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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 

Содержание 428B

Страница 1: ...ont cover OCR errors may exist and as such the user of this document should take care and use common sense when referencing this documentation Copyright Notice This documentation is copyrighted by Hewlett Packard and Jack Hudler hpdocs hudler org Permission to use and redistribute this documentation for non commercial and internal corporate purposes is hereby granted free of charge Any redistribut...

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Страница 5: ... 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 ...

Страница 6: ...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...

Страница 7: ... 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...

Страница 8: ... 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 ...

Страница 9: ...lip 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 Offic...

Страница 10: ...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 pow...

Страница 11: ...Model 428B Figure 3 1 Measurement Procedures 4 ...

Страница 12: ...edance 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 ...

Страница 13: ...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 energi...

Страница 14: ...d 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 ampli...

Страница 15: ...Model 428B Figure 4 1 Block Diagram 8 ...

Страница 16: ...oportional 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 ...

Страница 17: ...re 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 dire...

Страница 18: ...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 l...

Страница 19: ...plete 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 appr...

Страница 20: ...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 42...

Страница 21: ... 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...

Страница 22: ...he 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 ...

Страница 23: ...rom 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...

Страница 24: ... 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...

Страница 25: ...nt 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...

Страница 26: ...arly 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 ind...

Страница 27: ...Model 428B Figure 5 9 Detailed Troubleshooting Tree 20 ...

Страница 28: ...se 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 p...

Страница 29: ...Model 428B 22 ...

Страница 30: ...nsistor conn connection mA milliampere s 10 3 amperes farads V volt s dep deposited MHz megahertz 10 6 hertz piv peak inverse voltage vacw alternating current DPDT double pole double throw MΩ megohm s 10 6 ohms p o part of working voltage DPST double pole single throw metflm metalfilm pos position s var variable elect electrolytic mfr manufacturer poly polystyrene vdcw DC working voltage encap enc...

Страница 31: ...er 01 microfarad 20 1000 vdcw Not assigned C fxd my 0 1 microfarad 5 200 vdcw Not assigned C fxd mica 260 pF 1 C fxd mica 100 pF 5 Not assigned C fxd cer 1 microfarad 20 500 vdcw C fxd mica 620 pF 5 C fxd mica 6800 pF 5 C fxd mica 270 pF 5 Not assigned C fxd my 20 microfarads 50 10 450 vdcw C fxd my 20 20 microfarads 50 10 450 vdcw C fxd mica 220 pF 5 C fxd cer 01 microfarad 20 1000 vdcw C fxd 50 ...

Страница 32: ... 681 1 1 8 W R fxd flm 2 26 kilohms 1 1 8 W R fxd flm 6 98 kilohms 1 1 8 W R fxd flm 24 9 kilohms 1 1 8 W R fxd flm 66 5 kilohms 1 1 8 W R fxd comp 130 kilohms 5 1 4 W R fxd comp 43 megohms 5 1 2 W Not assigned R fxd comp 1000 ohms 10 1 4 W R fxd comp 1 megohm 10 1 4 W R fxd comp 10 kilohms 10 1 2 W R fxd comp 150 kilohms 10 1 4 W R fxd camp 750 ohms 5 1 4 W R fxd comp 150 kilohms 10 1 4 W R fxd c...

Страница 33: ... ohms ww Not assigned R var 2 kilohms 20 ww 1 5 W R fxd comp 5 6 kilohms 10 1 4 W R fxd comp 15 kilohms 10 1 4 W Not assigned R fxd flm 32 4 kilohms 1 1 2 W R var comp 50 kilohms 30 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 R var 300 ohms 20 ww 1 5 W R fxd 15 kilohms 5 2 W R fxd comp 100 kiloh ms 10 1 W R fxd prec 169 ohms ...

Страница 34: ... 12B4A miniature 9 pin Tube electron 6AU6 miniature 7 pin Cable AC power MISCELLANEOUS Dust Cover rack only Probe Assembly see Figure 6 1 Escutcheon Panel cabinet only Panel rack only Assembly cabinet Bumper rubber Handle leather Bail cabinet tilt Detent cabinet bail Assembly P C Board Bezel Board P C Boot rack only Brush nylon Bushing potentiometer Fuse clip Fuse holder Heat sink Knob round brown...

Страница 35: ...Model 428B Figure 6 1 Parts Breakdown current probe 28 ...

Страница 36: ...tionship between 7 7 COMPONENT LOCATION DIAGRAMS the assemblies of the instrument Signal flow between 7 8 The component location diagrams show the assemblies and significant portions of assemblies as well physical location of parts mounted on an assembly 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 ...

Страница 37: ...Model 428B Figure 7 1 Block Diagram 30 ...

Страница 38: ...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 ...

Страница 39: ...Model 428B Figure 7 5 Power Supply 32 ...

Страница 40: ...Model 428B Figure 7 6 Block Diagram 33 ...

Страница 41: ...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 ...

Страница 42: ...Model 428B Figure 7 10 Metering Circuit Note This schematic has been chopped and resectioned to reduce the 11x30 page to better fit on 8 x11 paper 35 ...

Страница 43: ...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 this Appendix 601 or below 36 ...

Страница 44: ...Model 428B Figure A 1 428B Side Views 37 ...

Страница 45: ...Model 428B Figure A 2 Backdating Schematics for 428B 38 ...

Страница 46: ...Model 428B Figure A 3 Backdating Schematics for 428B 39 ...

Страница 47: ... L7 L8 9140 0049 9110 0025 Inductor var 16 mh Inductor degaussing 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...

Страница 48: ...6K ohms 10 1 2 W R38 thru R40 R41 thru R44 0727 0184 Not Assigned fxd dep c 28 4K ohms 1 1 2 W R86 R87 thru R90 0687 1531 fxd comp 15K ohms 10 1 2 W Not Assigned 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 R92 2100 0013 var comp lin 50K ohms 20 R47 0689 3935 fxd comp 39K ohms 5 1 W R93 0687 1011 fxd comp 100 ohms 10 1 2 W R...

Страница 49: ...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...

Страница 50: ...tage 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 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 5 32 Disconnect FEEDBACK DISCON...

Страница 51: ...ack only 00428 64401 Assembly cabinet 5020 6849 Bezel 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 ...

Страница 52: ...Model 428B 00428 90003 PRINTED IN U S A 45 ...

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