HP 428B Service 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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About this Manual
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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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Copyright Notice
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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
MAINTENANCE 14 GENERAL INFORMATION 2
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
7
7
7
7
7
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
9
9
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
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.
9
9
11
11
11
4-28. 40 kHz INPUT/AMPLIFIER CIRCUIT. 11
4-30. SYNCHRONOUS DETECTOR AND FILTER (C24).
11
4-36. DC AMPLIFIER. 12
4-43. NEGATIVE FEEDBACK CURRENT CIRCUIT. 12
4-45. 40 kHz PHASE SHIFTER. 12
4-48. POWER SUPPLY. 12

Model 428B
LIST OF TABLES
Table 1-1 Specifications 1
Table 5-1. Recommended Test Equipment. 13
Table 6-1. Replaceable Parts 24
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 Absolute 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
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
1
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

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.
Figure 1-1. Model 428B Clip-On Milliammeter
2

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
Figure 3-1. Measurement Procedures
4

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.
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
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).
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
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.
7

Model 428B
Figure 4-1. Block Diagram
8

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.
Figure 4-2. Simplified Block.
9

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
Figure 4-5. 428B Flux Gate. Figure 4-6. Waveforms.
10

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.
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
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.
Figure 4-7. Detector Bridge.
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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.
Figure 4-8. Negative Feedback Figure 4-9. 90° Phase Shift
12

Model 428B
13
Table 5-1. Recommended Test Equipment.
EQUIPMENT REQUIRED CHARACTERISTICS USE RECOMMENDED MODEL
Meter Calibrator ± 0.2% of reading
±0.1% FS Range Check Meter Tracking
Output Calibration
AC Overload
-hp- Model 69208
AC/DC Meter Calibrator
Function Generator DC to 400 Hz
> 10 mA output Frequency Response AC Overload -hp- Model 3310A Function Generator
Oscilloscope DC to 40 kHz
100 mV/cm ± 3% Frequency Response AC Overload
Troubleshooting
-hp- Model 130C
200µV/cm Oscilloscope
Resistor 50 Ohms ± 1% Frequency Response
AC Overload -hp- Part No. 0698-3128 0698-8155
DC Voltmeter ± .25% at 730 mV Output Calibration -hp- Model 3430A 3469B
DC Digital Voltmeter
AC Voltmeter Resolves 2 mV
Battery operated Noise Check -hp- Model 4038 AC Portable Voltmeter
Resistor 1 kilohm ± 1% Output Calibration
Noise Check -hp- Part No. 11034-82601
Volt-Ohmmeter Input impedance: ≥1 megohm Troubleshooting -hp- Model 427 A
Multi-Function Meter
Oscilloscope Probe
Input Impedance: 10 megohms
All Troubleshooting -hp- Model 10001A
Resistive Divider Probes
Counter Reads 40 kHz ± 20 Hz Alignment
Troubleshooting -hp- Model 5321 B
Capacitor .0082 µF ± 10% 300 Vdc Alignment ——————————
Resistor 390 Ohms ± 5% 1/2 W Alignment ——————————