HP 428B Instruction Manual

Errata

Title & Document Type: 428B Clip-On DC Milliammeter
Manual Part Number: 00428-90003
Serial Prefixes: 995
Revision Date: Circa November 1970

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Hewlett-Packard's former test and measurement, semiconductor products and chemical analysis
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Changes to this Manual

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above).

If you wish to print on 11x17 or larger just print the page and scale it to fit the paper size.

About this Manual

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®

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OCR errors may exist and as such the user of this document should take care and use common
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Page intentionally left blank.

OPERATING AND SERVICE MANUAL

-hp- Part No. 00428-90003

MODEL 428B

CLIP-ON DC MILLIAMMETER

Serials Prefixed: 995- and above

Appendix A, Manual Backdating Changes

adapts this manual to instruments with

earlier serial prefixes.

Copyright Hewlett-Packard

Company 1970 P.O. Box 301,

Loveland, Colorado, 80537 U.S.A.

Printed: NOV 1970

Model 428B

TABLE OF CONTENTS

1-1. INTRODUCTION 2
1-7. INSTRUMENT AND MANUAL IDENTIFICATION.

2

INSTALLATION 3

2-1. UNPACKING AND MECHANICAL INSPECTION.

3

2-4. OPERATION CHECK. 3
2-6. INSTALLATION. 3

2-8. POWER REQUIREMENTS. 3
2-10. OPERATION ON 115 OR 230 VOLTS. 3

2-13. THREE CONDUCTOR POWER CABLE. 3
2-15. RACK MOUNT MODEL. 3

2-17. PREPARATION FOR STORAGE AND SHIPMENT.

3

2·20. STORAGE. 3

OPERATING INSTRUCTIONS 5

3-1. INTRODUCTION. 5
3-3 OPERATING PRECAUTIONS. 5

3-4. OPERATING CONSIDERATIONS. 5
3-5. INTERCHANGING PROBE HEADS. 5

3-7. EFFECT OF MEASUREMENT ON CIRCUIT. 5
3-12. EFFECT OF CIRCUIT ON MEASUREMENT. 5

3-19. Magnetic Fields. 6
3-26. OPERATING PRACTICES. 6

3-27. MECHANICAL OPERATION OF PROBE. 6
3-29. DEGAUSSING OF PROBE HEAD. 6

3-33. ELECTRICAL ZERO SET. 6
3-37. POLARITY OF CURRENT. 6

3-39. INCREASING THE ABSOLUTE SENSITIVITY 7
3-41. CURRENT CHECK LOOPS.

3-44. NULLING CURRENTS.
3-46. USE OF OUTPUT JACK.

3-48. With Oscilloscope.
3-50. With Recorder.

THEORY OF OPERATION

4-1. INTRODUCTION.
4-3. THEORY OF OPERATION.

4-9. CURRENT PROBE.
4-19. 20 kHz OSCILLATOR.

4-23. HEAD-DRIVE AMPLIFIER.
4-25. DETECTOR GATE AMPLIFIER.

4-28. 40 kHz INPUT/AMPLIFIER CIRCUIT. 11
4-30. SYNCHRONOUS DETECTOR AND FILTER (C24).

4-36. DC AMPLIFIER. 12
4-43. NEGATIVE FEEDBACK CURRENT CIRCUIT. 12

4-45. 40 kHz PHASE SHIFTER. 12
4-48. POWER SUPPLY. 12

7
7

7
7

7

9

9
9

9
11

11
11

11

MAINTENANCE 14 GENERAL INFORMATION 2

5-1. INTRODUCTION. 14
5-3. TEST EQUIPMENT REQUIRED. 14

5-5. IN-CABINET PERFORMANCE CHECKS. 14
5-7. CLEANING OF PROBE JAWS. 14

5-10. ELECTRICAL ZERO SET. 14
5-13. RANGE CHECK. 15

5-15. METER TRACKING. 15
5-17. OUTPUT CALIBRATION. 15

5-19. FREQUENCY RESPONSE. 15
5-21. AC OVERLOAD. 16

5-23. NOISE CHECK. 16
5-25. ADJUSTMENT PROCEDURE. 16

5-27. POWER SUPPLY. 16
5-29. MECHANICAL ZERO SET. 16

5-31. DC AMPLIFIER BALANCE. 16
5-34. ALIGNMENT. 17

5-35. OSCILLATOR BALANCE. 17
5-37. OSCILLATOR FREQUENCY. 17

5-39. OSCILLATOR LEVEL. 17
5-41. DETECTOR GATE. 17

5-43. TUNED AMPLIFIER. 17
5-44. Equipment Setup. 17

5-46. Input Alignment. 17
5-48. Interstage Alignment. 17

5-50. DETECTOR PHASE ADJUSTMENT. 17
5-53. Preliminary Adjustment. 18

5-54. Preset the controls as follows: 18
5-55. Drive Balance Adjustment. 18

5-59. TROUBLESHOOTING. 19
5-60. FRONT PANEL TROUBLESHOOTING. 19

5-63. DETAILED TROUBLESHOOTING. 19
5-64. Probe Check. 19

5-66. Power Supply Check. 19
5-71. Oscillator - Buffer Amp. Check. 21

5-80. Synchronous Detector. 21

REPLACEABLE PARTS 23

6-1. INTRODUCTION. 23
6-4. ORDERING INFORMATION. 23

6-6. NON-LISTED PARTS. 23

CIRCUIT DIAGRAMS 29

7-1. INTRODUCTION. 29
7-3. BLOCK DIAGRAM. 29

7-5. SCHEMATIC DIAGRAMS. 29
7-7. COMPONENT LOCATION DIAGRAMS. 29

MANUAL BACKDATING CHANGES 36

Model 428B

LIST OF TABLES

Table 1-1 Specifications 1
Table 5-1. Recommended Test Equipment. 13

LIST OF ILLUSTRATIONS

Figure 1-1. Model 428B Clip- On Milliammeter 2
Figure 3-1. Measurement Procedures 4
Figure 3-2. Polarity of Current. 6
Figure 3-3. Increasing The Absol ute Sensitivity. 7
Figure 4-1. Block Diagram 8
Figure 4-2. Simplified Block. 9
Figure 4-3. Magnetic Mechanical Analogy. 10

Figure 4-5. 428B Flux Gate. 10
Figure 4-4. Basic Flux Gate. 10
Figure 4-6. Waveforms. 10
Figure 4-7. Detector Bridge. 11
Figure 4-8. Negative Feedback 12
Figure 4-9. 90° Phase Shift 12
Figure 5-1. Cleaning Probe Jaws. 14
Figure 5-2. Electrical Zero Set. 14
Figure 5-3. Range Check. 15
Figure 5-4. AC Overload. 16
Figure 5-5. Oscillator Balance Probe. 17
Figure 5-6. Detector Phase Adjustment. 17

Table 6-1. Replaceable Parts 24

Figure 5-7. Detector Phase Display. 18
Figure 5-9. Detailed Troubleshooting Tree. 20
Figure 6-1. Parts Breakdown, current probe. 28
Figure 7-1. Block Diagram. 30
Figure 7-2. Component Locator For Circuit Board Part

No. 00428-66501 31

Figure 7-3. Front Panel Component Locator. 31
Figure 7-4 Rear Panel Component Locator. 31

Figure 7-5. Power Supply. 32
Figure 7-6. Block Diagram. 33
Figure 7-7. Component Locator for Circuit Board Part

No. 00428-66501 34
Figure 7-8. Front Panel Component Locator. 34
Figure 7-9 Rear Panel Component Locator. 34
Figure 7-10. Metering Circuit 35
Figure A-1. 428B Side Views. 37
Figure A-2 Backdating Schematics for 428B 38
Figure A-3 Backdating Schematics for 428B 39

Model 428B

428B SPECIFICATIONS

Table 1-1 Specifications

Current Range:

1 mA to 10 A full-scale, nine ranges.

Accuracy:

± 3% of full-scale ± 0.15 mA, from 0°C to 55°C.
(When instrument is calibrated to probe).

Probe Inductance:

Less than 0.5 µH.

Probe Induced Voltage:

Less than 15 mV peak (worst case at 20 kHz and
harmonics).

Output:

Variable linear output level with switch position
for calibrated 1 V into open circuit (corresponds

to full-scale deflection). 1.5 V Max. into open
circuit in uncalibrated position.

0.73 ± .01 V into 1 kΩin calibrated position.

Noise:

1 mA Range, < 15 mV rms across 1 kΩ.
3 mA Range, < 5 mV rms across 1 kΩ.

10 mA thru 10 A Ranges, <2 mV rms across 1
kΩ.

Frequency Range:

DC to 400 Hz (3 dB point).

AC Rejection:

Signals above 5 Hz with peak value less than
full-scale affect meter accuracy less than 2%.
(Except at 40 kHz carrier frequency and its

harmonics). On the 10 A range, ac peak value is
limited to 4 A.

Probe Insulation:

300 V Max.

AC Power:

115 or 230 V ±10%, 50 to 60 Hz, 71 W.

Operating Temperature:

- 20°C to + 55°C.

Cabinet Mount:

7½" wide, 11½" high, 14½" deep (190,5 x 292,1
x 368,3 mm).

Weight:

Cabinet Mount: Net 19 lbs (8,6 kg);
shipping 24 lbs (10,9 kg).

Rack Mount: Net 24 lbs (10,9 kg); shipping
35 lbs 15,9 kg).

Accessories Available:

-hp- Model 3528A Large Aperture Probe

-hp- Model 3529A Magnetometer Probe

-hp- Model 11035A Output Cable

-hp- Model 10110A Output Adapter

Dimensions:

Rack Mount

1

Model 428B

SECTION I

GENERAL INFORMATION

1-1. INTRODUCTION

1-2. The -hp- Model 428B Clip-On Milliammeter measures
the magnetic field, which exists around the wire carrying dc

current. Operating the instrument is simple. After zero
setting, the two jaws of the probe are clamped around wire

(arrow on probe head indicates direction of conventional
current flow) and the meter will indicate the current.

1-3. There are nine current ranges starting from 1 mA to 10
amp full-scale deflection. The sensitivity can be increased

even further by looping the wire several times through the
opening in the probe. The current indication is virtually

insensitive to superimposed ac signals and the series
loading of the circuit is less than 0.5 pH. A large amount of
feedback provides great stability.

1-4. OTHER PROBE HEADS.
1-5. Other probe heads are available to extend the

usefulness of your Clip-On DC Milliammeter. Write to the
nearest Sales and Service Office (listed in Appendix C) for

further information. At the time of publication of this manual,
the following accessory probe heads were available:

a. -hp-Model 3528A Large Aperture (2-1/2

inch probe head).

b. -hp- Model 3529A Magnetometer (1

gauss = 1 amp).

c. -hp- Model C11-3529A Magnetometer

(1 gauss = 1 mA).

1-6. Write to the nearest Sales and Service Office
(listed in Appendix C) stating your complete

requirements for information concerning special
applications.

1-7. INSTRUMENT AND MANUAL

IDENTIFICATION.

1-8. Hewlett-Packard uses a two-section serial
number. If the first section (serial prefix) of the serial

number on your instrument does not agree with
those on the title page of this manual, change

sheets supplied with the manual will define the
differences between your instrument and the Model

428B described in this manual. Some serial
numbers may have a letter separating the two

sections of the number. This letter indicates the
country in which the instrument was manufactured.

2

Figure 1-1. Model 428B Clip-On Milliammeter

Model 428B

SECTION II

INSTALLATION

2-1. UNPACKING AND MECHANICAL INSPECTION.

2-2. Inspect instrument for signs of damage incurred in
shipment. This instrument should be tested as soon as

it is received. If it fails to operate properly, or is
damaged in any way, a claim should be filed with the

carrier. A full report of the damage should be obtained
by the claim agent, and this report should be forwarded

to us. We will then advise you of the disposition to be
made of the equipment and arrange for repair or

replacement. Include model number and serial number
when referring to this instrument for any reason.

2-3. Hewlett-Packard Company warrants each
instrument manufactured by them to be free from

defects in material and workmanship. Our liability under
this warranty is limited to servicing or adjusting any

instrument returned to the factory for that purpose and
to replace any defective parts thereof. Any damage to

the instrument upon receipt is due to the carrier. File a
claim with the carrier as instructed in the preceding
paragraph.

2-4. OPERATION CHECK.

2-5. This instrument should be checked as soon as it is
received to determine that its electrical characteristics

have not been damaged in shipment. Refer to the
In-Cabinet Performance Checks of Section V of this
manual.

2-6. INSTALLATION.

2-7. See Paragraph 3-3 before operating this
instrument.

2-8. POWER REQUIREMENTS.

2-9. Power requirements are given in Specifications
table at t he front of this manual.

2-10. OPERATION ON 115 OR 230 VOLTS.

2-11. This instrument may be used with either a 115
volt or 230 volt supply with a frequency of 50 to 60 cps,

single phase. This instrument is shipped from the
factory ready for operation from a 115 volt source

unless otherwise indicated.

2-12. To operate from a 230 volt source, the 115-230
switch on the rear apron must be flipped to 230. First

turn the instrument off and pull the power cable from the
socket. Place a pointed tool, such as the sharpened end

of a pencil, in the slot of the switch and pull down.
Replace the fuse with the one given in Table 6-1 for 230

volt operation.

2-13. THREE CONDUCTOR POWER CABLE.

2-14. The three-conductor power cable supplied with
the instrument is terminated in a polarized, three-prong

male connector recommended by the National Electrical
Manufacturers' Association (NEMA). The third

conductor grounds the instrument chassis for the
PROTECTION OF THE OPERATING PERSONNEL.

When using a three-prong to two-prong adapter ground
third lead (green wire) externally.

2-15. RACK MOUNT MODEL.

2-16. This instrument is available in a rack mount
version in addition to the cabinet model shown in this

manual. The rack mount version is identical electrically
and similar physically except that the degausser has

been moved to the front panel for greater convenience.

2-17. PREPARATION FOR STORAGE AND SHIPMENT.

2-18. The best method for packing this instrument is in
the original shipping carton with the original fillers

packed in the same manner as when received from the
factory. Therefore, when unpacking, note carefully the

method of packing and save the original packing
material for possible future reuse.

2-19. If the original packing material is not available, and
it is desired to package the instrument for storage or

shipment, first wrap the instrument in heavy kraft paper
to avoid scratching the paint. Then pack in a cardboard

carton with a bursting strength of at least 150 lb per
square inch. Pad the instrument on all sides with at least

2 inches of rubberized hair or at least 4 inches of tightly
packed excelsior.

2·20. STORAGE.

2-21. No special precautions are necessary in storage
except the usual protection against mechanical damage,

salt air, etc.

3

Model 428B

4

Figure 3-1. Measurement Procedures

Model 428B

SECTION III

OPERATING INSTRUCTIONS

3-1. INTRODUCTION.

3-2. This section contains instructions and information
necessary for operation of the Model 428B clip-on

milliammeter.

3-3 OPERATING PRECAUTIONS.

CAUTION

a. BEFORE APPLYING OPERATING POWER TO

THE 428B, VERIFY THAT THE LINE VOLTAGE
SWITCH ON THE REAR PANEL INDICATES THE

LINE VOLTAGE TO BE USED AND THAT THE
INSTRUMENT IS PROPER L Y FUSED.

b. THE PROBE IS INSULATED TO WITHSTAND 300

VOLTS MAXIMUM. DO NOT USE THIS PROBE ON
A BARE WIRE WHICH IS MORE THAN 300 VOLTS

PEAK ABOVE GROUND.

c. DO NOT USE THE 428B PROBE IN THE

PRESENCE OF STRONG RF FIELDS.

d. DO NOT EXPOSE THE 428B PROBE TO

TEMPERATURES EXCEEDING 130° F (55°C). DO
NOT LAY THE PROBE ON TOP OF THE 428B

CABINET (OR ANY OTHER HOT SURFACE).
PROBE UNBALANCE AND EVENTUAL DAMAGE

WILL RESULT.

e. DO NOT DROP THE PROBE OR RELEASE THE

FLANGES ABRUPTLY SO THAT THE JAWS SNAP
TOGETHER.

f. DO NOT OPERATE THE DEGAUSSER FOR MORE

THAN THREE MINUTES CONTINUOUSLY.

g. BECAUSE THE 428B IS COOLED BY

CONVECTION" PLACE THE 428B WHERE AIR
CAN CIRCULATE FREELY THROUGH THE

INSTRUMENT.

h. DO NOT USE THE 428B TO MEASURE DC IN A

WIRE WHICH CARRIES MORE AC THAN
FULL-SCALE READING ON THE METER.

3-4. OPERATING CONSIDERATIONS.

3-5. INTERCHANGING PROBE HEADS.

3-6. Each probe is calibrated at the factory with a
particular instrument and carries the serial number of that

instrument (serial number appears on probe

connector) (NOTE: if your buying one with the probe,

make sure you verify this. The numbers are scribed
with a vibrating pen. Not very HP). If a probe has to be

replaced, a realignment and recalibration of the
instrument is necessary (see also Section V

Maintenance).

3-7. EFFECT OF MEASUREMENT ON CIRCUIT.

3-8. Reflected Impedance.
3-9. The probe will add a small inductance to the

circuit of less than 0.5 microhenries due to the
magnetic core and magnetic shield. This makes it

ideal for measuring current in very low impedance
paths such as ground loops where other instruments

would disturb the circuit.
3-10. Induced Voltage.

3-11. The gating signal, driving the core in and out of
saturation, will induce a voltage in the wire carrying

the dc current. This induced voltage is less than 15
millivolts peak. If more than one loop is passed

through the probe the induced voltage will be
multiplied by the number of loops.

3-12. EFFECT OF CIRCUIT ON MEASUREMENT.

3-13. Circuit Impedance.
3-14. The impedance of the circuit being measured

has practically no effect on the dc current
measurement. A shorted loop inserted along with a

wire carrying dc current will decrease the reading by
only 0.2% of full scale.

3-15. AC Fields & Superimposed AC Current.
3 -16. The instrument is designed to allow a high

amount of ac ripple in the dc being measured. The
presence of ac whose peak value equals full-scale

reading (limited to 4 amperes peak on 10-ampere
range) will cause less than 2% error in the dc reading.

Examples of such high ac currents are found in the
input of dc filter sections of power supplies.

3-17. Ac currents having frequency components of 40
kHz or harmonics thereof will cause error, as such

signals will interfere with the 40 kHz output signal of
the probe. The meter will indicate a beat reading if the

interfering frequency is within approximately 15 cycles
of 40 kHz or its harmonics. Although this situation is

very improbable, accurate dc current readings can be
obtained by shifting the frequency of the external ac

signal slightly.
3-18. The instrument as well as the pro be

head .should not be used in strong ac stray fields.
Such fields may exist in the vicinity of open core

power transformers, or large dc filter chokes, etc.

5

Model 428B

3-19. Magnetic Fields.

3-20. If the jaws of the probe are incompletely closed,
the magnetic shielding and the magnetic circuit will have

an air gap. The result is that dc fields, not associated with
the dc current being measured, will cause a shift in the

meter reading.
3-21. However, there will be an indication of a strong

external dc field even with the jaws perfectly closed.
Usually zero setting with the ZERO control compensates

such residual readings for a particular probe location.
3-22. EARTH'S MAGNETIC FIELD. The earth's magnetic

field will affect the reading if the jaws of the probe are not
completely shielded (jaws partially open). The effect of this

field is relatively strong - comparable to deflection due to
about 500 mA of current. Complete closure of the jaws

can be checked by switching to the 1 mA range with no dc
current input. If the jaws mate properly, the zero set

should stay within 0.1 mA while rotating the probe head
with respect to the earth's magnetic field.

3-23. If the zero shift is greater, the mating surfaces of the
jaws need to be cleaned or the probe wiring may be open

(see Section V).
3-24. FIELDS OF PERMANENT MAGNETS. Meter

magnets have strong stray fields, which can cause shift in
the current indication. Such fields are detected by bringing

the closed probe in the area where the measurement is to
be made and observing the zero shift (1 mA range).

3-25. FERROUS WIRE. Wires made out of magnetic
materials can cause a current reading of 2-3 mA without

any connection to the wire. This fact is important as leads
of most transistors are made out of magnetic material.

may be necessary after measuring current on the
1 thru 10 AMP RANGE.

3-32. Normally, it takes about 10 seconds to degauss
the probe when using the above method (see

Caution, Paragraph 3-3f).

3-33. ELECTRICAL ZERO SET.

3-34. If the instrument cannot be zero set electrically
(with ZERO control) there are two probable causes:

1) Incomplete closure of probe jaws, 2)
Magnetization of probe head.

3-35. Dust deposits on the lapped surfaces of the
probe jaws create an air gap. If the jaws are not

completely closed, the earth's magnetic field will
affect the reading. With the RANGE switch at 1 mA,

rotation of the closed probe should not vary the zero
set more than 0.1 mA. Cleaning of the jaws will

restore proper operation conditions (see Section V,
Cleaning of Probe Jaws).

3-36. Magnetic shields protect the probe head from
stray magnetic fields. However, excessive dc

currents (such as short circuit discharge currents
from electrolytic capacitors, etc.) will magnetize the

probe. For demagnetization of probe head, see
Paragraph 3-29, Degaussing of Probe Head.

3-37. POLARITY OF CURRENT.

3-38. The arrow on the probe head indicates the
direction of the conventional current flow for upscale

reading. Reversal of the current flow direction will
reverse the indication on the meter (see Figure 3-2).

3-26. OPERATING PRACTICES.
3-27. MECHANICAL OPERATION OF PROBE.

3-28. The probe jaws are opened by simply squeezing
together the two flanges on the probe body. An internal

spring returns the jaws to their proper position when the
flanges are released. (See Paragraph 3-3e.).

3-29. DEGAUSSING OF PROBE HEAD.

3-30. To demagnetize the probe, proceed as follows:
a. Insert probe into degausser at the rear of the

instrument (located on front panel of rackmount
models) with arrow on probe in same position as

arrow marked on chassis.

b. Depress degausser switch S3 to energize degausser.
c. Withdraw probe very slowly for the first few inches

while depressing the degausser switch until probe is
removed approximately one foot.

d. Zero instrument on 1 mA range with ZERO control

3-31. Under normal operating conditions, degaussing

Figure 3-2. Polarity of Current.

6

Model 428B

3-39. INCREASING THE ABSOLUTE SENSITIVITY

3-40. The sensitivity of the instrument can be increased by
looping the wire (carrying the dc current) several time

through the opening of the probe (see Figure 3-3). For
example, three turns increase the sensitivity three times.

With an increased sensitivity, however, the induced voltage
between the probe and the circuit under measurement will

increase also.

Figure 3-3. Increasing The Absolute Sensitivity.

3-41. CURRENT CHECK LOOPS.

3-42. In restricted situations such as printed circuit boards,
wire loops for the probe can be built into the circuit to allow

convenient current measurements with the Model 428B.
Here, currents can then be measured under operating

conditions with the same ease as voltage measurement.
3-43. Circuits can also be modified to accept an impromptu

loop for testing. As an example, to measure the collector
current of a transistor for troubleshooting purposes, the

collector lead can be removed from the board and a loop of
fine wire soldered between the collector lead and the board.

To measure current through a resistor, lift one lead and
install a series loop, clip the 428B probe around the loop and

measure current through the resistor. As an alternative, an
equivalent resistor with long leads can be installed to replace

the resistor in question.

3-44. NULLING CURRENTS.

3-45. The resolution of the 428B can be increased by nulling
one current against another and measuring the difference

between the two. To null the reading, clip the probe over
both wires at once with the wires so arranged that the

currents are going in opposite directions. The considerations
mentioned in Paragraph 3-39 also apply to current nulling.

For example, assume that a 0.6 A current source is to be
tested against a 0.4 A standard. The 0.6 A supply should be

looped twice through the probe jaws and the 0.4 A supply
should be looped three times through the jaws such that the

7

two currents oppose each other. It should be
remembered when making such a measurement,

that the absolute value of any deviations observed
have been multiplied. If, in the above example, the

0.6 A supply wavered by .01 A, the change would
be read as .02 A on the meter.

3-46. USE OF OUTPUT JACK.

3-47. The OUTPUT jack enables the 428B to be
used as a dc coupled: amplifier/I-E transducer

/isolator. The basic action of the 4 28B (considered
as an input/output device) is to sense the magnetic

field around a current carrying wire and deliver a
proportional voltage at the OUTPUT jack. The

value of the output can be varied by using the
OUTPUT LEVEL control to produce as much as 1

1/2 volts at 1 mA. While the 428B meter registers
average dc (ignoring ac), the output at the

OUTPUT jack contains both the dc and ac
components of the signal being measured.

3-48. With Oscilloscope.

3-49. To display the output of the 428B on an
oscilloscope:

a. If the oscilloscope is dc coupled, it can be

calibrated as in Paragraph 3-51.

b. Clip the probe around the wire which varies the

signal to be displayed.

c. Connect the oscilloscope input to the 428B

OUTPUT jack.

d. Adjust the 428B RANGE switch to the

appropriate range.

3-50. With Recorder.

3-51. To record the output of the 428B on a graphic
recorder:

a. Insure that the recorder's input impedance

exceeds 1400 ohms.

b. Connect the recorder input to the 428B

OUTPUT jack.

c. Zero the 428B on the 1 mA Range, turn

OUTPUT LEVEL to minimum output.

d. Zero the recorder.
e. Adjust the 428B ZERO control for full-scale on

the 428B meter.

f. Adjust the 428B OUTPUT LEVEL control for

full scale on the recorder.

g. Zero the 428B, switch to the appropriate range

and clamp the 428B probe around the wire
which carries the signal to be measured.

3-52. When recording current variations with the
428B, it should be borne in mind that the 428B

displays some long term zero drift. The 428B zero
drift normally amounts to about 300 µA (indicated)

per clay so periodic checks should be made to
determine whether or not the ZERO controls need

adjustment.

Model 428B

8

Figure 4-1. Block Diagram

Model 428B

SECTION IV

THEORY OF OPERATION

4-1. INTRODUCTION.

4-2. This section describes the overall operation of the Model 428B, the operating principle of the current probe and
the function of the different circuits of the instrument.

4-3. THEORY OF OPERATION.

4-4. The simplified block diagram of Figure 4-2 shows the basic operation of the Model 428B Clip-ON Milliammeter.
4-5. The probe clips around a wire carrying dc current and delivers a 40 kHz output signal which is proportional to the

dc current. For transducing the dc current into a 40 kHz signal, the probe requires a 20 kHz gating signal, as described
in detail under Paragraph 4-9, Current probe.

4-6. The 40 kHz output signal of the probe is amplified, detected and fed back as negative feedback current to the
probe head cancelling the effect of the measured dc current and thus reducing the 40 kHz output signal almost to zero.

The negative feedback current, being proportional to and magnetically almost equal to the dc current of the inserted
wire, is used to indicate the measured dc current.

4-7. The 20 kHz oscillator has two functions: First, it supplies a 20 kHz signal for driving the probe head, and also
provides a 40 kHz (second harmonic) signal for gating the 40 kHz Synchronous Detector.

4-8. Due- to slight unbalances, the probe head output contains a small 40 kHz signal, even with no dc current being
measured. A 40 kHz phase-shifter output cancels such residual 40 kHz signal (zero-set controls).

4-9. CURRENT PROBE.

4-10. The probe head is a specially designed second harmonic flux gate type of a magnetometer used to measure the
magnetic field around a wire carrying direct current.

4-11. The flux gate principle is easily understood by referring to the mechanical model shown in Figure 4-3.
4-12. Coil A (representing wire through probe), is energized with dc, producing a dc flux in the core. Armature is

rotating at a constant rate (F), gating the flux 2F times per second inducing a voltage of 2 F frequency in coil B. The
amplitude is determined by the dc in coil A.

9

Figure 4-2. Simplified Block.

Model 428B

Figure 4-3. Magnetic Mechanical Analogy.

4-13. The Model 428B head uses this principle in a
similar way. Figure 4-4 shows the basic concept of a

saturable flux gate.
4-14. A magnetic core in saturation loses permeability

and, therefore, is comparable to a core that has been
mechanically opened (low permeability due to air gap).

4-15. Coil C saturates the core periodically with a 20 kHz
signal, driving the small cores in and out of saturation

twice per cycle or once for each peak (positive or
negative) of the input current.(See Figure 4-6) The only

function of the 20 kHz signal is to gate the dc flux in the
core of the current probe.

Figure 4-4. Basic Flux Gate.

4-16. The 428B probe head is actually analogous to the
flux gate shown in Figure 4-5. The energizing dc current

produces flux path "A". Flux path "A" is periodically
interrupted by saturation of the (transformer type) core, a

result of the two flux paths "C". Note that the current
enters L3 and L4 from the same end and that the coils are

wound in opposite directions causing opposite magnetic
polarities and the consequent circular flux path (c).

4-17. The four coils in the 428B probe head serve 3
purposes: (a) To saturate the cores, a result of the 20 kHz

current that flows between pins 1 and 2. This current is
generated by the 20 kHz oscillator-amplifier circuits. (b)

To act as a secondary, picking up a chopped signal from
the

10

Figure 4-5. 428B Flux Gate.

Figure 4-6. Waveforms.

Model 428B
wire that is clamped in the probe jaws. (c) To conduct the

dc feedback current that tends to annul the energizing dc
current from the wire being measured.

4-18. Because the coils are electrically arranged in a
balanced bridge circuit, the 20 kHz signal is balanced at

the output of the bridge (pins 3 and 4); and there is no 20
kHz differential signal at this point. The 40 kHz signal and

the dc feedback current are also nulled out by the
balanced bridge so that these signals do not appear as a

differential voltage across pins 1 and 2. The dc feedback
current is isolated from the 40 kHz amplifier by capacitor

C11. The 40 kHz is kept out of the dc circuitry by RF
choke L6.

4-19. 20 kHz OSCILLATOR.

4-20. The function of the 20 kHz oscillator is to generate a
balanced 20 kHz signal which, after amplification, is used

for driving the probe head in and out of saturation.
4-21. The circuit of the 20 kHz oscillator is shown in

Figure 7-10. The oscillator V7 is operating in push-pull
having a plate circuit tuned to 20 kHz. Transformer

coupling provides positive feedback through resistor R94
and R95 to the oscillator control grids. The control grids of

oscillator V7 supply the drive signal for the push-pull head
drive amplifier V8. The oscillator level is adjusted by

controlling the cathode current of V7.
4-22. The common cathodes of oscillator V7 supply the

40 kHz signal (2 pulses per 20 kHz cycle) needed for the
synchronous detector gate amplifier V5 and the 40 kHz

phase shifter.

4-23. HEAD-DRIVE AMPLIFIER.

4-24. The head-drive amplifier V8 supplies the balanced
20 kHz signal for the probe head. Drive balance

adjustment R98 controls the current ratio of the two triode
sections, and hence the second harmonic output. The dc

bias voltage for the oscillator and the head-drive amplifier
is obtained from reference tube V11.

signal passes through a voltage divider SI B, which
keeps the loop gain constant for all current ranges by

maintaining a constant input level range to stage VI. The
output of the 40 kHz amplifier VI is band-pass coupled to

the 40 detector driver stage V2. The output signal of V2
is isolated from ground by transformer T2, and fed to the

40 synchronous detector.

4-30. SYNCHRONOUS DETECTOR AND FILTER (C24).

4-31. The Synchronous Detector detects the amplitude
and the phase of the 40 kHz signal. Phase detection is

necessary to preserve negative feedback at all times.
Since the probe may be clipped over the wire in either of

two ways the phase of the signal may vary by 1800. If
phase detection were not present this 1800 phase

reversal would cause positive feedback and the
instrument would oscillate. With phase detection the

polarity of the feedback will change also, maintaining the
feedback negative around the system at all times.

4-32. The synchronous detector requires a large 40 kHz
gating signal, having the frequency of the desired signal.

Figure 4-7 shows the synchronous detector drawn as a
bridge circuit.

4-33. On one half-cycle, with A much more positive than
E and with B equally more negative that E, the balanced

circuit ACB conducts hard, and C becomes effectively
equal to point E. Circuit BDA is opened at this time by its

back-biased diodes, and only the signal that appears
across the conducting half of the T2(FC) will charge C24.

4-34. On the next half-cycle BDA conducts, ACB
becomes open, and the signal across FD will charge C24.

If signal F is positive with respect to C on the first
half-cycle, signal F will be positive with respect to D on

the second half-cycle, and the top of C24 will consistently
be charged positive. If the signal at F changes phase by

1800 with respect to the gating signal at T3, the top of
C24 will consistently be charged negative.

4-25. DETECTOR GATE AMPLIFIER.

4-26. The 40 kHz resonant circuit C1, C2, and L5
increases the level of the gate signal and filters out all

signals except 40 kHz. It also allows phase adjustment of
the signal to correspond to the phase of the Synchronous

Detector.
4-27. The operation of the Synchronous Detector requires

a high level 40 kHz signal. The 40 kHz output signal of
the oscillator V7 passes through a tuned circuit and drives

the gate amplifier V5. The output of V5 delivers a 40 kHz
gate signal to the Synchronous Detector.

4-28. 40 kHz INPUT/AMPLIFIER CIRCUIT.

4-29. The 40 kHz output voltage of the probe head is
resonated by a 40 kHz series resonant circuit (L5 and

C1/C2). Resistor R1 broadens the resonance response
by lowering the Q to minimize drift problems. The 40 kHz

11

Figure 4-7. Detector Bridge.

Model 428B

4-35. In summary then, C and D are alternately grounded,
and the polarity of the signal across T2 changes as C and

D are switched to produce an output wherein the polarity
is dependent on the phase of the input. Where C is in

phase with A, F will be negative when C and D are
grounded. Where C is 180° out of phase with A, F will be

positive when C and D are grounded.

4-36. DC AMPLIFIER.

4-37. The dc amplifier supplies a negative dc feedback
current to the probe proportional to the output of the

synchronous detector. The polarity of the negative
feedback current changes if the polarity of the dc current

(measured in the probe) changes. In this way the
feedback of the system remains negative at all times thus

maintaining the stability of the instrument.
4-38. In addition, this local negative feedback loop

stabilizes the gain of the DC Amplifier.
4-39. Tube V6 is a differential amplifier in which a signal

of approximately 1 volt (for full-scale deflection) is fed to
pin 7 and compared with the signal on pin 2. The output

of V6 is fed to the base of Q3.
4-40. Transistor Q3 drives the current-amplifiers Q1 and

Q2 which are used as emitter-followers in a push-pull
NPN-PNP pair combination.

4-41. The output current from the complimentary pair, Q1
and Q2, goes through the meter circuit to the current

divider S1A which feeds a portion of this current,
appropriate for the range this instrument is working on, to

the probe head as negative current feedback.
4-42. After passing through S1A and the probe head, the

combined current goes through the parallel resistor
network R60-64. This develops a voltage at the junction

of R61 and R62 which is proportional to the feedback
current. This voltage is applied to pin 2 of V6 to complete

the local feedback loop of the DC Amplifier. This circuit
makes the output current of the DC Amplifier proportional

to the voltage applied to the input grid, pin 7, of V6.

4-43. NEGATIVE FEEDBACK CURRENT CIRCUIT.

4-44. The negative feedback current path is shown in
Figure 4-8 . Current divider S1 A divides the feedback

current in proportion to the dc current being measured*.
For a dc input of 10 A, approximately 50 mA feedback

current is fed to the probe head. Since an equal number
of ampere-turns are necessary for canceling the main dc

flux, the feedback coil inside the head requires
approximately 200 turns.

* Maintaining the current through meter M1 constant (5
mA maximum) for all current ranges. Inductance L6

isolates the 40 kHz signal from the dc current circuit.

4-45. 40 kHz PHASE SHIFTER.

The output of the 40 kHz phase shifter is fed to the head
of the probe to cancel any residual 40 kHz output signal

which exists when zero dc is being measured. The
canceling signal is obtained by adding two voltages

which are 90° out of phase and variable in amplitude.
Figure 4-9 shows the circuit and the idealized phase

relationship of the two output voltages with respect to the
signal from the oscillator.

By adding the two output voltages (vector A and B) a 40
kHz signal is obtained, having phase angle and

amplitude to cancel exactly the residual 40 kHz signal
from the probe (vector C). Once the residual 40 kHz

signal of the probe has been canceled, the ZERO control
compensates for any normal variations of zero shift. This

control is necessary only on the lower ranges.

4-48. POWER SUPPLY.

4-49. A single series-regulated power supply of the
conventional type provides 280 volts regulated for the

circuits of the instrument. Voltage reference tube V11
provides a constant cathode potential at control tube V10,

and this is the reference potential for the control grid of
V10.

12

Figure 4-8. Negative Feedback

Figure 4-9. 90° Phase Shift

Model 428B

Input Impedance: 10 megohms

Table 5-1. Recommended Test Equipment.

EQUIPMENT REQUIRED CHARACTERISTICS USE RECOMMENDED MODEL

Meter Calibrator ± 0.2% of reading

±0.1% FS

Function Generator DC to 400 Hz

> 10 mA output

Oscilloscope DC to 40 kHz

100 mV/cm ± 3%

Resistor 50 Ohms ± 1% Frequency Response

DC Voltmeter ± .25% at 730 mV Output Calibration -hp- Model 3430A 3469B

AC Voltmeter Resolves 2 mV

Battery operated

Resistor 1 kilohm ± 1% Output Calibration

Volt-Ohmmeter Input impedance: ≥1megohm Troubleshooting -hp- Model 427 A

Oscilloscope Probe

Counter Reads 40 kHz ± 20 Hz Alignment

Capacitor .0082 µF ± 10% 300 Vdc Alignment ——————————
Resistor 390 Ohms ± 5% 1/2 W Alignment ——————————

Range Check
Meter Tracking

Output Calibration
AC Overload

Frequency Response
AC Overload

Frequency Response
AC Overload

Troubleshooting

AC Overload

Noise Check -hp- Model 4038

Noise Check

All Troubleshooting -hp- Model 10001A

Troubleshooting

-hp- Model 69208
AC/DC Meter Calibrator

-hp- Model 3310A
Function Generator

-hp- Model 130C
200µV/cm Oscilloscope

-hp- Part No. 0698-3128

0698-8155

DC Digital Voltmeter

AC Portable Voltmeter

-hp- Part No. 11034-82601

Multi-Function Meter

Resistive Divider Probes

-hp- Model 5321 B

13

Model 428B

SECTION V

MAINTENANCE

5-1. INTRODUCTION.

5- 2. This section contains the service information which is
required to properly maintain the 428B Clip-On

Milliammeter. Included are performance checks,
adjustment and calibration procedures, and servicing and
troubleshooting information.

5-3. TEST EQUIPMENT REQUIRED.

5-4. The test equipment required to perform the operations
outlined in this section is listed in Table 5-1. This table

includes the type of equipment, critical specifications,
purpose, and recommended model. If the model

recommended is not available, equipment which meets or
exceeds the critical specifications listed may be
substituted.

5-5. IN-CABINET PERFORMANCE CHECKS.

5-6. The performance checks described in Paragraphs 5-7
through 5-24 are front panel procedures designed to

compare the Model 428B with its published specifications.
These tests may be incorporated into periodic

maintenance, post repair, and incoming quality control
checks. The performance checks should be conducted

before any attempt is made to adjust or calibrate the
instrument. During the in-cabinet performance checks, the

Model 428B should be connected to the ac line through a
variable voltage device so that line voltage may be varied

± 10% from 115 or 230 Vac to assure that the instrument
operates correctly at various ac line voltages.

5-7. CLEANING OF PROBE JAWS.

5-8. When the probe shows excessive sensitivity to the
magnetic fields around it, the probe jaws should be

cleaned (to determine excessive sensitivity, see
Paragraph 5-24 Step a). Excessive sensitivity to external

fields can be caused by foreign material between the
probe jaws or by overheating (see Paragraph 3-3 Step d).

The probe jaws should always be cleaned prior to
calibration, adjustment or repair of the 428B.

5-9. To clean the probe jaws, open the probe and brush off
the mating surfaces with the brush which is supplied with

the instrument. If the brush will not remove the
contaminant, disassemble the probe and clean it with a

pencil eraser as illustrated. When reassembling the probe,
be sure that the arrow on the body of the probe points to

the heavy red wire as shown in Figure 6 -1 .

Figure 5-1. Cleaning Probe Jaws.

5-10. ELECTRICAL ZERO SET.

5-11. In the following procedure the numbers in
parentheses refer to Figure 5-2.

a. Connect the 428B to the power line through a

variable transformer .

b. Turn on power (1) and allow about two minutes of

warmup time.

c. Place probe (2) away from any stray fields such as

meter magnets, or open core transformers.

d. Insure that the probe jaws (3) are completely

closed (Paragraph 5-24 Step a, outlines a way to
do so).

e. Set RANGE switch (4) to 1 mA.
f. Zero-set the 428B with the ZERO (5) control. If the

meter cannot be zeroed, degauss the probe
(Paragraph 3-29) or clean the probe jaws

(Paragraph 5-8).

g. Change the line voltage from 103 to 127 volts (±

10%) and read zero drift on the 428B meter. Zero
drift should not exceed ± .03 mA.

14

Figure 5-2. Electrical Zero Set.

Model 428B
5-12. Allow a 30 minute warmup and stabilization

period before conducting the following performance
checks.

5-13. RANGE CHECK.

5-14. Figure 5-3 illustrates the test arrangement
recommended. A de meter calibrator is required for

this test.
a. Zero the 428B on the 1 mA range.

b. Switch the 428B and the meter calibrator to the 10

amp range. Set the meter calibrator for 5 amps of
output (maximum output for the recommended

6920B). Connect a test lead between the output
terminals of the meter calibrator and loop the test

lead through the jaws of the 428B twice (see
Figure 3-3). The 428B should read 10 amps ± 0.3

amp.

c. Switch the meter calibrator to the 1 amp range and

adjust it for 1 amp of output. Remove the test lead
from the 428B Probe jaws. Zero the 428B, then

loop the test lead 3 times through the 428B probe
jaws. Switch the 428B to the 3 amp range. The

428B should read 3 amps ± 90 mA.

d. Remove the test lead from the 428B probe jaws

and zero set the 428B. Clamp the jaws over the
unlooped lead after switching the 428B to the 1

amp range. The 428B should read 1 amp ± 30 mA.

e. Continue to downrange the 428B and the meter

calibrator as in Steps c and d and as guided by the
Performance Check Test Card. Where possible,

keep the calibrator at full output to take advantage
of the greater accuracy; this consideration applies

to any meter calibrator which is specified at full
range (or full-scale) output. Note that there will be

a small residual magnetism induced in the probe
by the 10 amps of current that have been passed

through the probe, and that it is good practice to
degauss the probe before using the 1 mA range.

5-15. METER TRACKING.

5-16. The test arrangement and equipment required
for this test are the same as for Paragraph 5-13.

a. Zero the 428B on the 1 mA range.
b. Switch the 428B to the 100 mA range. Switch the

meter calibrator to the 10 mA range and adjust for
10 mA of output. Connect a long, thin test lead to

the output terminals of the meter" calibrator and
loop the lead ten times through the jaws of the

428B probe. Read 100 mA ± 3 mA on the 428B.
Remove the test lead from the jaws of the probe

one loop at a time and check the linearity of the
readings. Each reading should be within 3 mA of

nominal (e.g. 90 ± 3 mA, 80 ± 3 mA. etc.).

5-17. OUTPUT CALIBRATION.

5-18. A meter calibrator and a one kilohm resistor are
required for this test.

a. Switch the 428B and the meter calibrator to

the 100 mA range. Connect a test lead
between the meter calibrator output leads and

clip the 428B probe to the lead.

b. Set the meter calibrator to 100 mA output and

read full-scale on the 428B.

c. Connect a voltmeter to the output jack of the

428B. In parallel with the 1000 ohm resistor.

d. The voltmeter should read 0.73 V ± 10 mV.

5-19. FREQUENCY RESPONSE.

5-20. A 50 ohm resistor and a function generator are
required to complete this check.

a. Connect a 50 ohm resistor to the high output

jack of the function generator.

b. Turn the 428B RANGE switch to 100 mA then

clip the 428B probe around the resistor lead.

c. Set the function generator to:

Range: .0001 Function: Square
DC Offset: 0 Frequency Dial: 5

d. Upon the first up-scale alternation to the 428B,

set the function generator Output Level control
for a full-scale reading on the 428B.

e. Connect the vertical input of the oscilloscope

to the output jack of the 428B.

f. Set the coupling of the oscilloscope input to

DC. Set sensitivity and vernier to display
variations of the 428B output as a 10

centimeter peak -to-peak signal.

g. Set the function generator to the sine function.
h. Slowly raise the frequency of the function

generator output using the frequency dial and
function switch as necessary to bring the

output to 400 Hz. Watch the oscilloscope,
looking for the lowest amplitude of peak

deflection.

i. The peak-to-peak level of the waveform

should not drop below 7 centimeters (-3 dB).

15

Figure 5-3. Range Check.

Model 428B

5-21. AC OVERLOAD.

5-22. An oscillator or function generator, a 50 ohm
resistor, an ac voltmeter or oscilloscope, and the meter

calibrator are required to complete this check (a
filament transformer and suitable current limiting

resistors can be substituted for the oscillator).

a. Set the meter calibrator for 10 mA. With the

428B on the 10 mA range clip the 428B probe
onto the lead which connects the calibrator

terminals.

b. Note the 428B reading.
c. Set the function generator to 60 Hz, its output

to generate a 10 mA peak sine wave into a 50
ohm resistor (1 volt peak-to-peak or 0.35 volt

rms across a 50 ohm resistor).

d. Clip the 428B probe onto BOTH the wire from

the meter calibrator and the resistor lead from
the function generator (see Figure 5-4).

e. The 428B should read within 2% of the

reading noted in Step b.

Figure 5-4. AC Overload.

5-23. NOISE CHECK.

5-24. A battery operated ac voltmeter and a 1 kilohm
resistor are required to complete this check.

a. Point probe East and West and rotate about its

axis.

b. Note peak-to-peak change in meter reading, if it

doesn't exceed 0.1 mA on the 1 m A range,
proceed. If the change is excessive, perform

Paragraph 5-8.

c. Zero the Model 428B on the 1 mA range.
d. Connect the output of the 428B to the input of a

battery operated 403 B with a 1 kilohm resistor in
parallel with the 403B input.

e. Section V

f. With the 428B on the 1 mA Range check for a

maximum of 15 m V reading on the AC Voltmeter.

g. Switch the 428B to the 3 mA range. Check for a

maximum of 5 m V reading on the AC Voltmeter.

h. Switch the 428B to the 10 mA range. Check for a

maximum of 2 mV reading on the AC Voltmeter.

i. Check each range from 10 mA to 10 amps using

the procedure of Step f. No ac output reading
should exceed 2 mV.

5-25. ADJUSTMENT PROCEDURE.

5-26. When the instrument shows signs of defective
components, use the troubleshooting procedure to find

and correct the problem. It is quite easy to compound
a 428B trouble by misaligning the instrument in an

attempt to adjust out the effects of a defective
component.

5-27. POWER SUPPLY.

5-28. Connect an electronic dc voltmeter to test point 5
(Pin 1, V9) of the 428B. The voltage at this point

should be 272 ± 6 V if not, adjust R109 for 272 V.

5-29. MECHANICAL ZERO SET.

5-30. When meter is properly zero set, pointer rests
over the zero calibration mark on the meter scale when

instrument is (1) at normal operating temperature, (2)
in its normal operating position, and (3) turned off.

Zero set as follows to obtain best accuracy and
mechanical stability:

a. Allow instrument to operate for at least 20 minutes;

this allows meter movement to reach normal
operating temperature.

b. Turn instrument off and allow 30 seconds for all

capacitors to discharge.

c. Rotate mechanical zero adjustment screw

clockwise until meter pointer is to left of zero and
moving upscale toward zero..

d. Continue to rotate adjustment screw clockwise;

stop when pointer is right on zero. If pointer
overshoots zero, repeat Steps c and d.

e. When pointer is exactly on zero, rotate adjustment

screw slightly counterclockwise to free adjustment
screw from the meter suspension. If pointer moves

during this step, repeat Steps c through e.

5-31. DC AMPLIFIER BALANCE.

5-32. Open the 428B and short across the brass pins
on the circuit board which are connected to the ends of

C1 0 (these pins are located on either side of C14).
Connect a 1500 ohm resistor between Pin 2 of V6 and

the 6.3 V filament supply. Disconnect the feedback
disconnect (see Figure 7-3). Zero the 428B meter with

R46.

16

Model 428B
5-33. Remove the clip lead and resistor. Replace the

feedback disconnect jumper.

5-34. ALIGNMENT.

5-35. OSCILLATOR BALANCE.

5-36. Construct a probe as shown below:

Figure 5-5. Oscillator Balance Probe.

Connect the ends of the probe to the T5 side of R94
and R95. Connect an ac voltmeter between ground

and the junction of the two probe resistors. To adjust
oscillator balance, pad C53 or C54 for minimum

reading (on instruments with serial prefixes of 601- or
below, adjust the slug of T5). The ac reading at the

junction of the probe resistors should be 50 m V or less.
Remove the probe and meter before proceeding.

5-37. OSCILLATOR FREQUENCY.

5-38. Connect an ac coupled electronic counter to test
point 4. Adjust the slug of T5 to set the frequency

measured to 40 kHz ± 200 Hz. If T5 doesn't have
enough range, pad C52 (in instruments with serial

prefixes of 601- or below do not use T5; use C52 only).
Remove counter.

5-39. OSCILLATOR LEVEL.

5-40. Connect an ac voltmeter to Test Point 6 (Pin 7,
V7). Adjust R92 for 8 ±0.4 V. Remove meter.

5-41. DETECTOR GATE.

5-42. Connect an ac voltmeter to Test Point 4 (Pin 7 of
V5). Adjust L7 for a peak reading.

5-43. TUNED AMPLIFIER.
5-44. Equipment Setup.

5-45. Set an ac voltmeter to the 1 volt range and
connect it to Test Point 3 (Pin 7, V2) of the 428B. Clip

the 428B probe around a wire which is carrying 35 mA
rms ac. This current can be generated by a filament

transformer, as in Figure 5-6, or by an oscillator
operating into its rated load as in Paragraph 5-20. Set

the 428B RANGE switch to 100 mA. This arrangement
will be used for input, interstage, and detector phase

adjustments.

Figure 5-6. Detector Phase Adjustment.

5-46. Input Alignment.

5-47. Short TP1 and TP2 to ground, open the
feedback disconnect shown in Figure 7-3, then adjust

C2 for a maximum ac reading at Test Point 3 (Pin 7 of
V2).

5-48. Interstage Alignment.

5-49. Set up equipment as in Paragraph 5-45, then:
a. Connect a .0082 microfarad capacitor across the

brass pins which are connected to L10. Adjust L9 for
maximum ac at Test Point 3 (Pin 7 of V2).

b. Remove the capacitor. Connect the .0082
microfarad capacitor across the brass pins which are

connected to L9. Adjust L10 for maximum ac at TP3.
c. Remove the capacitor and voltmeter.

5-50. DETECTOR PHASE ADJUSTMENT.

5-51. With the bases of Q1 and Q2 (TP1 and TP2)
grounded, open the feedback disconnect jumper (see

Figure 7 -3), and install a 390 ohm resistor between
the feedback wire and the pin on the circuit board to

which it normally connects. Connect the horizontal
input of an oscilloscope to TP4 (Pin 7, V5). Connect

the vertical to TP3 (Pin 7, V2). Except for the ac
voltmeter, set up equipment as in Paragraph 5-45 (a

proper setup is shown in Figure 5-6). Turn slug of L7
until the pattern looks like one of the correct patterns

shown in Figure 5-7. If there is a diamond in the center
of the pattern, the diamond should contain no traces.

Remove the 390 ohm resistor and the voltmeter.

17

Model 428B

Figure 5-7. Detector Phase Display.

5-53. Preliminary Adjustment.

5-54. Preset the controls as follows:

a. Ground the bases of QI and Q2 (TP1 and TP2).

Open the feedback disconnect (see Figure 7-3).

a. Switch the 428B to the 300 mA range.
b. Adjust R84 for minimum ac at the wiper

connection of R84. The reading should be less
than 0.1 volt.

c. Adjust R82 for minimum ac at the wiper

connection of R8·2. The reading should be
approximately 0.02 volt.

d. Thoroughly clean probe head jaws (see Paragraph

5-8).

e. Degauss probe head (see Paragraph 3-29).

5-55. Drive Balance Adjustment.

5-56. With the 428B set to the 1 mA range, and with no
input to the current probe, set R98 for a minimum ac

reading at Test Point 3 (Pin 7 of V2). The minimum
reading should be less than 0.5 volt.

5-57. Residual 40 kHz Cancellation.
5-58. The residual output of the probe head is

cancelled by means of the circuit discussed under
Paragraph 4-45. This procedure assumes that the

preliminary adjustments listed in Paragraph 5-54 have
been completed.

a. Reconnect the feedback disconnect which

was opened in Paragraph 5-54 Step a.

b. Remove the short between the bases of Q1

and Q2 and ground (installed in Paragraph
5-54 Step a).

c. Degauss the probe.
d. Zero the 428B meter with front panel ZERO

control.

e. Adjust R84 for a minimum ac reading at Test

Point 3 (Pin 7 of V2).

f. Continue to alternate Steps d and e until the

adjustment cannot be further refined. The
minimum reading in Step e should be

approximately 0.02 volts or less.

18

Model 428B

5-59. TROUBLESHOOTING.

5-60. FRONT PANEL TROUBLESHOOTING.

5-61. The Front Panel Troubleshooting Tree was
designed for use as a means to guide the repairman to

a block of circuitry. This procedure, in combination with
the adjustment, calibration, and performance checks

should be adequate to repair most 428B's.
5-62. In any case where the front panel troubleshooting

does not lead to the problem, it will be necessary to
troubleshoot the instrument in detail.

5-63. DETAILED TROUBLESHOOTING.

5-64. Probe Check.

5-65. Resistance between pins 1-3, 1-4, 2-3, and 2-4 of
the probe cable connector should be nearly identical

and in the area of approximately 4 1/2 to 5 ohms. Where
a resistance measurement between any of the above

pairs of pins is significantly higher than between the other
pairs (by about 3 times), an open probe coil is indicated.

Any open circuit indicates a broken conductor between
the plug and the bridge. The broken conductor is probably

a broken wire in the cable. Either condition could indicate
a broken wire at the Probe Terminal Section (see wiring

diagram, Figure 6-1).

5-66. Power Supply Check.

5-67. To test the power supply:
a. Measure +272 ±6 Vdc between Test Point 5 and

ground.

b. Measure +12 ±1 Vdc between the cathode of CR10

and ground.

c. Measure -7 ±1 Vdc between the anode of CR9 and

ground.

5-68. Where the +272 V and the +12 V have both failed,
check the +12 first. Check CR10, Q1, C66, and C69.

19

Model 428B

20

Figure 5-9. Detailed Troubleshooting Tree.

Model 428B
5-69. Where the + 272 V is at fault, try replacing V9, V10,

or V11. If the tubes are not defective, check the associated
circuitry, especially CR11 (V11 in some instruments).

5-70. Where the -7 V has failed, measure -17 Vdc at
junction of R102 and C65. If less than -7 volts is measured

at this point, check F3 and C65. If the junction of R102 and
C65 measures more than - 7 Vdc, check CR9 and Q2.

5-71. Oscillator - Buffer Amp. Check.

5-72. Short Test Point 1 and Test Point 2 to ground.
5-73. Compare the waveform at pin 2 of Jl to the waveform

shown at this point of the schematic diagram. If these
waveforms are not comparable, check the waveform at

Test Point 7 and follow the Troubleshooting Tree. If the
waveform at pin 2 of J1 is acceptable, continue to the next

paragraph.
5-74. Set up equipment as described in Paragraph 5-51.

Compare the oscilloscope display to Figure 5-7.
5-75. If the display is unacceptable, clip the probe around

100 mAdc and downrange to the 1 mA range. Find a 2.5 V
pk-pk sine wave at the junction of R1 and L5. If this

waveform is unacceptable, adjust or repair the input tuning
circuit (L5, C1, C2, and R1).

5-76. If the waveform described in Paragraph 5-75 is
alright, use the same test setup to find a 2.5 V pk-pk sine

wave at pin 7 of VI. If this waveform is unacceptable, check
VI and the range switch.

5-77. If the waveform described in Paragraph 5-76 is
alright, use the same test setup to look for a 1 V pk-pk sine

wave at pin 7 of V2. Follow this branch of the
Troubleshooting Tree to its conclusion.

5-78. Detector Gate Amp. Check.
5-79. No test equipment need be set up for these checks

except an oscilloscope and a wire to ground TP1 and TP2.

5-80. Synchronous Detector.

5-81. With TP1 and TP2 still grounded and on the 100
mA range, perform the test indicated. Beyond a tedious

ohmmeter test of the circuitry between T3 and pin 7 of
V6, little can be done to check the detector circuit. It

should be noted, however, that R46 should be able to
provide the + 0.5 Vdc which is required at pin 7 of V6

during normal operation.
5-82. DC Amplifier Check.
5-83. Remove the ground wire from Test Point 1 and

Test Point 2 (installed for previous checks). Disconnect
the probe from the 428B front panel. Set the 428B to the

300 mA range. Zero the 428B with the front panel ZERO
control. Measure and note the voltages at pin 7 and pin

2 of V6. Turn the ZERO control to deliver a ± 1 Vdc
swing around the voltage noted for pin 2. The difference

between the voltages at pins 7 and 2 of V6 should not
exceed 0.05 Vdc at any point during this swing.

5-84. If the DC Amplifier tracks properly as tested in
Paragraph 5-83, proceed with the performance checks.

If the DC Amplifier has failed, continue to perform the
tests in the Troubleshooting Tree.

5-85. CALIBRATION.
5-86. Zero the 428B on, the 1 mA range using the front

panel ZERO control. Switch to the 100 mA range. Clip
the probe around an accurate 100 mA current (as in

Paragraph 5-14). Adjust Meter Cal, R69, for a meter
reading of 100 mA. Connect a 1000 ohm ± 1 % resistor

in parallel with an accurate (0.25%) voltmeter to the
output jack of the 428B. Turn the front panel Output

Level control full counterclockwise past the detent.
Adjust the Output Cal control, R63, for a reading of

exactly 0.73 V on the voltmeter.
5-87. Perform the entire In-Cabinet Performance Tests

(Paragraphs 5-5 through 5-24) to insure that the 428B
meets all of its specifications.

21

Model 428B

22

Model 428B

silverIDinside diameter

seconds

impregnated

throw

ampere(s)

order by description

ohms

hertz

toggle

inductor

printed circuit

logarithmic taper

amperes

faradsVvolt(s)

hertz

alternating current

ohms

position(s)

polystyrene

volts

fixed

wirewound

hertz

volts

amps

optimum value selected at

henry(ies)

mercury

no standard type number

special type

hertz (cyc1e(s) per second)

SECTION VI

REPLACEABLE PARTS

6-1. INTRODUCTION.

6-2. This section contains information for ordering
replacement parts. Table 6-1 lists parts in alphabetic

order of their reference designators and indicates the
description, -hp- part number of each part, together with
any applicable notes, and provides the following:

a. The TQ column lists the total quantity of any

part listed in the manual. The total quantity of
a part is given the first time the part number

appears.

b. Description of the part. (See list of

abbreviations below.)

c. Typical manufacturer of the part in a

five-digit code. (See Appendix B for list of
manufacturers.)

d. Manufacturer's part number

6-3. Miscellaneous parts are listed at the end of
Table 6-1.

6-4. ORDERING INFORMATION.

6-5. To obtain replacement parts, address
order or inquiry to your local Hewlett-Packard Field

Office. (See Appendix C for list of office locations.)
Identify parts by their Hewlett-Packard part

numbers. Include instrument model and serial
numbers.

6-6. NON-LISTED PARTS.

6-7. To obtain a part that is not listed, include:
a. Instrument model number.

b. Instrument serial number.
c. Description of the part.

d. Function and location of the part.

Ag
AI

A
Au
C
cer
coef
com
comp
conn
dep

DPDT double-pole double­DPST double-pole single­elect
encap
F
FET
fxd
GaAs
GHz Gigahertz=10
gd

Ge
grd
H
Hg
Hz

impg
incd
Ins
kΩ kilohm(s) 10
kHz kilohertz= 10
L
lin
log
mA milliampere(s) = 10
MHz megahertz = 10

MΩ megohm(s) = 10
metflm
mfr
ms
mtg
mV millivolt(s)= 10
µF
µs

+9

µV microvolt(s) = 10
my

nA nanoampere(s) = 10
NC
Ne
NO
NPO

(zero temperature coefficient) Si Silicon

Symbols Multiplier Prefix Symbols Multiplier

tera T 10
giga G 10

mega M or Meg 10
kilo K or k 10
hecto h 10
deca da 10 femto f 10
deci d 10

ABBREVIATIONS

ns nanosecond(s) = 10
nsr

Ω

obd

+3

OD

+3

P
pA
pc

-3

pF picofarad(s) 10

+6

piv

+6

p/o
pos
poly
pot
p-p peak-to-

-3

ppm
prec w/o

precision (temperature coefficient,

-6

R

-9

Rh
rms root-Olean­rot
Se
sect

sl slide

long term stability, and/or

DECIMAL MULTIPLIERS

+12
+9

+6
+3
+2

-1

centi c 10
milli m 10

micro µ 10
nano n 10
Pico p 10

atto a 10

-9

SPDT single-pole double­SPST single-pole single-

Ta
Te
Ti02
tog
tol
trim

-12

TSTR
vacw

var
vdcw
W
wi
wiv

ww
*

factory, avg value shown

**

-2

-3

-6

-9

-12

-15

-18

23

A assembly FL filter Q transistor TS terminal strip

DESIGNATORS

B motor HR heater OCR transistor-diode V vacuum tube, neon bulb,
BT battery IC integrated circuit R resistor photocell, etc.
C capacitor J jack RT thermistor W cable

CR diode K relay S switch X socket
DL delay line L inductor T transformer XDS lamp holder
DS lamp M meter TB terminal board XF fuse holder
E misc. electronic part MP mechanical part TC thermocouple Y crystal
F fuse P plug TP test point Z network

Model 428B

Diode:

Diode:

Diode:

Table 6-1. Replaceable Parts

REFERENCE

DESIGNATOR

A1, A2

A3
Cl

C2
C3 thru Cl0

C11
C12

C13
C14
C15

C16, C17
C18
C19

C20
C21

C22, C23
C24

C25
C26
C27

C28, C29
C30

C31
C32 thru C40
C41

C42
C43 thru C50
C51

C52
C53, C54

C55
C56 thru C60

C61
C62
C63

C64
C65

C66
C67
C68

C69
C70
C71

CR1
CR2

CR3
CR4

CR5
CR6
CR7

CR8
CR9

CR10
CR11, CR12
CR13 thru CR16

CR17
DS1

-hp-

PART NO.

428B-26A

428B-268

0160-0938
0131-0004

0150-0014
0180-0050

0160-0269
0140-0179
0150-0012

0160-0269
0140-0179
0150-0012

0180-0059

0170-0078
0180-0058

0150-0012
0140-0179

0150-0012

0170-0019

0140-0223
0160-2204

0160-0269
0160-0363

0140-0164
0140-0210

0180-2336
0180-2337
0160-0134

0150 .. 0012

0180-0058
0180-0104

0160-0269
0180·0104

0160-2203
0160-0134

1901-0036

1901-0036

1901-0036

1901·0036

1902-0013
1902-0211

1901-0040
1910-0016

1902-3404
1450-0049

TQ DESCRIPTION MFR. MFR. PART NO.

2

Assembly: resistor ww

A1 includes R3 thru R6
A2 includes R7 thru R 10

1

Assembly: resistor ww, includes R2, Rll

1

C: fxd 1000 pF 5%

1

C: var mica 16-150 pF 175 vdcw
Not assigned

1

C: fxd cer .005 microfarad 500 vdcw

1

C: fxd 40 microfarads +75% -10% 50 vdcw

5

C: fxd cer .1 microfarad 20% 500 vdcw

3

C: fxd mica 1000 pF 2%

6

C: fxd cer .01 microfarad 20% 1000 vdcw
C: fxd cer .1 microfarad 20% 500 vdcw

C: fxd mica 1000 p F 2%
C: fxd cer .01 microfarad 20% 1000 vdcw

Not assigned

1

C: fxd 10 microfarads + 75% -10% 25 vdcw
Not assigned

1

C: fxd my .47 microfarad 5% 150 vdcw

2

C: fxd 50 microfarads +75% -10% 25 vdcw
C: fxd cer .01 microfarad 20% 1000 vdcw
C: fxd mica 1000 pF 2%

C: fxd cer .01 microfarad 20% 1000 vdcw
Not assigned

1

C: fxd my 0.1 microfarad 5% 200 vdcw
Not assigned

1

C: fxd mica 260 pF 1 %

1

C: fxd mica 100 pF 5%
Not assigned
C: fxd cer .1 microfarad 20% 500 vdcw

1

C: fxd mica 620 pF 5%

2

C: fxd mica 6800 pF 5%

1

C: fxd mica 270 pF 5%
Not assigned

1

C: fxd my 20 microfarads +50% -10% 450 vdcw

1

C: fxd my 20-20 microfarads +50% 10% 450 vdcw

2

C: fxd mica 220 pF 5%
C: fxd cer .01 microfarad 20% 1000 vdcw

C: fxd 50 microfarads + 75 -10% 25 vdcw
C: fxd 200 microfarads +75% -10% 15 vdcw

2

Not assigned
C: fxd cer .1 microfarad 20% 500 vdcw

C: fxd 200 microfarads +75% -10% 15 vdcw
C: fxd mica 91 pF 5%

1

C: fxd mica 220 pF 5%
Diode: si 1000 piv

4

Not assigned
Diode: si 1000 piv

Not assigned
Diode: si 1000 piv

Not assigned
Diode: si 1000 piv

Not assigned
Diode: breakdown 6.81 V 10%

1

Diode: breakdown 12.1 V 10%

1
2
4

1
1

si 30 wiv 50 mA 2 pF 2ns
germanium 60 wiv 1 microsecond Trr

breakdown 82.5 V 5%

Light Indicator: clear

-hp-

-hp­14655

72136

04222
56289

56289
14655
56289

56289
14655
56289

56289

84411
56289

56289
14655

56289

56289

14655
72136

56289
72136

14655
00853

56289
56289
14655

56289
56289

56289

56289
56289

14655
14655

04713

04713

04713

04713

04713
04713

01295
08257

04713
72765

RDM15E102J1C obd
T51410-9

D1-4 obd
30D406G050DD2-DS

41C9285-CDH
RDM19F102G3C obd
29C214A3-CDH

41C9285-CDH
RDM19F102G3C obd
29C214A3-CDH

30D106G025882-DSM

663UW
30D506G025CC2-DSM

29C214A3-CDH
RDM19Fl02G3C obd

29C214A3-CDH

192P10452-PTS

RDM15F261F3C obd
RDM15F101J3C obd

41C92B5-CDH
RDM15F621 J3C obd

RDM30F682J5S obd
RDM15F271 J3C obd

68010168
68D20060
RDM15F221J3C obd

29C214A3-CDH
30D506G025CC2-DSM

30D207G015DF2-DSM

41C92B5-CDH
30D207G015DF2-DSM

RDM15F910J3C obd
RDM15F221 J3C obd

SR1358-12

SR1358-12

SR1358-12

SR1358-12

SZ1521-7
SZ1521-79

PG512
obd

SZ10939-446
599-125( clear)

24

Model 428B

REFERENCE

DESIGNATOR

F1
F1

F2
F3

J1
J2

J3
L5
L6

L7
L8

L9, L10
M1
Q1

Q2
Q3

R1
R2 thru R11
R12

R13
R14

R15
R16
R17

R18
R19

R20
R21
R22

R23
R24

R25
R26
R27

R28
R29

R30
R31
R32

R33
R34

R35
R36
R37

R38 thru R40
R41 thru R44

R45
R46
R47

R48
R49

R50
R51
R52

R53 thru R 59
R60

-hp-

PART NO.

2110-0202
2110-0312

2110-0312
2110-0012

1251-0089
1251-2357

1250-0118
9100-1487
9100-1488

9100-1489
9110-0025

9100-1482
1120-0904
1854-0039

1853-0016
1854-0003

0684-6801

0757-0346
0757-0386

0757-0397
0698-4420

0757-0419
0698-4433

0698-4470
06984486

0698-4503
0683-1345
0698-7683

0684-1021
0684-1051

0687-1031
0684-1541

0683-7515
0684-1541

0687-5621
0757-0353
0698-4965

0698-4712
0684-4701

06874731
0686-9115
0684-4711

0757-0283
0757-0327

2100-0331
0689-3935

0684-4711
0698-4594

0684-5631
0757-0982
0698-0058

0683-2015

Table 6-1. Replaceable Parts (Cont'd)

TQ DESCRIPTION MFR. MFR. PART NO.

1

Fuse: .5 AT 250 V (for 230 V operation)

2

Fuse: 1 AT 250 V (for 115 V operation)
Fuse: 1 AT

1

Fuse: .5 A 250 V

1

Connector: female 4 pin

1

Connector: male 3 pin power

1

Connector: bulkhead, BNC female

1

Choke

1

Choke

1

Coil: gate input filter

1

Reactor: degausser

2

Coil: interstage

1

Meter: 0 - 5 microamps

1

TSTR: si NPN 2N3053

1

TSTR: si PNP 2N3638

1

TSTR: si NPN

1

R: fxd comp 68 ohms 10% 1/4 W
NSR: part of A 1, A2, and A3

1

R: fxd flm 10 ohms 1% 1/8 W

1

R: fxd flm 24.3 ohms 1% 1/2 W

1

R: fxd 68. 1 0 h ms 1 % 1/8 W

1

R: fxd flm 226 ohms 1 % 1/8 W

1

R: fxd flm 681 1% 1/8 W

1

R: fxd flm 2.26 kilohms 1 % 1/8 W

1

R: fxd flm 6.98 kilohms 1% 1/8 W

1

R: fxd flm 24.9 kilohms 1 % 1/8 W

1

R: fxd flm 66.5 kilohms 1% 1/8 W

1

R: fxd comp 130 kilohms 5% 1/4 W

1

R: fxd comp 43 megohms 5% 1/2 W
Not assigned

4

R: fxd comp 1000 ohms 10% 1/4 W

2

R: fxd comp 1 megohm 10% 1/4 W

1

R: fxd comp 10 kilohms 10% 1/2 W

2

R: fxd comp 150 kilohms 10% 1/4 W

1

R: fxd camp 750 ohms 5% 1/4 W
R: fxd comp 150 kilohms 10% 1/4 W

1

R: fxd comp 5.6 kilohms 10% 1/2 W

1

R: fxd flm 249 kilohms 1 % 1/2 W

1

R: fxd flm 324 kilohms 1 % 1/2 W

1

R: fxd flm 12.4 kilohms 1 % 1/4 W

2

R: fxd comp 47 ohms 10% 1/4 W

1

R: fxd comp 47 kilohms 10% 1/2 W

1

R: fxd comp 910 ohms 5% 1/2 W

4

R: fxd comp 470 ohms 10% '1/4 W
Not assigned

4

R: fxdflm 2 kilohms 1% 1/8W

1

R: fxd flm 499 kilohms 1 % 1/4 W

1

R: var 5 kilohms 10% ww 1.5 W

1

R: fxd comp 39 kilohms 5% 1 W
R: fxd comp 470 ohms 10% 1/4 W

1

R: fxd flm 487 ohms 1 % 1/4 W

1

R: fxd comp 56 kilohms 10% 1/4 W

1

R: fxd fl m 245 kilohms 1 % 1/2 W

1

R: fxd flm 4.02 kilohms 1% 1/4 W
Not assigned

1

R: fxd comp 200 ohms 5% 1/4 W

71400
71400

-hp-

71400
02660

-hp-

24931

-hp-

-hp-

-hp-

-hp-

-hp-

-hp-

04713
04713

01295

-hp-

91637
14674

14674
14674
14674

14674
14674

14674
01121
01121

01121
01121

01121
01121

01121
01121

01121
91637
91637

91637
01121

01121
01121
01121

14674
91637

79727
01121

01121
91637

01121
91637
91637

01121

MDL-1/12
MDL-1

AGC1/2
91-PC4F

28J R128-1

2N3053
2N3638

SM0843
obd

CMF-1/10-32 T-1 obd
C4 T-0 obd

C4 obd
C4 T-0 obd
C4 T-0 obd

C4 T-0 obd
C4 T-0 obd

C4 T-0 obd
CB1345
EB4365

CB1021
CB1051

EB1031
CB 1541

CB7515
CB1541

EB5621
MFF-1/2-10 T-1 obd
MFF-1/2-10 T-1 obd

MFF -1/8-32 T-1 obd
CB4701

EB4731
EB9115
CB4711

C4 T-O
MFF-1/8-32, T-1 obd

E-870
GB3935

CB4711
MFF-1/8-32 T-1 obd

CB5631
MFF-1/2-10 T-1 obd
MFF-1/8-32 T-1 obd

CB2015

25

Model 428B

S1

S2

S3

S4

S5

T1

T2

T3

T4

T5

REFERENCE

DESIGNATOR

R61
R62

R63
R64

R65
R66

R67
R68

R69
R70
R71

R72
R73

R74
R75
R76

R77
R78

R79
R80
R81

R82
R83

R84
R85
R86

R87 thru R90
R91

R92
R93
R94, R95

R96
R97

R98
R99
R100, R101

R102
R103

R104
R105
R106

R107
R108

R109
R110
R111, R112

R113
R114

R115
R116
R117 thru R120

-hp-

PART NO.

0757-1039
0757-1047

2100-0328
2100-0270

0684-2711
0684-4701

0812-0015
0757-1040

2100-1560
0684-1001
0764-0028

0764-0003
0698-4746

0698-4711
0811-0007

0684-6821
0698-4953

0684-1021

0683-6225
2100-0197

2100-0282
0684-5621
0684-1531

0698-4931
2100-0094

0684-1011
0684-1021

0684-4711
0684-1011

2100-0394
0698-3647
0690-1041

0811-0041

0684-2251
0684-4711
0757-0857

0698-4153
0698-4793

2100-0094
0698-4772
0683-0475

0687-3331
0684-1041

0698-4752
0684-1051
0764-0006

00428-61901
3101-0001

3101-0018

3101-1234

9100-1481
9100-1483

9100-1484
9100-1485

Table 6-1. Replaceable Parts (Cont'd)

TQ DESCRIPTION MFR. MFR. PART NO.

1

R: fxd flm 45 ohms 1% 1/4 W

1

R: fxd flm 90 ohms 1% 1/4 W

1

R: var 500 ohms 10% ww 1.5 W

1

R: var 50 ohms 10% ww 2 W

1

R: fxd comp 270 ohms 10% 1/4 W
R: fxd comp 47 ohms 10% 1/4 W

1

R: fxd prec 8 ohms 3% ww 3 W

1

R: fxd flm 50 ohms 1% 1/4 W

1

R: var 30 ohms 10% ww 1.5 W

1

R: fxd comp 10 ohms 10% 1/4 W

1

R: fxd 100 kilohms 5% 2 W
Not assigned

1

R: fxd 3.3 kilohms 5% 2 W

1

R: fxd flm 53.6 kilohms 1 % 1/4 W

1

R: fxd flm 11.8 kilohms 1% 1/4W

1

R: fxd prec 10 kilohms 1% ww 5 W

1

R: fxd comp 6.8 kilohms 10% 1/4 W

1

R: fxd flm 124 kilohms 1 % 1/2 W
R: fxd comp 1000 ohms 10% 1/4 W

Not assigned

1

R: fxd 6.2 comp kilohms 5% 1/4 W

1

R: var 20 kilohms/2000 ohms ww
Not assigned

1

R: var 2 kilohms 20% ww 1.5 W

1

R: fxd comp 5.6 kilohms 10% 1/4 W

1

R: fxd comp 15 kilohms 10% 1/4 W
Not assigned

1

R: fxd flm 32.4 kilohms 1 % 1/2 W

2

R: var comp 50 kilohms 30%

2

R: fxd comp 100 ohms 10% 1/4 W
R: Fxd comp 1000 ohms 10% 1/4 W

R: Fxd comp 470 ohms 10% 1/4 W
R: Fxd comp 100 ohms 10% 1/4 W

1

R: var 300 ohms 20% ww 1 .5 W

1

R: fxd 15 kilohms 5% 2 W

2

R: fxd comp 100 kiloh ms 10% 1 W

1

R: fxd prec 169 ohms 1% ww 4 W
Not assigned

1

R: fxd comp 2.2 megohm 10% 1/4 W
R: fxd comp 470 ohms 10% 1/4 W

1

R: fxd 82.5 kilohms 1% 1/2 W

1

R: fxd flm 9.31 kilohms 1% 1/2 W

1

R: fxd flm 402 kilohms 1 % 1/4 W
R: var comp 50 kilohms 30%

1

R: fxd flm 178 kilohms 1% 1/4 W

2

R: fxd comp 4.7 ohms 5% 1/4 W

1

R: fxd comp 33 kilohms 10% 1/2 W

1

R: fxd comp 100 kilohms 10% 1/4 W

1

R: fxd flm 84.5 kilohms 1% 1/4 W
R: fxd comp 1 megohm 10% 1/4 W

4

R: fxd 18 kilohms 5% 2 W

1

Assembly: Range Switch (incl. resistors)

1

Switch: toggle SPST 3A 250 vdcw

1

Switch: toggle (momentary) SPST
Not assigned

1

Switch: slide DPDT
Not assigned

1

Detector: signal trans.

1

Gate transformer

1

Zero balance trans

1

Coil: osc.

14674
19701

79727
12697

01121
01121

91637
14674

11236
01121
14674

14674
91637

91637
91637

01121
91637

01121

01121
11237

79727
01121
01121

91637
11236

01121
01121

01121
01121

79727
14674
01121

91637

01121
01121
91637

75042
91637

11236
91637
01121

01121
01121

91637
01121
14674

-hp-

04009

OOLAF

82389

-hp-

-hp-

-hp-

-hp-

C5 T-0 obd
MF52C T-0

E-870 obd
obd

CB2711
CB4701

RS2B-95
C5T-0 obd

110
CB1001
C-42S obd

C-42S obd
MFF-1/8-32 T-1 obd

MFF-1/8-32 T-l obd
RS5-78

CB6821
MFF-1/2-10 T-1 obd

CB 1021

CB6225
C252-45

E-870
CB5621
CB1531

MFF-1/2-10 T-1 obd
UPM 7ORE obd

CB1011
CB1021

CB4711
CB1011

E-870 obd
FP-2 obd
GB 1041

RS2-64 obd

CB2251
CB4711
MFF-1 /2-10 T-1 obd

CEC, T-O obd
MFF-1/8-32 T-1 obd

UPM 7ORE obd
MFF-1 /8-32 T-1 obd
CB47G5

EB3331
CB1041

MF F-1 /8-32 T-1 obd
CB1051
FP-2

80994-HB
8906K499

11A-1242A

26

Model 428B

REFERENCE

DESIGNATOR

T6
T7

-hp-

PART NO.

9100-1486
9100-3228

Table 6-1. Replaceable Parts (Cont'd)

TQ DESCRIPTION MFR. MFR. PART NO.

1

Transformer: head drive

1

Transformer: power

-hp-

-hp-

V1, V2
V3, V4

V5
V6, V7

V8
V9

V10

W1

1923-0017

1923-0017
1932-0022

5080-0041
1921-0010

1923-0021

8120-1348

428A-1C

428A-21A
428A-83A

428B-2A
428B-2R

428B-44

0403-0004
1440-0002
1490-0015

5040-0616

00428-66501

5020-0653

00428-26501

0330-0029
8520-0017

1410-0052
2110-0269

0510-0748
1205-0011

0370-0822
0370-0031

0370-0035
0370-0064
0370-0823

0370-0824
0370-0825

0370-0826
7121-0192
2950-0034

1200-0062
1200-0083

3100-0268

00428-90003

3

Tube: electron 6AH6
Not assigned

Tube: electron 6AH6
Tube: electron 6DJ8

2

Tube: electron selected 12AU7

1

Tube: electron 12B4A miniature 9 pin

1

Tube: electron 6AU6 miniature 7 pin

1

Cable: AC power

1

MISCELLANEOUS

Dust Cover (rack only)

1

Probe Assembly (see Figure 6-1 )

1

Escutcheon

1

Panel (cabinet only)

1

Panel (rack only)

1

Assembly: cabinet

1

Bumper: rubber

4

Handle: leather

1

Bail: cabinet tilt

1

Detent: cabinet bail

2

Assembly: P .C. Board

1

Bezel

1

Board: P.C.

1

Boot (rack only)

1

Brush: nylon

1

Bushing: potentiometer

1

Fuse: clip

2

Fuse: holder

1

Heat sink

1

Knob: round, brown

1

Knob: 1" dia. black conc.

1

Knob: bar 1" dia. black

1

Knob: 3/4" dia. red w/arrow

1

Knob: red (for R63)

1

Knob: orange (for R69)

1

Knob: yellow (for R46)

1

Knob: green (for R98)

1

Nameplate (cabinet only)

1

Nut: hexagonal

1

Socket: 9 pin miniature tube

4

Socket: 7 pin miniature tube

4

Switch: rotary

1

Operating and Service Manual

1

33173

33173
73445

-hp-

33173
33173

-hp-

-hp-

-hp-

-hp-

-hp-

-hp-

-hp-

OOOMM

0000Z

OOLAG

-hp-

-hp-

-hp-

-hp-

76545

OOLAH

32016
91506

75915
98978

-hp-

-hp-

-hp-

-hp-

-hp-

-hp-

-hp-

-hp­91345
91961

71785
91662

76854

-hp-

6AH6

6AH6
obd

12B4A
6AU6A

obd
obd
obd

62 obd
obd

obd
6008-32CN

357002
TXBF-032-025B obd

obd
obd

121-51-11-060
05-0730-02

216481-HZ obd

27

Model 428B

Figure 6-1. Parts Breakdown, current probe.

28

Model 428B

SECTION VII

CIRCUIT DIAGRAMS

7-1. INTRODUCTION.

7-2. This section contains the circuit diagrams necessary
for the operation and maintenance of the Model 428B
Clip-on DC Milliammeter. Included are a block diagram,
schematics and component location diagrams.

7-3. BLOCK DIAGRAM.

7-4. The block diagram shows the relationship between
the assemblies of the instrument. Signal flow between

assemblies and significant portions of assemblies as well
as major feedback paths are shown. The block diagram Each part is identified by a reference designator.
is used to develop an understanding of the basic theory
of operation.

SCHEMATIC NOTES

7-5. SCHEMATIC DIAGRAMS.

7-6. The circuitry contained within each assembly is
shown in the schematic diagrams. These diagrams are
used to develop an understanding of the detailed theory
of operation of each assembly and as an aid in isolating
troubles within an assembly.

7-7. COMPONENT LOCATION DIAGRAMS.

7-8. The component location diagrams show the
physical location of parts mounted on an assembly.

29

MEASUREMENT CONDITIONS.

Line voltage: 115 V, 60 Hz
Range: 100 mA

Input: 100 mA

All voltages were measured with a high input impedance
electronic voltmeter. A 10 megohm oscilloscope probe was

used to measure the waveforms. The oscilloscope should be
externally synchronized from pin 2 of V7 (through a 47 µF

capacitor to avoid disturbing the circuit.

Model 428B

30

Figure 7-1. Block Diagram.

Model 428B

Figure 7-2. Component Locator For Circuit Board Part No. 00428-66501

Figure 7-3. Front Panel Component Locator.

Figure 7-4 Rear Panel Component Locator.

31

Model 428B

32

Figure 7-5. Power Supply.

Model 428B

33

Figure 7-6. Block Diagram.

Model 428B

Figure 7-7. Component Locator for Circuit Board Part No. 00428-66501

Figure 7-8. Front Panel Component Locator.

Figure 7-9 Rear Panel Component Locator.

34

Model 428B

Figure 7-10. Metering Circuit

(Note: This schematic has been chopped and resectioned to reduce the 11x30 page to better fit on 8½x11 paper)

35

Model 428B

APPENDIX A

MANUAL BACKDATING CHANGES

A-1. The changes which appear in this Appendix make A-2. Schematic diagrams, photographs, and a parts
this manual applicable to those 428B Clip-on DC list for the earlier instruments have been included in

Milliammeters which bear serial number prefixes of
601- or below.

this Appendix.

36

Model 428B

Figure A-1. 428B Side Views.

37

Model 428B

38

Figure A-2 Backdating Schematics for 428B

Model 428B

39

Figure A-3 Backdating Schematics for 428B

Model 428B

Circuit

Reference

A1, 2 428B-26A Assembly: ww resistor, C65 0180-0058 fxd, elect, 50µf -10% + 100%, 25 vdcw

A3 428B-26B Assembly: ww resistor, includes R2, 11 C68 0170-0022 fxd, my, 0.1 pf +20%, 600 vdcw
C1 0140-0099 fxd, mica, 1K pf +1%, 500 vdcw C69 0180-0104 fxd, elect, 200 µf, 15 vdcw

C2 0131-0004 var, mica, 14-150 pf, 175 vdcw CR1 thru CR8 1901-0028 Diode, Si
C3 thru C10 Not Assigned CR9 1902-0013 Diode, Si
C11 0150-0014 fxd, cer, 5K pf, 500 vdcw CR10 1902-0014 Diode, Si

C12 0180-0050 fxd, elect, 40 uf -15% + 100%, 50 vdcw DS1 2140-0012 Lamp: indicating, #12, 2 pin base
C13 0170-0022 fxd, my, 0.1 uf +20%, 600 vdcw F1, 2 2110-0007 Fuse: 1 amp, s-b ( F1 115 V operation)

C14 nsr, part of T1 assembly 2110-0008 Fuse: 1/2 amp, s-b (F1 230 V operation)
C15 0150-0012 fxd, cer, 10K pf +20%, 1000 vdcw F3 2110-0012 Fuse: 1/2 amp
C16, 17 0170-0019 fxd, my, 0.1 of +5%, 200 vdcw J1 1251-0089 Connector: female, 4 pin

C18 nsr, part of T1 assembly J2 Not Assigned
C19 0150-0012 fxd, cer, 10K pf +20%, 1000 vdcw J3 1250-0118 Connector: BNC, female
C20 Not Assigned L1 thru L4 nsr; part of probe assembly (see misc.)

C21 0180-0059 fxd, elect. 10 uf -10% + 100%, 25 vdcw L5, 6 428B-60K Assembly, choke
C22, 23 0150-0015

C24 0170-0078 fxd, my, 0.47 uf +5%, 150 vdcw Ml 1120-0116 Meter: 0-5 ma, 6-10 ohms
C25 0180-0058 fxd, elect, 50 of -10% + 100%, 25 vdcw P1 1251-0090 Connector: male, 4 pin
C26 0150-0012 fxd, cer, 10K pf ±20% 1000 vdcw P2 8120-0050 Cable, power

C27 0140-0099 fxd, mica, 1K pf +1%, 500 vdcw Q1 1854-0039 Transistor: Si NPN 2N3053
C28, 29 0150-0012 fxd, cer, 10K pf +20%, 1000 vdcw Q2 1853-0016 Transistor: Si PNP 2N3638
C30 Not Assigned Q3 1854-0003 Transistor: 2N1564

C31 0170-0019 fxd, my, 0.1 of +5%, 200 vdcw R1 0687-6801 fxd, comp, 68 ohms ±10%, 1/2 W
C32 0140-0034 fxd, mica, 22 pf +5%, 500 vdcw R2 nsr; part of A3 assembly
C33 thru C40 Not Assigned R3 thru R10 nsr; part of A1, A2 assembly

C41 0140-0108 fxd, mica, 253 pf +2%, 300 vdcw R11 nsr; part of A3 assembly
C42 0140-0041 fxd, mica, 100 pf +5%, 500 vdcw R12 0727-0335 fxd, dep c, 10 ohms +10%, 1/2 W

C43 thru C50 Not Assigned R13 0757-0002 fxd, mfg, 24.3 ohms ±1% 1/2 W
C51 0170-0019 fxd, my, 0.1 of +5%, 200 vdcw R14 0727-0035 fxd, dep c, 68.4 ohms ±1/2%, 1/2 W
C52 0140-0149 fxd, mica, 470.pf +5%, 300 vdcw R15 0727-0060 fxd, dep c, 225 ohms ±1%, 1/2 W

C53, 54 0140-2102 fxd mica, 8.2K*pf +2%, 500 vdcw R16 0727-0085 fxd, dep c, 680 ohms ±1%, 1/2 W
C55 0140-0147 fxd, mica, 180 pf +5%, 500 vdcw R17 0727-0120 fxd, dep c, 2250 ohms ±1%, 1/2 W
C56 thru C60 Not Assigned R18 0727-0145 fxd, dep c, 6960 ohms ±1%, 1/2 W

C61 0180-0011 fxd, elect, 20 uf 450 vdcw R19 0727-0178 fxd, dep c, 24.7K ohms ±1%, 1/2 W
C62A/B 0180-0012 fxd elect, 2 sect, 20 of sect 450 vdcw
C63 0140-0031 fxd, mica, 220 pf +10%, 500 vdcw

Stock No. Description #

A1 includes, R3 thru R6 C66 0180-0104 fxd, elect, 200 µf 15 vdcw
A2 includes, R7 thru R10 C67 Not Assigned

fxd, TiO2 2.2 pf +10%, 500 vdcw
2

Reference

C64

L7
L8

0150-0012 fxd cer, 10K pf +20%, 1000 vdcw

9140-0049
9110-0025

Inductor: var, 16 mh
Inductor: degaussing

Circuit Stock No. Description

40

Model 428B

Circuit

Reference-hp-

R20
R21
R22, 23 0687-2261 fxd, comp, 22M ±10%, 1/2 W R73 0764-0003 fxd, mfg, 3300 ohms ±5%, 2 W

R24 0687-1021 fxd, comp, 1K ohms ±10%, 1/2 W
R25 0687-1051 fxd, comp, 1M ±10%, 1/2 W R75 0727-0163 fxd, dep c, 11. 88K ohms ±1%, 1/2 W
R26 0687-1031 fxd, comp, 10K ohms ±10%, 1/2 W R76 0816-0008 fxd, ww, 10K ohms ±10%, 10 W

R27
R28
R29 0687-1541 fxd, comp, 150K ohms ±10%, 1/2 W R79 0687-1021 fxd, comp, 1K ohms ±10%, 1/w

R30 0687-5621 fxd, comp, 5.6K ohms ±10%, 1/2 W
R31 0727-0226 fxd, dep c, 2 50K ohms ±1%, 1/2 W R81 0686-6225 fxd, comp, 6.2K ohms ±5%, 1/2 W
R32 0727-0201 fxd, dep c, 71.56K ohms ±1%, 1/2 W R82A/B 2100-0197 var comp, dual pot, ±10%, 2 W

R33 0730-0032 fxd, dep c, 12. 3K ohms ±1%, 1 W
R34 0687-4701 fxd, comp, 47 ohms ±10%, 1/2 W R82B, 200 ohms
R35 0690-4731 fxd, comp 47K ohms ±10%, 1 W R83 Not Assigned

R36 0689-9115 fxd, comp, 910 ohms ±5%, 1 W
R37 0687-4711 fxd, comp, 470 ohms ±10%, 1/2 W R85 0687-5621 fxd, comp, 5.6K ohms ±10%, 1/2 W

R38 thru R40
R41 thru R44
R45 0727-0244 fxd, dep c, 500K ohms ±1%, 1/2 W R91 0727-0195 fxd, dep c, 50K ohms ±1%, 1/2 W

R46 2100-0006 var, ww, 5K ohms ±10%, 2 W
R47 0689-3935 fxd, comp, 39K ohms ±5%, 1 W
R48 0687-4711 fxd, c .p, 470 ohms ±10%, 1/2 W R94, 95 0687-1021 fxd, comp, 1K ohms ±10%, 1/2 W

R49
R50
R51 0730-0080 fxd, dep c, 245K ohms ±1%, 1 W R98 2100-0038 var, ww, 300 ohms

R52
R53 thru R59
R60 0687-6811 fxd, comp, 680 ohms ±10%, 1/2 W R102 0811-0041 fxd, ww, 169 ohms ±1%, 3 W

R61 0727-0021 fxd, dep c, 45 ohms ±1%, 1/2 W
R62 0727-0038 fxd, dep c, 90 ohms ±1%, 1/2 W R104 0687-2251 fxd, comp, 2.2M ±10%, 1/2 W
R63 2100-0022 var, ww, lin, 500 ohms ±20%, 1 W R105 0687-4711 fxd, comp, 470 ohms ±10%, 1/2 W

R64 2 100-0270 var, ww, lin, 50 ohms ±10%, 2 W
R65 0687-2711 fxd, comp, 270 ohms ±10%, 1/2 W R107 0727-0154 fxd, dep c, 9380 ohms ±1%, 1/2 W
R66 0687-4701 fxd, comp, 47 ohms ±10%, 1/2 W R108 0727-0240 fxd, dep c, 405K ohms ±1%, 1/2 W

R67 0812-0015 fxd, ww, 8 ohms ±3%, 2 W
R68 0727-0023 fxd, dep c, 50 ohms ±1%, 1/2 W

Stock No. Description #

0727-0198
0686-1345

0687-1541
0689-7515

0727-0184

0727-0075
0687-4721

0727-0132 fxd, dep c, 4K ohms ±1%, 1/2 W

fxd, dep c, 66K ohms ±1%, 1/2 W
fxd, comp, 130K ohms ±5%, 1/2 W

fxd, comp, 150K ohms ±10%, 1/2 W
fxd, comp, 750 ohms ±5%, 1 W

Not Assigned
fxd, dep c, 28.4K ohms ±1%, 1/2 W

fxd, dep c, 490 ohms ±1%, 1/2 W
fxd, comp, 4.7K ohms ±10%, 1/2 W

Not Assigned

R69
R70

R71
R72

R74 0727-0196 fxd, dep c, 52. 6K ohms ±1%, 1/2 W

R77
R78

R80 Not Assigned

R84 2100-0153 var, comp, 2K ohms ±20% 1/3 W
R86

R87 thru R90
R92 2100-0013 var, comp, lin, 50K ohms ±20%

R93 0687-1011 fxd, comp, 100 ohms ±10%, 1/2 W
R96

R97
R99

R100, 101
R103 0819-0011 fxd, ww, 4.5K ohms ±10%, 20 W

R106 0730-0063 fxd, dep c, 83K ohms ±1%, 1 W

R109 2100-0013 var, comp, lin, 50K ohms ±20%

2100-0002
0687-1001

0693-1041
0690-1241

0687-6821
0730-0072

0687-1531 fxd, comp, 15K ohms ±10%, 1/2 W

0687-4711
0687-1011

0767-0010
0690-1041

var, ww, 50 ohms ±10%, 2 W
fxd, comp, 10 ohms ±10%, 1/2 W

fxd, comp, 100K ohms ±10%, 2 W
fxd, comp, 120K*ohms ±10% 1 W

fxd, comp, 6.8K ohms ±10%, 1/2 W
fxd, dep c, 123K ohms ±1%, 1 W

R82A, 2K ohms

Not Assigned

fxd, comp, 470 ohms ±10%, 1/2 W
fxd, comp, 100 ohms ±10%, 1/2 W

fxd, mfg, 15K ohms ±5%, 3 W
fxd, comp, 100K ohms ±10%, 1 W

Circuit

Reference

41

L Stock No. Description#

Model 428B

Circuit

Reference

R110 0727-0218 fxd, dep c, 180K ohms ±1%, 1/2 W 428B-75E Assembly: resistor board "E" includes
S1 428B-19A Assembly: range switch, includes, C27 thru C29 C51 R47 thru R52

S2 3101-0001 Switch: tog, SPST R94 thru R96
S3 3101-0018 Switch: tog, SPST, momentary 428B-75F Assembly: resistor board "F" includes
S4 Not Assigned C53, C54 C68 CR1 thru CR8 R106

S5 3101-0033 Switch, sl DPDT R108 R110
T1 9120-0052 Assembly: transformer, interstage, includes, G-74AW Knob: red, 3/4" w/arrow

T2 428A-60G Transformer, detector signal G-74N Knob: bar, RANGE
T3 428A-60C Transformer, gate 1200-0003 Socket, tube: 9 pin miniat
T4 428B-60J Transformer, zero balance 1200-0017 Socket, tube: 7 pin miniat

T5 9120-0051 Transformer, osc 1400-0008 Fuseholder
T6 428A-60D Transformer, head drive 1400-0084 Fuseholder
T7 9100-0104 Transformer, power 1450-0020 Jewel, pilot light

V1, 2 1923-0017 Tube, electron: 6AH6 8520-0017 Electric shaver brush
V3, 4 1930-0013 Tube, electron: 6AL5 1205-0011 Heat dissipater, semiconductor

V5 1923-0017 Tube, electron: 6AH6
V6, 7 1932-0022 Tube, electron: 6DJ8 / ECC88
V8 1932-0029 Tube, electron: 12AU7

V9 1921-0010 Tube, electron: 12B4A
V10 1923-0021 Tube, electron: 6AU6
V11 1940-0001 Tube, electron: 5651

-hp- Stock
No.

R2 thru R20, R61, R62 R72 R91

C14, C18 G-74J Knob: black, 1", concentric shaft ZERO

MISCELLANEOUS

428A-21A Assembly: probe, includes

L1 thru L4, P1

428B-65C Assembly: circuit board "C" includes

R65, R67, P68

428B-75H Assembly: circuit board "A" includes

C32, C66, C69, CR9, CR10, Q1 thru Q3, R70,
R71, R74, R75, R102

428B-75B Assembly: resistor board "B" includes

C13 R26, 27
C15 R35, 36
C21 R41, 42

C25 R45
R21 thru R23 R73

428B-75C Assembly: circuit board "C" includes

C16, C17, C24, C41, C42, R30 thru R33
R43, R44 R77 thru R79, R81, R85, R86

428B-75D Assembly: resistor board "D" includes

C1, C2 C11, C12 R1 R25

Description#

Circuit

Reference

-hp- Stock No. Description#

42

Model 428B

MANUAL CHANGES

Manual Part No. 00428-90003

►New or Revised Item

ERRATA:
Pages 1-0 and 2-1. The power frequency should be 48

- 440 Hz.
Page 1-1. Delete Paragraph 1-5a (the 3528A is

obsolete).

►Page 3-2, Paragraph 3-22. Change last sentence to

read as follows:

10 April 1974

►Page 5-3, Paragraph 5-32. Change C10 to L10.

Change C14 to C18.
The 6.3 V filament supply is available at pin 4 or 5 of

V6.
Page 5-3, Paragraph 5-32. Replace entire paragraph

with the following:

"If the jaws mate properly, the zero set should stay
within 0.2 mA, while rotating the probe head with

respect to the earth's magnetic field (probe aligned in
an East-West direction)."

►Page 3-2, Paragraph 3-35. Change the third

sentence to read a follows: "With the RANGE switch at
1 mA, rotation of the closed probe should not vary the
zero set more than 0.2 mA."

Pages 4-0,7-2 and 7-4. Change V3, V4 to CR13
through CR16.

Page 4-3, Paragraph 4-29. Change all "40" to "40
kHz".

Page 4-4, Figure 4-9. Change R87 to R81 . Delete
C12 and return C42 directly to common.

Paragraph 4-49. Delete the words "tube V11" and
insert "Zener diodes CR10, CR17.".

Page 5-0. Change the 50 Ω resistor to 0698-8155.
Change the recommended DC voltmeter to -hp- Model

3469B.
Add: Resistors (2),10 K, 1 %, matched, for alignment,

-hp- Part No. 0757-0442.
Page 5-1, Paragraph 5-2. Add: See the Performance

Test Card at the end of this section."

►Page 5-1, Paragraph 5-11 c. Add the following

sentence: "Point probe in an East-West direction."
Page 5-2. Set the OUTPUT LEVEL control to CAL for

Paragraph 5-18.

►Page 5-3, Paragraph 5-24a. In second sentence,

change reference to 0.1 mA to read 0.2 mA

►Page 5-3, Paragraph 5-28. Replace entire

paragraph with the following:

5-28a. Connect an electronic dc voltmeter to test
point 5 (Pin 1, V9) of the 428B. The voltage at this
point should be 272 ± 6 V; if not, adjust R 109 for

272 V.

b. Measure +12 ±1 V dc between the cathode
of CR 10 and ground.

c. Measure -7 ±1 V dc between the anode of
CR9 and ground.

43

5-32. Disconnect FEEDBACK DISCONNECT (See
Figure 7-3); red wire. Place range switch in the 10 amp

position. Adjust fine and course zero controls for
center of travel. Adjust R46 for zero on front panel

meter.

►Page 5-4, Paragraph 5-33. Replace entire

paragraph with the following:
5-33. Reconnect FEEDBACK DISCONNECT.

►Page 5-4, Paragraph 5-42. Add to last sentence"

(2.5 - 3.3 V rms)."

►Page 5-4, Paragraph 5-45. Add the following to

sentence two: “.... carrying 35 mA rms ac, f ≤400 Hz."

►Page 5-4, Paragraph 5-47. Replace the feedback

disconnect before proceeding to Paragraph 5-48.

►Page 5-4, Paragraph 5-47. Add to last sentence: "(.2

V - .4 V peak)"
Para. 5-51. Replace the feedback disconnect before

proceeding to Paragraph 5-52.

►Page 5-5, Paragraph 5-54. Replace entire

paragraph with the following:

5-54. Preset the controls as follows:

a. Set RANGE switch to 100 mA
b. Adjust R84 for minimum ac at the

wiper connection of R84. The reading
should be less than 0.1 volts.

c. Adjust R82 for minimum ac out at the

wiper of R82a. The reading should be
≤0.02 volts.

d. Thoroughly clean probe head jaws

(See Paragraph 5-7),

e. Degauss probe head (See Paragraph

3-29).

►Page 5-5, Paragraph 5-58. Remove Steps a and b.

Re-letter Steps c to f to read a to d respectively. I n
newly designated Step d, Step d is now Step b; Step e

is now Step c.

►Page 5-5. The following paragraph is added

following Paragraph 5-58 to complete the adjustment
procedure:

Model 428B
5-58.1. OUTPUT ADJUSTMENT.

5-58.2. A meter calibrator and a one kilohm resistor
are required for his test.

a. Place RANGE switch in 10 amp position
and adjust R46 for zero volts DC ± 0.2 mV at

the· Output Level jack.
b. Switch the 428B and the meter calibrator to

the 100 mA range. Connect a test lead
between the meter calibrator output leads and

clip the 428B probe to the lead.
c. Set the meter calibrator to 100 mA output

and read full-scale on the 428B. Adjust R69 if
necessary.

d. Connect a voltmeter to the output jack of the
428B in parallel with the 1000 ohm resistor.

e. The voltmeter should read 0.73 V ± 10m V.
Adjust R63 if necessary.

Page 5-8, Paragraph 5-69. Replace V11 by Zener
diodes CR10, CR17.

Page 6-2. Change CR13 through CR16 to 1906-0034,
Duo-Quad, -hp-, for all replacements.

Change C63 part no. to 0160-2025.
Pages 6-2/6-3. Change C24, C52, C55, C70, and R60

to "starred" (selected) components.
Nominal values are as follows:

►Page 6-6,Fig. 6-1. Change Part No. of Coil Spring by

probe nose section to 1460-0600 and change PIN of
Cable Clamp by connector plug to 00428-41201.

Page 7-1. Note that resistors are in ohms and
capacitors are in pF unless otherwise marked, and that

selected ("starred") components marked wit an
asterisk may not even be present.

Page 7-3, figure 7-5. TP5 is connected to pin 1 of V9.
Reverse F1 and F2.

Page 7-3, Figure 7-2; Page 7-5, Figure 7-7. Change
CR13 (near TP2) to CR11.

Interchange: R26/R30; R27/R29 and C14/C18.
Page 7-5, Figure 7-10. R8 is 40 ohms. R68 is 50

ohms.
R61 is 45 ohms.

R62 is 90 ohms.
R 12 through R20 may be any mixture of the values

shown on Pages 7-5 or A-D.
Nominal value of C52* is 560 pF.

Nominal value of C55 * is 180 pF.
C2 is 16 - 150 pF.

R91 is not a "starred" component; it is no longer a
selected value.

Page 7-5/A-0. Add another contact to S1A as shown:

C24, 0160-0138, .39 µF, 200 Vdcw.
C52, 0140-D178, 560 pF, 300 Vdcw.

C55, 0140-0197, 180 pF, 300 Vdcw.

Page 6-3. M1 is a 0 - 5 mA meter.
Pages 6-3 and 7-5. Change R 15 to 216 Ω.

Page 6-4. R82 is 2 K/200 ohms.
Page 6-5. Part Numbers in the Operating and Service

Manual are for blue instruments; part numbers for
brown instruments are listed below.

00428-00204 Dust Cover (rack only)
00428-62101 Probe Assembly

00428-00201 Panel (cabinet only)
00428-00203 Panel (rack only)

00428-64401 Assembly: cabinet
5020-6849 Bezel

.

Page A-0. R41 is 28.4 K. Change R78 (near CR11) to
R71. Add R65 between the junction of R61, R62 and

54 (as in Figure 7-10). Change R60 to R60*. Change
the LINE switch (230 V-115 V) from S3 to S5. Change

the power transformer from +7v to T7. R33 is 12.3 K.
Page A-2. Add CR11, CR12 as on page 6-2.

Pages A-2, A-3. R 12 through R20 may be individually
replaced by the parts shown on Page 6-3.

Page A-4. Add R111, R112 as on page 6-4.
CHANGE NO. 1: For Serial 0995A06603 and Greater.

Page 6-2. Add C33*, 0140-0149 C: fxd 470 pF 300 V.
Add C72* 0160-0174 C: fxd .47 µF 25 V.

Page 7-5. Add C33*, 470 pF, between base and
collector of Q3. Add C72* .47 µF between emitter and

collector of Q3

10 April 1974

44

Model 428B

90003

U.S.A.

45