HP 421A Service manual

,
НР 427A
SM: 94)— opp YP
OPERATING AND SERVICE MANUAL
PROPERTY OF
ELECTRICAL ENGINEERING
VANDERBILT UNNER SITY VOLTMETER
421A
HEWLETT Åp; PACKARD
Vilt dH

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OPERATING AND SERVICE MANUAL
-hp- Part No. 00427-90004
MODEL 427A
VOLTMETER
Serials Prefixed: 947-
Appendix C, Manual Backdating Changes,
С % adapts manual to serials prefixed 550-, 621-, 731-
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Copyright Hewlett-Packard Company 1969
P.O. Box 301, Loveland, Colorado, 80537 U.S.A.
Printed: JAN 1970

a a
e
e
Table of Contents
TABLE OF CONTENTS
Model 427A
Section Page
I GENERAL INFORMATION . . ...... 1-1
1-1. Description. ......... .. 1-1
1-5. Вайегу.............. 1-1
1-7. Instrument and Manual
Identification. . . ....... 1-1
1-10. Available Accessories ...... 1-1
1-12. Cables and Adapters ..... 1-1
1-13. Probes, “22524 04 asa a Jal
Section Page
п INSTALLATION ............. 25-1
2-1. Introduction. ........... 2-1
2-3 Initial Inspection . . . . . 557221
2-9. Power Requirements, . . . . . , 2-1
2-7. Grounding Requirements. . ... 2-1
2-10. Installation. ........... 2-1
2-12, Bench Mounting ........ 2-1
2-14. Rack Mounting. ........ 2-1
2-16. Combination Mounting .... 2-1
2-18. Repackaging for Shipment .... 2-1
Section Page
ІП OPERATING INSTRUCTIONS ...... 3-1
3-1. Introduction. ........... 3-1
3-3. Front and Rear Panel
Description .........
Operating Instructions ,....
Mechanical Zero Adjustment
Turn-On Procedure. . . . .
1 DC Measurements. ,....
2. Resistance Measurements
3. AC Measurements. .... А
6
Section
IV THEORY OF ОРЕБВАТІОМ,......
4-1. General. ............
4-5. DC Орегаһоп..........
Ohms Operation ........
. AC Operation. .........
2.
Circuit
Descriptions,
.....
3. DC Amplifier (A1). . . .,.
6. DC Offset (А2) ... .
9. AC Metering Circuit (A2) ния
3. Battery Regulator Al....
Section
У MAINTENANCE ............
5-1, Introduction ..........
9-3. Required Equipment ,......
5-5. Performance Checks
DB Measurements. ,.....
.
У ФА) Q3 C2 Q3 023 2 C3 CO
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Section
У MAINTENANCE (Сопе' 4)
9-7. АС Accuracy and Frequency
Page
Response Check. .... 5-1
5-14. Resistance 2. Check. 5-2
5-16, DC Accuracy. 4 4.50...» 5-2
5-18. AC Rejection Check . sé aaa des
5-20. Input Impedance Check .... 5-3
5-25. Adjustment and Calibration
LIST OF TABLES
Number Page
1-1. Specifications ..,............. 1-0
3-1. Effects of Harmonic Distortion. . . . . 3-2
3-2. DB Range Identification. . . . . .. .. 3-2
5-1. Required Test Equipment, ....... 5-0
5-2. Resistance Accuracy Check. . . . . . . 5-2
01791-2
Procedures ......... . 5-4
5-21. Cover Removal ........ 5-4
5-30. Amplifier Balance and Zero
Offset Adjustment. . . . . . 5-5
5-31. Regulator Adjust ....... 5-5
5-32. AC Calibration. ........ 5-5
5-36, Resistance Infinity Adjustment 5-6
5-38. DC Calibration. ........ 5-6
9-40. Alternate Performance Checks
and Calibration Procedures. . 5-6
5-42. AC Сїтсїв.......... 5-6
5-45. Resistance Circuits. . . . .. 5-6
5-47, DC Circuitfs 32,22 xxx 5-7
5-50. АС Rejection ......... 5-7
5-52. Input Impedance Check .... 5-7
5-54. Battery Replacement. . . .... 5-7
5-56. Troubleshooting. ......... 5-7
5-60. Power Supply ......... 5-8
5-63. DC Amplifier ......... 5-8
5-67. AC Metering Circuit, ..... 5-8
5-71. Adjustment of Factory Selected
Components ...... .... 5-8
5-13. MRI 1.15114. xw ca кез 5-8
5-75. A2C2* så xg Xov 5-9
5-77. A2R14* T . 5-9
5-80. A2R16* ..... 4-8 ызык va 10-9
5-82. Etched Circuit Board Repair. 5-9
Section Page
VI CIRCUIT ПЛАСВАМ$.,,....... 2. 6-1
6-1, Introduction ............ 6-1
Section Page
ҮП REPLACEABLE PARTS.......... 7-1
1-1. Introduction ............ 7-1
7-4. Ordering Information. . . . . . . . 7-1
7-6. Non-Listed Parts. . . ... века 41-1
Appendix
А CODE LIST OF MANUFACTURERS
B SALES AND SERVICE OFFICES
С MANUAL BACKDATING CHANGES
Number Page
5-3. 1% Resistors............ Ux. 5-7
5-4. Troubleshooting. .......... .. 5-7
5-5. Power Supply Voltages ,........ 5-8
5-6 AC Metering Circuit Voltages ..... 5-8
7-1. Replaceable Parts. .. ......... 7-1
iii

Table of Contents
Model
427A
Voltmeter
, .
.
11096A
High
Frequency
Probe.
.
Location
of
Front
Panel
Controls
and
Indicators.
.
.
Impedance
Correction
Graph
Simplified
Block
Diagram
.
.
DC
ОрегаПоп.........
Ohms
Operation
.......
AC
Operation
DC
Offset
Circuit,
,.
» .
+
+=
AC
Accuracy
and
Frequency
Response
Check.
.
.
.
Моде! 427А
OF
ILLUSTRATIONS
Page
Number
Page
.
1-0
5-9,
Resistance
Accuracy
Сһеск.......
5-2
‚
1-1
5.3.
DC
Accuracy
and
Linearity
Check.
...
5-2
5-4,
AC
Rejection
Сһеск...........
5-3
.
3-0
5-5,
Input
Impedance
Check
.........
5-4
.
3-3
5-6.
Location
of
Internal
Adjustments
....
5-5
.
4-0
5-7,
Battery
Керісетепі...........
5-7
. 4-1
.
4-2
6-1.
Location
of
Switch
Components
....
-
6-2
.
4-2
6-2.
Component
Госапоп,..........
6-3
4-3
6-3.
Schematic
Diagram
...........
6-3
.
5-0
1-1.
Location
of
Mechanical
Рагів......
7-0
01791-2
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а
da

Section
I
Model
427A
в
a ын
| |
ҮМЕТЕН
HEWLETT
·
PACKARD
@
4274
VOLI
» FUNCTION
3
EEE
Figure
1-1.
Model
427A
Voltmeter
Table
1-1.
Specifications
DC VOLTMETER
Voltage
Ranges:
+100
mV
to
+1000
У
full
scale
па
1,
3,
10
sequence
(9
ranges).
AC
VOLTMETER
(Cont'd)
==
а
bl
си
ОПТ
а)
Input
Impedance;
10
megohms
shunted
by
40pF
on
10
mV
to
1
V
ranges;
20
pf
on
3
V
to
300
V
ranges.
Response:
Responds
to
the
average
value
of
the
in-
put;
calibrated
in
rms
voltsfor
а
sine
wave
input.
Overload:
300
V/rms
momentarily,
1
V
range
and
Accuracy:
+2%
of
full
scale
on
any
range
(09C
to
50°C),
Input
Resistance:
10
megohms
on
all
ranges.
Ре
у
below
AC
Rejection:
Superimposed
peak
ac
voltages
(60
M
:
Hz
and
above)
100
times
greater
than
full
scale
425
V/rms
maximum
above
1
V
range.
affects
reading
less
than
1%.
Maximum
450
OHMMETER
volts peak.
Overload:
1200
Vdc
on
any
range.
AC VOLTMETER
Voltage
Ranges:
10
mV
to
300
V
rms
full
scale
in
a
1,
3,
10
sequence
(10
ranges).
Frequency
Range:
10
Hz
to
1
MHz.
Accuracy:
(0°C
to
50°C),
RANGE
+2%
OF
FULL
SCALE
0.01
V
-
30
V
10
Hz
-
1
МН?
100
V
-
300
V
10
Hz
-
100
KHz
Frequency Response:
Resistance
Ranges:
10
ohms
center
scale
to
10
meg-
ohms
center
scale
(7
ranges).
Accuracy:
+5%
of
reading
at
midscale
(00C
to
+50°C),
Polarity:
Common
terminal
negative,
source Current:
OPEN
CIRCUIT
|
SHORT
CIRCUIT
RANGE
VOLTAGE
CURRENT
X10
3d8
edB
IdB
О
GENERAL
Floating
Input:
May
be
operated
up
to
500
Vdc
above
ground.
(Ohms
input
open
in
any
function
except
ohms--volts
input
open
when
instrument
is
in
off
position.)
Power:
22-1/2
volt
dry
cell
battery.
(Eveready
No.
763
ог
RCA
VS102.)
Option
01:
Battery
operation
and
ac
line
opera-
tion
(selectable
on
rear
panel).
115
or
230
V
+20%,
50
Hz
to
400Н2,
1/2
W.
IHz
SHz
lOHz
IMHz
eMHz 3MHz
4MHz
Frequency
response
10
mV
to 30
V
ranges,
1-0
01791-2
44

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p =
Model 427A
Section I
SECTION 1
GENERAL
1-1. DESCRIPTION.
1-2. The Hewlett-Packard Model 427A is a versatile,
compact, self-contained voltmeter. It is capable of
making de measurements from 1 mV to 1000 volts, ac
measurements from 0. 3 туѓо 300 volts at frequencies
from 10 Hz to 1 MHz, and resistance measurements
from 0.2 ohms to 500 megohms. With the 01 option,
the Model 427A may be powered either by a 115 or
230 volt line or by an internal 22-1/2 volt dry cell
battery.
1-3, The use of solid state components throughout
gives the Model 427A both ruggedness and reliability,
Battery operation makesthe instrument ideal for field
use or isolation from common mode ground loops.
1-4, Figure 1-1showsthe Model427A, and Table 1-1
contains а list of the Model 427A Specifications,
1-5. BATTERY.
1-6. The battery used in the Model 427A is a 22-1/2
volt dry cell, Eveready No. 763 or an RCA VS102.
Typical battery life is morethan 300 hours continuous
operation or 700 hours intermittent operation.
1-7. INSTRUMENT AND MANUAL
IDENTIFICATION.
1-8. Hewlett-Packard uses a two-section eight-digit
serial number (000-00000). If the first three digits
(prefix) of the serial number on your instrument do
not agree with those on the title page of this manual,
INFORMATION
change sheets supplied with the manual will define
differences between your instrument and the Model
427A described in this manual.
1-9. If a letter prefixes the serial number, the
instrument was manufactured outside the United
States.
1-10. AVAILABLE ACCESSORIES.
1-11. The following accessories are available to in-
crease the test capabilities of the Model 427A.
1-12. CABLES AND ADAPTERS.
-hp- 110014 45" test lead, dual banana to BNC
male
-hp- 11002А 5' test lead-dual banana to alligator
clips
-hp- 110034 5" test lead-dual banana to pencil
probe and alligator clip
-hp- 10111A shielded BNC female to banana
plug adapter
1-13. PROBES.
-hp- Model 11074A 10:1 Voltage Divider Probe
extends the voltage range of Model 427А by a
factor of 10.
-hp- Model 11039A 1000:1 Capacitive Voltage
Divider for measuring voltages up to 24 KV.
-hp- Model 11096A High Frequency Probe ex-
tends the frequency range ofthe 427A to 500 MHz.
The voltage range is 0.25V to 30V.
Figure 1-2. 11096А High Frequency Probe
01791-2
1-1

e^
e
Mode1 427А
Section П
SECTION II
INSTALLATION
2-1. INTRODUCTION.
2-2. This section contains information and instruc-
tions necessary for installation and shipping of Model
427A Voltmeter. Included are initial inspection pro-
cedures, power and grounding requirements, install-
ation, and instructions for repackaging for shipment,
2-3. INITIAL INSPECTION.
2-4. This instrument was carefully inspected both
mechanically and electrically before shipment. It
should be physically free of mars or scratches and in
perfect electrical order upon receipt. To confirm
this, the instrument should be inspected for physical
damage in transit. Also, test the electrical perform-
ance of the instrument using the procedure outlined in
Paragraph 5-5. If there is damage or deficiency, see
the warranty on the inside front cover of this manual,
2-5. POWER REQUIREMENTS.
2-6. The Model 427A uses а 22. 5 volt dry cell battery
for its primary power source. However, if Option01
is included, the Model 427A can be operated from any
source of 115 or 230 volts (420%) at 50 to 400 Hz.
With the instrument disconnected from the ac power
source, move the slide switch (located on the rear
panel) until the desired line voltage appears. Power
dissipation is less than 1/2 watt maximum.
2-7. GROUNDING REQUIREMENTS.
2-8. To protect operating personnel, the National
Electrical Manufacturers' Association (NEMA) recom-
mends that the instrument panel and cabinet be ground-
ed. The Option 01 427A is equipped with a three-
conductor power cable which, when plugged into an
appropriate receptacle, grounds the instrument. The
offset pin on the power cable three-prong connector
is the ground wire.
2-9. To preserve Ше protectionfeature when operating
the instrument from atwo-contact outlet, use а three-
prong to two-prong adapter and connect the pigtail on
the adapter to ground.
2-10. INSTALLATION.
2-11. The Model 427A is fully transistorized; there-
fore, no special cooling is required. However, the
instrument should not be operated where the ambient
temperature exceeds +559С (1310F). For additional
information, address inquiries to your local -hp-
Sales and Service Office. (See Appendix B for office
locations. )
2-12. BENCH MOUNTING,
2-13, Model 427A is shipped with plastic feet and tilt
stand in place, ready for use as а bench instrument.
01791-3
2-14. RACK MOUNTING.
2-15. Model 427A may be rack mounted by using an
Adapter Frame (-hp- Part No. 5060-0797). The adapt-
er frame is a rack frame that accepts any combination
of submodular units. It can be rack mounted only.
2-16. COMBINATION MOUNTING,
2-17. The Model 427A may be mounted incombination
with other submodular units by using a Combining Case
(-һр- Model 1051A). If the 427A is equipped witha
carrying handle, it will be necessary to remove the.
top cover on 427A before inserting it into the Com-
bining Case, The Combining Case is a full-module
unit which accepts various combinations of submodular
units. Being a full-module unit itself, itcan be bench
or rack mounted and is analogous to any full-module
instrument.
2-18. REPACKAGING FOR SHIPMENT.
2-19. The following paragraphs contain а general guide
for repackaging ofthe instrumentfor shipment, Refer
to Paragraph 2-20 if the original container is to be
used; 2-21 if it is not,
NOTE
If the instrument is to be shipped to
Hewlett-Packard for service or repair,
attach a tag to the instrument identifying
the owner and indicating the service or
repair to be accomplished; include the
model number and full serial number of
the instrument. In any correspondence,
identify the instrument by model number,
serial number, and serial number prefix.
2-20. If original container is to be used, proceed as
follows:
a. Place instrument in original container. If
original container is not available, a suitable
container can be purchased from your near-
est -hp- Sales and Service Office.
b, Ensure that container is well sealed with
strong tape or metal bands.
2-21, И original container is notto ре used, proceed
as follows:
a. Wrap instrument in heavy paper or plastic
before placing in an inner container.
b. Place packing material around all sides of in-
strument and protect panel face with cardboard
strips.
c. Place instrument and inner container inaheavy
cartonor wooden box and seal with strong tape
or metal bands.
d. Mark shipping container with "DELICATE
INSTRUMENT," "FRAGILE" etc,
2-1

Қ шек TET сесия таче tn oe wama =
Section
Ш
Model
427A
a
í
MODEL 427A FRONT PANEL MODEL 427А REAR PANEL (OPTION Ol)
HEWLE1) - PACKARD //7 427А VOLTMETER
4)
FUNCTION RANGE
АСУ хок
IO XIOOK
5
ре 30 хм
( IOO хюм
N
300
>
1000 — ону
ос ZERO/I 00
427А-8-0529
BATT/LINE
(option
01
only)
slide
switch:
Se-
COM
terminal:
Connects
to
instrument
com-
lects either battery or line operation. mon. (Common input for ACV, OHMS and
DCV.)
115/230
slide
switch
(option
01
only):
Selects
either
115
Vac
or
230
Vac
for
line
operation.
VOLTS
terminal:
Applies
unknown
ac
or
dc
voltage
to
instrument.
Line input (option 01 only): Connects ac
erati er to instrument.
op
ing
pow
жігі:
2
FUNCTION
switch:
Selects
mode
of
operation.
Selections are OFF, BATT, +DC, -DC, OHMS
and AC.
RANGE switch: Selects appropriate range of
unknown input.
e ©
DC
ZERO/2
«thumbwheel:
Usedto
electrical-
Mechanical
zero:
Mechanically
zeroes
the
ly
zerothe
instrument
in
DC
mode
and
to
infinity-
meter,
set
the
instrument
in
OHMS
mode.
©
Meter face: Displays the magnitude of unknown
OHMS
terminal:
Connects
unknown
resistance
resistance
or
voltage
in
ohms
or
volts
respec-
to instrument. tively.
о 00000
е ғ
Figure
3-1.
Location
of
Front
Panel
Controls
and
Indicators
3-0
01791-2

жө
Model 427A
SECTION
INSTRUCTIONS
OPERATING
3-1. INTRODUCTION.
3-2. The Model 427А may be operated ав а дс volt-
meter, ac voltmeter, ohmmeter or dB meter. This
section contains operating instructions for each mode
of operation.
3-3. FRONT AND REAR PANEL DESCRIPTION.
3-4. Figure 3-1 shows the location of all the Model
427A controls and indicators and explainsthe function
of each. Тһе Option 01 rear panel is shown, Тһе
standard rear panel is blank.
3-5. OPERATING INSTRUCTIONS.
NOTE
To obtain maximum battery life,
set the FUNCTION switch of OFF
when the instrument is not in use.
3-6. MECHANICAL ZERO ADJUSTMENT.
3-7. Before any measurements are made, complete
the Mechanical Zero Adjustment inthe following steps.
а. Be sure instrument has been off for at least
one minute.
b. Rotate Mechanical Zero Adjustment screw
CLOCKWISE until meter pointer istothe left
of zero and moving upscale toward zero.
c. Continue to rotate adjustment screw clockwise,
STOP when needle is exactly on zero, If
needle overshoots, repeat step b.
а. When pointer is exactly over zero, rotate
adjustment screw slightly COUNTERCLOCK-
WISE to relieve tension on suspension. И
the pointer іпоуев to the left, repeat whole
procedure, but make the counterclockwise
rotation less.
3-8. TURN-ON PROCEDURE.
3-9, STANDARD INSTRUMENT,
a, Rotate the FUNCTION Switch to BATT/1. 5
MIN position,
b. Тһе meter should read 1. 5 or higher on the
0-3 scale on any range, indicating that the
battery voltage is 15 volts or higher. И the
reading is below 1.5, replace the battery
according to the steps in Paragraph 5-54.
01791-2
Section ІП
3-10. OPTION 01 INSTRUMENT.
а.
3-11.
а.
b.
с.
е,
3-12.
а.
Select either battery ог line operation withthe
rear panel BATT/LINE slide switch. H bat-
tery operation is selected, check the battery
according to Paragraph 3-9.
If line operation is selected, set the 115/230
slide switch to indicate the proper line volt-
age,
Rotate the FUNCTION switch to the desired
function. During line operation, the BATT/
1.5 MIN check position displays the output of
the Option 01 power supply. The reading
should be 1,5 or higher on the 0-3 scale оп
any range, indicating а power supply output
of 15 volts or more. This serves as а соп-
venient check of the Option 01 power supply.
DC MEASUREMENTS.
Rotate the FUNCTION switch to +DCV or
-DCV depending on the polarity of the input.
Short the VOLTS input tothe COM input, го-
tate RANGE to 0. 1, and adjustthe DC ZERO/
Өс thumbwheel for zero meter deflection.
Remove shorting connection. Н there is a
zero offset with COM and VOLTS open, refer
to Paragraph 4-16 and Paragraph 5-30.
Select approximate range of input with RANGE
switch,
DO NOT APPLY MORE THAN
1200 VDC TO ANY DC RANGE.
Connect input across VOLTS and COM ter-
minals and read magnitude of input on meter.
RESISTANCE MEASUREMENTS.
DAMAGE ТО 427A INPUT CIR-
CUIT МАҮ RESULT IF DC OR
AC VOLTAGE IS APPLIED TO
OHMS TERMINAL.
Rotate the FUNCTION switch to OHMS.
Selectthe approximate range withthe RANGE
switch; and withthe input terminals open, ай-
just the DC ZERO/Qe thumbwheel for an =
indication onthe ohms scale. (Pointer should
rest on the mark just to the left of х).
3-1

Section Ш
с. Connect the unknown resistance across the
OHMS and COM terminals. Read the resis-
tance value on the ohms scale.
NOTE
For best accuracy, select an
ohms range ‘that will place the
meter pointer near the center
of the scale,
3-13. AC MEASUREMENTS.
3-14, The Model 427А responds to the average value
о the ac input and is calibrated in rms volts for a
sine wave input.
value of а non-sinusoidal signal are different, any
distortion on the input will affect the accuracy of the
reading. Table3-1 shows the effect of harmonic dis-
tortion on å reading.
NOTE
The following table is uni-
versal in application since
these errors are inherent in
all average-responding volt-
meters. The error shown above
may vary with the phase relation-
ship between the harmonic and
fundamental.
Table 3-1. Effects of Harmonic Distortion
INPUT VOLTAGE TRUE RMS METER
CHARACTERISTICS VALUE INDICATION
umaman = 10 | юш _| ко |
Fundamental + 10%
second harmonic
Fundamental + 20% 102
second harmonic
Fundamental + 50% 112 100 - 110
second harmonic
Fundamental + 10%
third һагтпопіс
Fundamental + 20%
third harmonic
3-15. Use the following stepsto make ап ас measure-
Fundamental + 50%
ment.
third harmonic
а, Rotate FUNCTION switch to АСУ,
NOTE
With the input shorted, there
may be а zero offset of about
two minor divisions. This is
caused by the bias current through
the meter bridge and does not affect
the accuracy of ac measurements as
the meter moves upscale,
Since the average value and the rms
b.
с.
Model 427А
a м
Rotate RANGE switch to approximate range
of input voltage.
DO NOT APPLY MORE THAN 425V
RMS WHEN THE INSTRUMENT IS
ON RANGES ABOVE 3, OR MORE
THAN 300V RMS ON RANGES BE-
LOW 3.
Connectthe signal to be measured to the
VOLTS and COM terminals and read the
magnitude on the voltage scale.
3-16. DB MEASUREMENTS.
а,
Making а dB ог dBm measurement is essen-
tially the same as making an ac voltage mea-
surement, Follow Ше steps іп Paragraph 3-13, $
but read the magnitude on the dB scale.
The 1 volt position of the RANGE switch is
the 0 dBm range. Each position above 1 volt ?
is а 10 dB increase, and each position below
1 volt is а 10 АВ decrease. Table 3-2 lists
the dB value of each range.
Table 3-2. DB Range Identification
Å given dB reading is equal to the algebraic
sum of the range andthe meter reading. For
example, if the meter reading were -6 and
the instrument were on the 10 volt (+20 dB)
range, the final reading would be 20 dB -
6 dB - 14 dB.
The 427A meter is calibrated in dBm. 0dBm
is equivalent to 0.775 volt dropped acrossa
6002 load. Consequently, any dBm measure-
ments must be made acrossatotal impedance
of 6002. Measurements across other imped-
ances will be in dB, not dBm.
To convert a dB reading to dBm, usethe Im-
pedance Correction Graph (Figure 3-2). For
example, to convert a +30 dB reading made
across а 502 load to dBm, locate the 500
load impedance on the bottom of the graph.
Follow the impedance line to the heavy
black line and read the meter correction
at that point. Тһе correction for 500 is
+10.5 dBm, and the corrected reading is
+40. 5 dBm.
01791-2

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01791-1 3-3
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Model 427A
Section IV
SECTION IV
THEORY OF
4-1. GENERAL.
4-2. The Model 427A measuresac voltage, dc voltage,
and resistance. It is battery operated, but with Op-
tion 01, can be powered by line voltage. Figure 4-1
shows a simplified block diagram of the 427A.
4-3. The Battery Regulator (А1) regulates the bat-
tery output and provides +6. 7 and -6. 7 volt bias sup-
ply to the amplifiers. The 427A uses two amplifiers,
the DC Amplifier (А1) and the AC Metering Circuit
(A2). The former is a high input impedance unity
gainamplifier used toamplify de and resistance inputs.
It also serves as å preamplifier for ac signals. The
AC Metering Circuit amplifies ac signals from the
preamplifier, converts them to dc signals proportional
to the average ac, and connects them to the meter.
The meter is calibrated to display the rms value of
the ac input.
4-4, The DC Offset Adjust (A2) compensates for
leakage current from the de amplifier, and DC CAL and
OHMS CALareresistive circuits used for calibration.
4-5. DC OPERATION.
4-6. Figure 4-2 shows Ше Model 427A in the DC Mode
of Operation. The ас input is first applied to the DC
Range Attenuator where it is attenuated by 10dB for
each stepoftheattenuator. Thedc signal from the at-
tenuator goes to the DC Filter, and the filter rejects
any ac superimposed noise that may be present on the
input. The dc output of the filter goes to the DC Am-
рііНег (А1) and then to the meter. The DC Amplifier
matches the high impedance of the attenuator to the
low impedance of the meter. The DC CAL circuits
ATTENUATOR
Figure 4-2.
01791-2
OPERATION
are resistive circuits in series with the meter used
to adjust the meter current to calibrate the lower
ranges.
4-7. OHMS OPERATION.
4-8. Figure 4-3 15 a block diagram of the Model 427A
in the OHMS Mode of Operation. With the input open,
Ra and Rp forma voltage divider. The voltage across
R,, causes full scale current toflow through the meter.
The OHMS CAL circuit adjusts the meter current for an
indication of » with the input open. When R, is equal
tothe parallel combination of Ва and Rp, the total re-
Bistance from the OHMS terminal to ground will be
half the parallel combination of R4 and Ву, the voltage
into the amplifier will be halved, and the meter indi-
cation will be half scale. The Model 427A is designed
во that thefull RANGE setting will be displayed in the
center of the scale. For example, 10 Поп the X10
range is a center scale reading.
4-9. AC OPERATION.
4-10. Figure 4-4 shows a block diagram of the 427A
in the AC Mode of Operation, The input signal goes to
the AC Range Attenuator. On the 1 volt range and be-
low, the signal is not affected by the AC Range Attenuator;
but on all the higher ranges, the signal is attenuated
by 504В, Capacitor C3adjusts the frequency response
of the attenuator with а 3 volt 100 КН? input. Тһе
signal from the AC Range Attenuator goes through
the DC Amplifier to the AC Post Attenuator where it
is attenuated by 10 dB for each step of the RANGE
selector. The DC Amplifier matches the low impe-
dance of the Post Attenuator to the high impedance of
the Range Attenuator, acting ав а preamplifier.
ос ос
FILTER AMPLIFIER
FEEDBACK
DC Operation

Section IV
OHMS
ATTENUATOR
427А-НО
ty
Model 427A
DC
AMPLIFIER
FEEDBACK
Figure 4-3. Ohms Operation
AC RANGE
AT TENUATOR
DC
AMPLIFIER
FEEDBACK
427A—ROA
AC POST
ATTENUATOR
AC
METERING
CIRCUIT
AC FEEDBACK
Figure 4-4. AC Operation
4-11. The AC Metering Circuit contains both a feedback
Stabilized ac amplifier andan averaging meter circuit,
The meter circuit converts the ac signal to a dc volt-
age proportional to the average of the ac amplifier
output. A2R17 adjusts the current through the meter
so that the scale reading is in rms volts.
4-12. CIRCUIT DESCRIPTIONS.
4-13. DC AMPLIFIER (А1).
4-14. Figure 6-3 is the schematic diagram of the
Model 427A. The input to the DC Amplifier (А1) is
applied through Pin 2 to the impedance converter
A1Q6. A field effect transistor is used as the impe-
dance converter because of its characteristicaily high
input impedance. Transistors А107 and A1Q9 make
пра two-stage amplifier, with 4109 as ап emitter
follower output stage. The signal from the emitter
of А109 is fed back to the base of А107 for gain sta-
bilization.
4-15. A1Q8actsasa constant current source for A1Q6.
The constant current from A1Q8 assures linearity in
A1Q6 and helps minimize drift.
4-2
4-16. DC OFFSET (A2).
4-17. The gate leakage from А196 (Iggg) amounts to а
fraction of a nanoamp. This current is insignificant
with the input shorted; but with the input open, it flows
through the DC Range Attenuator resistance of 10 M2
resulting in a few millivolts deflection on the lower
ranges. Consequently, the meter indications with the
input open and with the input shorted can be different.
When measuring across a low impedance, Iggg flows
mostly through the low impedance; and this difference
does not effect the measurement. Unless 1065 is
compensated for, it will effect measurements made
across high impedances (>1 М 9).
4-18. The DC Offset Circuit (A2) (see Figure 4-5)
compensates for the leakage from A1Q6. A positive
voltage tapped from A2R7 reverse biases A2CR1. The
reverse leakage current from A2CR1 (Ir) meets ап
extremely high resistance at the gate of A1Q6 and
passes through the much lower resistance of the at-
tenuator, іп a direction opposite to that of Iqgg. Ву
adjusting A2R7, Ip can be set so that it and Теса cancel
eachother. The net voltage dropacross the attenuator
01791-2
ғ 4.

ж +
ж ~
Model 427A
РУ А2
RG
OF 4127
<, RESISTANCE
<, OF ATTENUATOR
<, (IO MEGOHMS)
Y
4274-8-0560
Figure 4-5. DC Offset Circuit
would then be zero. Ап alternative method of mini-
mizing the effects of Іс would beto zero the instru-
ment with the impedance of the measurement source
connected to Ще input terminals rather than with the
input shorted, A2CR1 has а temperature charac-
teristic quite similar to that of A1Q6. "Therefore,
01791-2
Е Iu | | |
Section IV
both Теса and Ig change by approximately the same
amount with temperature and the amount of additional
error at higher temperatures is lessened.
4-19. AC METERING CIRCUIT (A2).
4-20. Figure 6-3 contains the schematic of the AC
Metering Circuit. The ac input from the ac post at-
tenuator is applied through A2C1 tothe base of A2Q1.
A2Q2 is an emitter follower that provides impedance
matching to common emitter output stage A2Q3.
Capacitor A2C4 provides an ac feedback path for gain
stabilization.
4-21. The output from the collector of A2Q3 is recti-
fied by A2CR3 and A2CR4 and applied to the meter
movement.
4-22. Resistor A2R17 is used to calibrate the ampli-
Нег at low frequency ага is adjusted for full scale
meter deflection witha 10 ту, 400 Hz input. Capacitor
А2С3 is used to calibrate the amplifier at high fre-
quency. With а 10 mV, 1 MHz input, A2C3 is adjusted
for full scale,
4-23. BATTERY REGULATOR Ai (See Figure 6-3).
4-24. The Battery Regulator regulates the dc voltage
fromthe 22, 5 volt battery or from the optional power
supply. The voltage drop across A1CR2 and the
emitter-base junction of A1Q3 provides the -6, 7 volt
referencefor the power supply andthe -6. 7 volt output.
A1Q5 senses changes in the +6. 7 volt output and соп-
trols A1Q2, the positive series regulator. А101 isa
current source for A1Q5, and diodes A1CR3, CR4,
and CR5 provide temperature stabilization. Тгап-
sistor A1Q4 provides a current shunt for A1CR2 to
keep its voltage constant under heavy current loads,

Section
У
Model
427A
Table 5-1, Required Test Equipment
INSTRUMENT
REQUIRED
RECOMMENDED
TYPE
CHARACTERISTICS
USE
MODEL
DC
Voltage*
Range:
0-300
V
Performance
Checks
~hp-
Model
738BR
уой-
Standard
Accuracy:
+0.2%
meter
Calibrator
Test
Oscillator*
|
Frequency
Range:
10
Hz
to
1
MHz
Performance
Checks
|
-hp-
Model
652A
Oscil-
Output:
0
to
3
Vac
lator
Variable
Line
Frequency
Range:
50
to
60
Hz
Performance
Checks
Powerstat
Type
116B
Transformer
Voltage
Range:
0
to
120
V
Decade
Resistor
|
Range:
102
to 10
MQ
Performance
Checks
|
General
Radio
Model
Accuracy:
+0.5%
1432Z
Decade
Resistor
DC
Voltmeter
Range:
0-50
Vdc
Troubleshooting
-hp-
Model
427A
Volt-
Accuracy:
+2%
meter
AC
Voltmeter
Range:
0-50
Vac
Troubleshooting
-hp-
Model
427A
Volt-
Accuracy:
+2%
meter
Resistors
100
ко
+,
1%
Performance
Checks
-hp-
Part
Ко.
0811-0460
1
МО
+
.1%
-hp-
Part
No,
0811-0473
*
Contained
іп
-hp-
E02-738BR
VTVM
Calibration
System
VOLTMETER CALIBRATION SYSTEM
hpE02-7386R
hp
427A
VOLTMETER
VOLTMETER CALIBRATOR
Артзавв
TEST OSCILLATOR
Apes2a
4274-B-1180
Figure 5-1. AC Accuracy and Frequency Response Check
5-0
01791-2
A^ а

ж o
Model 427A
EE ==
Section V
SECTION V
MAINTENANCE
5-1. INTRODUCTION.
5-2. This section contains information necessary to
maintain the Model 427A. The following paragraphs
describe the Performance Checks, the Calibration
Procedures, and the Troubleshooting Procedures.
5-3. REQUIRED EQUIPMENT.
5-4. Table 5-1 15а list of the equipment needed to
properly maintain the Model 427A. If the recommended
model is not available, use any substitute that meets
the required characteristics.
5-5. PERFORMANCE CHECKS.
5-6, The Performance Checks are "in cabinet" tests
that compare the Model 427A with its specifications.
These procedures can be used both for incoming in-
spection and periodic inspection. A Performance
Check Test Cardis providedat the endof this section
for recording the performance of the Model 427A during
the performance checks. The card canbe removed
from the manual and used as a permanent record of
the incoming inspection or of a routine performance
check. if the Model 427A does not meet its specifi-
cations, refer tothe Adjustment and Calibration Pro-
cedures, Paragraph 5-25,
5-7. AC ACCURACY AND FREQUENCY RESPONSE
CHECK.
5-8. The AC Accuracy and Frequency Response Check
requires a test oscillator that is flat within + 0.5%
from 10 Hz to 1 MHz. The absolute value of the ap-
plied voltage must be accurate within +0.2%. The
-hp- Model 738BR Voltmeter Calibrator produces a
400 Hz signal that is within less than 0.2% of the in-
dicated output. The -hp- Model 652A Test Oscillator
can be adjusted to within less than0. 25% of a desired
reference from 10 Hz to 10 MHz.
NOTE
Before beginning the Performance
Checks, be sure to adjust the me-
chanical meter zero according to
the steps in Paragraph 3-6.
5-9. AC ACCURACY CHECK.
5-10. Connect the test oscillator, and the voltmeter
calibrator as shown in Figure 5-1.
a. Set the voltmeter calibrator for an output of
0.01 volts at 400 Hz.
b, Set 51 in Figure 5-1 to position A.
c. Set the Model 427A FUNCTION to ACV and
the range to 0.01,
d. The 427A should read 0.01 volts rms + 2%
(2 minor scale division).
01791-3
е. Repeat steps а through d for each ас RANGE
selection through 300 V by setting the volt-
meter calibrator output tothe full scale value
for each range. Тһе Model 427A indication
should not vary from the known input by more
than 2% оп any range. 2% corresponds to 2
minor divisions on the 0-1 scale and 1-1/2
minor divisions on the 0-3 scale,
f. Set the Model 427A to the 1 volt range and
apply 0.9, 0.8, 0.7, 0,6, 0,5, 0.3, 0,2 and
0.1 volt signals. In each case the reading
should be within two minor divisions of the
known input signal.
е. Set the Model 427A to the 3 volt range and
apply 2.5, 2, 1.5, 1, and 0.5 volt signals.
In each case the reading should be within
1-1/2 minor divisions of the knowninput sig-
nal.
9-11. FREQUENCY RESPONSE CHECK.
9-12. The frequency range of the -hp- Model 652A
із 10 Hz to 10 MHz. The set level indication men-
tioned in the following steps is used to re-establish
the proper amplitude each time the frequency is
changed.
a. Set S1 in Figure 5-1 to position B. Set the
Model 427A RANGE to 0.01.
b. Setthe test oscillator to 400 Hz and adjust
the output for a 0.95 indication on the 427A
0-1 scale.
с. Set test oscillator to expand, and adjust the
set level control (REF SET) for convenient
meter indication.
d. Record set level indication for use as a re-
ference in steps e and f.
e. Change the oscillator frequency to 10 Hz and
reset the oscillator am plitude to the set level
indication establishedinstepc. DO NOT re-
adjust the set level control on the oscillator.
The 427A indication should not vary by more
than +2 minor scale divisions (x 296).
f. Repeat step e for 100 Hz, 1 kHz, 10 kHz,
100 kHz, 500 kHz and 1 MHz. in each case
the 427A indication should not vary by more
than +2 minor scale divisions (+ 29),
5-13. Кереаі Paragraph 5-11 with the 427А RANGE
set to 3 volts, and test oscillator output set to give
3 V reading оп 427A іп step b. Тһе 427A reading
should not vary by more than +2% (+1-1/2 small
scale divisions).
5-1

Section У
5-14. RESISTANCE ACCURACY CHECK.
5-15, To check the resistance accuracy, precision
resistances are needed. Figure 5-2 shows the resis-
tance accuracy check using а General Radio Model
14322 Decade Resistor. The resistance used should
be accurate to within +0. 5% and should have а range
of 10 ohms to 10 М ohms,
a. With the input open, set the Model 427A
FUNCTION to OHMS and the RANGE to
X10. Adjust the DC ZERO/Sx adjustment
for an indication of « on the meter face.
(The needle should rest on the mark just to
the left of о).
b. Connect the decade resistor and the Model
421A as shown in Figure 5-2, and set the
decade for 10 0.
c. Тһе 427A meter should read within +5% of
the known resistance (- one small scale divi-
sion, + one-half small division).
d. Repeat steps a through c using the RANGE
and decade resistor settings listed in Table
5-2.
DECADE RESISTOR
GENERAL. RADIO 14322
990090090869
— —
Model 427А
т =
МОТЕ
The DC ZERO/ adjustment
need only be made on the X10
and X1K ranges.
Table 5-2, Resistance Accuracy Check
RANGE
5-16. DC ACCURACY.
5-11. The DC Accuracy Check requires а дс voltage
standard that is accurate to within +0. 2% of its indi-
cated output from 0.1 volt to 300 V. The -hp- Model
738BR Voltmeter Calibrator is accurate to within
+0, 1% of the indicated output, Figure 5-3 shows the
test setup for the DC Accuracy Check. ~
а. Set the Model 427A FUNCTION to «ОСУ and
the RANGE to 0.1.
hp 427A
VOLTMETER
@@@ =
427А4-8-0533
Figure 5-2. Resistance Accuracy Check
VOLTMETER CALIBRATOR
hp 7388R
hp 427A
VOLTMETER °
427A-B-0330
ғ «ғ.
Figure 5-3. DC Accuracy and Linearity Check
01791-3

p o.
= ~
Model 427A
b. Adjust the DC ZERO/Q« for zero, This ad-
justment need only be made on the 0.1 volt
range.
c. Connect the voltmeter calibrator and the
Model 427A as shown in Figure 5-3.
а. Set ће voltmeter calibrator output to 0.1
volts dc. The Model 427A should read 0.1
volt «275,
e. Repeat steps a through e for each RANGE
selection to 300 V by setting the voltmeter
calibrator output to the full scale value for
each range. The Model 427A indication should
not vary from the known input by more than
+2% on any range. 2% corresponds to 2 min-
or divisions on the 0-1 scale and 1-1/2 minor
division on the 0-3 scale.
1. Set the dc RANGE to 1000 V and the voltmeter
calibrator to 300 V. The Model 427A should
read 300 V +2% of full scale on the 1000 volt
range.
г. Set the Model 427A to the 1 volt range and
apply 0.9, 0.8, 0.7, 0,6, 0.5, 0.4, 0.3, 0.2,
and 0,1 volt dc signals from the voltmeter
calibrator, 1n each case the reading should
be within two minor divisions of the known
input voltage.
В. Set the Model 427A to the 3 volt range and
apply 2.5, 2, 1,5, 1, and 0.5 volt dc signals.
In each case the reading should be within
1/2 minor division on the 0-3 scale.
i. Repeat setps a through f with the FUNCTION
set to -DCV and the voltmeter calibrator set
for a negative output. Тһе test results should
be the same.
5-18. AC REJECTION CHECK.
3-19. A peak ac superimposed noise signal 100 times
the full scale input should affect the Model 427A read-
ing less than 1%. Figure 5-4 shows the test setup
using a variable line transformer as a noise generator.
А 7.07 volt rms output from the variable line trans-
former corresponds to a 10 volt peak noise signal.
The 10 volt noise signal will be applied to the 0. 1 volt
dc range.
VARIABLE
LINE
TRANSFORMER
42TA-B-O53t
Section V
а. Set the 427A FUNCTION to -DCV and short
the input, Set the RANGE to 0. 1 and adjust
the DC ?ЕНО/Фо for 0 meter indication.
b. Switch the FUNCTION to ACV and the RANGE
to 10. Connect the variable line transform-
er and the Model 427A as shown in Figure 5-4.
c. Adjust the transformer output for a reading
of 7, 07 volts rms on the 427A.
d. Switch the 427A FUNCTION to +DCV andthe
RANGE to 0.1.
e. Тһе meter reading shouldbe 0 +1% (1 minor
scale division).
NOTE
The meter may move upscale
momentarily and then return to
zero. This indicatesthe charging
of the DC Filter capacitors and
is normal.
1. Repeat steps a through e using the 1 volt dc
range and 70, 7 volt rms signal from thetrans-
foriner.
5-20, INPUT IMPEDANCE CHECK.
5-21. INPUT RESISTANCE CHECK.
5-22. Figure 5-5 shows the setup for the input resis-
tance check, А 1 МО resistor is connected in series
with the input, and the voltage drop across the input
resistance will be:
Rin
Ев = Fapplied | Reeries + Rin
With 1 volt applied, Ев will be 0.91 volt if the input
resistance is 10 МО. Ед varies directly with changes
in Rin.
а, Connect the Model 427A and the voltmeter
calibrator to position Å illustrated in Figure
5-5. Set the Model 427A FUNCTION to+DCV
and RANGE to 1 V.
b. Set the voltmeter calibrator for +1 V output.
с. Connect the voltmeter calibrator to position
B shown in Figure 5-5,
hp 427A
VOLTMETER
Figure 5-4. AC Rejection Check
01791-3