1 pin 36 RT1, R1, CR10, VR1, CR8, CR9, R3, U1-36
clamps
U1 pin 3 RT1, R1, CR10, VR1, CR8, CR9, R2, C1,
Z1(9.997 M), U1-3 clamps
U1 pin 1 Z2, R85, U1-1 clamps
G
Diode Test
S1(1,3)(29,5) RT1, R1, CR10, VR1, CR8, CR9
S1(7,8)(7,6) F1, CR1, CR2Voltage applied to
mA/µAinput jack in
any of the above
functions or OFF
U2 pin 3 after F1 opens R7, CR5
R5 F1, CR1, CR2, R6mA/A
U2 pin 3 after F1 opens R7, CR5
R4 F1, CR1, CR2µA
U2 pin 3 after F1 opens R7, CR5
R6 F2 (assuming enough current is available)Voltage applied to A
input jack in any
function including OFF
U2 pin 2 after F2 opens R10, CR6