IET Labs, Inc. 1864-1644 User And Service Manual

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♦ PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT ♦

1864-1644

Positive Polarity

Megohmmeter

User and Service Manual

Visit www.ietlabs.com for manual revision updates

1864-1644 im/January 2018

IET LABS, INC.

www.ietlabs.com

Email: info@ietlabs.com

TEL: (516) 334-5959 • FAX: (516) 334-5988

♦ PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT ♦

IET LABS, INC.

www.ietlabs.com

Email: info@ietlabs.com

TEL: (516) 334-5959 • FAX: (516) 334-5988

WARRANTY

We warrant that this product is free from defects in material and workmanship and, when properly used, will perform in accordance with applicable IET speciﬁ cations. If within one year after original shipment, it is found not to meet this standard, it will be repaired or, at the option of IET, replaced at no charge when returned to IET. Changes in this product not approved by IET or application of voltages or currents greater than those allowed by the speciﬁ cations shall void this warranty. IET shall not be liable for any indirect, special, or consequential damages, even if notice has been given to the possibility of such damages.

THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO, ANY IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE.

WARNING

OBSERVE ALL SAFETY RULES

WHEN WORKING WITH HIGH VOLTAGES OR LINE VOLTAGES.

Dangerous voltages may be present inside this instrument. Do not open the case

Refer servicing to qualiﬁ ed personnel

HIGH VOLTAGES MAY BE PRESENT AT THE TERMINALS OF THIS INSTRUMENT

WHENEVER HAZARDOUS VOLTAGES (> 45 V) ARE USED, TAKE ALL MEASURES TO

AVOID ACCIDENTAL CONTACT WITH ANY LIVE COMPONENTS.

USE MAXIMUM INSULATION AND MINIMIZE THE USE OF BARE

CONDUCTORS WHEN USING THIS INSTRUMENT.

WHEN WORKING WITH HIGH VOLTAGES, POST WARNING SIGNS AND

Use extreme caution when working with bare conductors or bus bars.

KEEP UNREQUIRED PERSONNEL SAFELY AWAY.

CAUTION

DO NOT APPLY ANY VOLTAGES OR CURRENTS TO THE TERMINALS OF THIS

INSTRUMENT IN EXCESS OF THE MAXIMUM LIMITS INDICATED ON

THE FRONT PANEL OR THE OPERATING GUIDE LABEL.

1864-1644 Positive Polarity Megohmmeter

Contents

Condensed Operating Instructions ..........................................................iv

Speciﬁ cations .............................................................................................v

1864-1644 Speciﬁ cations ............................................................................................... v

Chapter 1: INTRODUCTION ....................................................................... 1

1.1 Description ............................................................................................................... 1

1.2 Opening and Tilting the Cabinet .............................................................................. 1

1.3 Controls, Connectors and Indicators ........................................................................ 1

1.4 Symbols.................................................................................................................... 5

1.5 Connections .............................................................................................................. 5

Chapter 2: INSTALLATION .........................................................................7

2.1 Initial Inspection ...................................................................................................... 7

2.2 Dimensions .............................................................................................................. 7

2.3 Repackaging for Shipment ....................................................................................... 7

2.4 Storage ..................................................................................................................... 8

2.5 Bench Setup ............................................................................................................. 8

2.6 Rack Mounting ......................................................................................................... 8

2.7 Power Connections .................................................................................................. 8

Chapter 3: OPERATION ............................................................................. 9

3.1 Measurement Setup ................................................................................................. 9

3.1.1 Ground-Link Connection ............................................................................... 9

3.1.2 Test Voltage Selection .................................................................................... 9

3.1.4 Connection of Unknown ................................................................................ 9

3.2 Measurement Procedure ................................................................................... 10

3.2.1 General ........................................................................................................... 10

3.2.2 Search Procedure ........................................................................................... 10

3.2.3 Sort Procedure ............................................................................................... 10

3.2.4 Shock Hazard ................................................................................................ 10

3.3 Output Jack .............................................................................................................. 11

Chapter 4: APPLICATIONS ........................................................................13

4.1 Insulation Testing ..................................................................................................... 13

4.2 Test Sample Resistivity Measurements .................................................................... 14

4.3.1 General ........................................................................................................... 14

4.3.2 Charging Time Constant ................................................................................ 14

CONTENTS, FIGURES, AND TABLES

1864-1644 Positive Polarity Megohmmeter

4.3.3 Measurement Time Constant ......................................................................... 15

4.3.4 Discharge Time .............................................................................................. 15

4.3.5 Large Capacitors, Very High Resistance ....................................................... 15

4.4 Resistance Measurements ........................................................................................ 16

4.5 Measurement of Voltage Coefﬁ cient ........................................................................ 16

4.6 Guarded 3-Terminal Measurements ......................................................................... 17

4.7 Remote Shielded Measurements .............................................................................. 17

4.8 Measurements Under Humid Conditions ................................................................. 17

Chapter 5: THEORY ....................................................................................19

5.1 General .................................................................................................................... 19

5.2 Circuit Description ................................................................................................... 19

5.2.1 General ........................................................................................................... 19

5.2.2 Type 1863 Megohmmeter (Figure 7.6) .......................................................... 19

5.2.3 Type 1864 Megohmmeter (Figure 7.9) .......................................................... 20

Chapter 6: SERVICE AND MAINTENANCE ..............................................21

6.1 Service ...................................................................................................................... 21

6.3 Cabinet Removal ...................................................................................................... 23

6.4 Troubleshooting ....................................................................................................... 23

6.4.1 General ........................................................................................................... 23

6.4.2 Test Voltages .................................................................................................. 24

6.5 Calibration Procedure .............................................................................................. 24

6.5.1 General ........................................................................................................... 24

6.5.2 Meter Tracking ............................................................................................... 24

6.5.3 Voltage Accuracy .................................................................................................. 24

6.5.4 Range-Resistor Accuracy ............................................................................... 25

6.5.5 Coarse  Adjustment ..................................................................................... 25

6.6 Knob Removal ......................................................................................................... 26

6.7 Knob Installation ...................................................................................................... 27

6.8 Meter Cover Care ..................................................................................................... 27

Chapter 7: PARTS LISTS AND DIAGRAMS ..............................................29

CONTENTS, FIGURES, AND TABLES

1864-1644 Positive Polarity Megohmmeter

Figures

1864-1644 Front-panel Controls, Connectors and Indicators ................................. iv

Figure 1-1. Type 1864-1644 Front-panel Controls, Connectors and Indicators ..... 2

Figure 1-2. Type 1864-1644 rear-panel controls and connectors .......................... 4

Figure 1-3. Methods of connection to the measurement terminals ........................ 5

Figure 2-1. Dimensions of the GR/IET 1864-1644 Megohmmeters ...................... 7

Figure 3-1 Ground-link connection to GUARD and to -UNKNOWN terminals ... 9

Figure 4-1. Electrode arrangement for resisitivity measurements ......................... 14

Figure 4.2 Basic megohmmeter circuit ................................................................... 15

Figure 4-3. Guarded measurement of a three-terminal resistor .............................. 16

Figure 5-1 Megohmmeter block diagram ............................................................... 20

Figure 6-1. Connections for measuring standard resistors ..................................... 22

Figure 6-2. Top interior view of 1864 Megohmmeter ........................................... 26

Figure 6-3. Bottom interior view of 1864 Megohmmeter ...................................... 26

Figure 7-1. Replaceable mechanical parts on the 1864-1644 ................................. 31

Figure 7-3. Type 1864 rectiﬁ er circuit etched-board assembly .............................. 32

Figure 7-2. Regulator and ampliﬁ er circuits etched-board assembly ..................... 32

Figure 7-4 Type 1864 switching diagram ............................................................... 33

Figure 7-5. Type 1864 schematic diagram .............................................................. 34

Figure 7-6. Complete cabinet assembly .................................................................. 35

CONTENTS, FIGURES, AND TABLES

iii

1864-1644 Positive Polarity Megohmmeter

Condensed Operating Instructions

POWER-OFF

SWITCH

SET ∞

HIGHEST RANGE

SET ∞

GUARD

GROUND

- UNKNOWN +

MULTIPLIER

DIAL

FUNCTION

SWITCH

1864-1644 Front-panel Controls, Connectors and Indicators

1. Determine which ground link connection is to be used (paragraph 3.1.1 ).

2. Set the TEST VOLTAGE switch(es) to the proper voltage (paragraph 3.1.2).

3. Set the

∞ adjustments (paragraph 3.1.3).

TEST-VOLTAGE

SWITCHES

4. Connect the unknown to the UNKNOWN terminals.

5. Measure the unknown with either the search (paragraph 3.2.2) or sort (paragraph 3.2.3) procedure.

CONTENTS, FIGURES, AND TABLES

1864-1644 Speciﬁ cations

(10% of scale)

("5" rdg)

(2.5% of scale)

("20" rdg)

10 Vdc to 50 Vdc 50 k 500 G 2 T*7

50 Vdc to 100 Vdc 200 k 5 T 20 T 8

100 Vdc to 500 Vdc 500 k 5 T 20 T*7

500 Vdc to 1090 Vdc 5 M 50 T 200 T 8

*Rec ommended Limit

Rmin

(Full Scale left end)

("0.5" rdg)

Rmax (right end)

Voltage

Setting

Useful

Ranges

1864-1644 Positive Polarity Megohmmeter

Speciﬁ cations

Resistance Accuracy (min reading 0.5):

Range 1-5:

Range 6:

Range 7:

Range 8:

Accuracy applies for >100 V; For 100 V add 2%.

±2 (meter reading+1)%

Where meter reading is the actual number indicated at the scale; e.g. for a reading of 900 k on the 1 G scale, the accuracy is ±2(.9+1)% or 3.8%

add ±2% to accuracy above

add ±3% to accuracy above

add ±5% to accuracy above

Meter Display:

Full mechanical zero at right end, so 2.5 % fullscale is near right end and full-scale is at left end. However, resistance values read naturally, increasing from left to right.

Voltage Accuracy (across unknown):

For 100 V ± 2% For <100 V ± (3% + 0.5 V)

Short-Circuit Current:

Approximately 5 mA

Power:

100 - 125 or 200 - 250 V

50 - 400 Hz 13 W

Fuse:

For 100 to 125 V operation:

Fuse: T 0.25A, 250V (PN 0034.3111) For 200 to 250 V operation: Fuse: T 0.125A, 250V (PN 0034.3108) Fuse holder is located under the IEC receptacle and holds a 5 x 20 mm time-delay fuse.

Dimensions:

6.63 x 10 x 6.75 in.

Weight:

9.5 lb.

CONTENTS, FIGURES, AND TABLES

1864-1644 Positive Polarity Megohmmeter

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CONTENTS, FIGURES, AND TABLES

1864-1644 Positive Polarity Megohmmeter

Chapter 1

INTRODUCTION

WARNING

High voltage is applied to the measurement terminals of 1864-1644 Megohmmeters at all times, except when the function switch is set to DISCHARGE. While the current is limited to a value that is not dangerous under most conditions, the energy stored in a capacitor connected to the terminals may be LETHAL. Always set the function switch to DISCHARGE before you connect or disconnect the unknown.

1.1 Description

The 1864-1644 (Figure 1-1) indicates resistance from 50 k to 200 T. These ranges are suitable for leakage-resistance measurements of most types of insulation used in electrical instruments, electronic devices and components, etc (Section 4). The 1864 has a voltage range from 10 to 1090 V that can be set in 1 Vdc steps from 10 to 109 V, and in 10 V steps from 100 to 1090 V by using the TEST VOLTAGE switches on the front panel.

The 100-volt setting is the EIA standard for measurement of composition, ﬁ lm, and wire-wound resistors above 100 kilohms. The 500-volt setting is a standard value in the measurement of the insulation resistance of rotating machinery, transformers, cables, capacitors, appliances, and other electrical equipment.

A regulated power supply and charging circuit permit rapid and accurate measurement of the leakage resistance of capacitors.

1.2 Opening and Tilting the Cabinet

To open the cabinet, refer to the pictorial graphic on the rear panel of the unit; see Figure 1-2. The Flip-Tilt cabinet can be opened by placing the instrument on its rubber feet with the handle away from you. Push down on the handle and the instrument, located in the upper part of the case, will rotate to a vertical position. While holding the handle down with one hand, rotate the instrument to the desired position with the other

hand and then slowly release the handle.

1.3 Controls, Connectors and Indicators

Figure 1-1 shows the front-panel controls, connectors and indicators of the 1864. Table 1-1 lists and identiﬁ es them. Figure 1-2 shows the rear panel controls and connectors, and Table 1-2 lists and identiﬁ es them.

Guard and ground terminals permit measurement of grounded or ungrounded two-or three-terminal resistors.

A panel warning light indicates when voltage is applied to the test terminals and alerts users to the safe operation of the instrument.

INTRODUCTION

1864-1644 Positive Polarity Megohmmeter

Figure 1-1. Type 1864-1644 Front-panel Controls, Connectors and Indicators

INTRODUCTION

1864-1644 Positive Polarity Megohmmeter

Figure 1.1

Name Type Function

Reference

2 Meter

4 SET ∞

5 GUARD

6 Ground

7 UNKNOWN -

8 UNKNOWN +

9 DANGER

10 Multiplier

12 TEST VOLTAGE

POWER OFF

SET ∞ HIGHEST RANGE

MEASURE CHARGE DISCHARGE

2-position toggle switch

4-in. meter with plastic cover

Knob-rotated control

Insulated binding post

Uninsulated binding post

Insulated binding post

Indicated light shaded red

8-position rotary switch

3-position toggle switch

3 rotary switches:

10-position 9-position 2-position

Turn power on and off.

Indicates the value to be multiplies by the multiplier switch.

Adjusts high end of meter scale on highest resistance range to compensate for offset current.

Adjusts high end of meter scale on highest resistance range to compensate for offset voltage in the voltmeter.

For guarded measurements. It can accept a shorting link to the ground post.

Grounds the Unknown- or guard. Contains a shorting link.

Connects the - side of the unknown to the megohmmeter.

Connects the + side of the unknown to the megohmmeter.

Glows red when the function switch is in the CHARGE or MEASURE position.

Selects resistance range.

Selects the operating position applied to the unknown.

Select voltage in 1-V steps from 10 to 109 V and in 100-V steps from 100 to 1090 V.

INTRODUCTION

1864-1644 Positive Polarity Megohmmeter

Figure 1-2 Reference

Figure 1-2. Type 1864-1644 rear-panel controls and connectors

Table 1.2.

Name Type Function

Power Input

Output

Line Voltage

1/8 Amp

IEC Standard Power input receptacle.

Phone jack Provides a dc voltage output for recorder

2-position slide switch

Integral fuse holder

Power input and circuit protection

operation

Connects wiring of power transformer for either 100 to 125 V or 200 to 250 V input

Holder for 5 x 20 mm

For 100 to 125 V operation: 1/4 A fuse For 200 to 250 V operation: 1/8 A fuse

INTRODUCTION

1864-1644 Positive Polarity Megohmmeter

1.4 Symbols

These instruments indicate the resistance of the unknown in multiples of ohms. The relationship between ohms (), kilohms (k), megohms (M), gigaohms (G), and teraohms (T) is as follows:

1 MΩ= 10

I GΩ= 10

1 TΩ = 10

Ω = 10

Ω = 106 kΩ= 103 MΩ

Ω = 109 kΩ = 106 MΩ= 103GΩ

kΩ

1.5 Connections

The UNKNOWN, GUARD and ground terminals are standard 3/4-in. spaced binding posts that accept banana plugs, standard telephone tips, alligator clips, crocodile clips, spade terminals and all wire sizes up to number eleven (Figure 1-3).

When several measurements of components with leads are to be made, consult IET for an appropriate test jig or ﬁ xture.

Figure 1-3. Methods of connection to the measurement terminals

INTRODUCTION

1864-1644 Positive Polarity Megohmmeter

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INTRODUCTION

1864-1644 Positive Polarity Megohmmeter

Chapter 2

INSTALLATION

Dimensions in inches

Figure 2-1. Dimensions of the GR/IET 1864-1644 Megohmmeters

2.1 Initial Inspection

IET instruments receive a careful mechanical and electrical inspection before shipment. Upon receipt, verify that the contents are intact and as ordered. The instrument should then be given a visual and operational inspection.

If any shipping damage is found, contact the carrier and IET Labs. If any operational problems are encountered, contact IET Labs and refer to the warranty at the beginning of this manual.

Save all original packing material for convenience in case shipping of the instrument should become

necessary.

2.2 Dimensions

The dimensions of the 1864-1644 are shown in both the rack and bench-mounted conﬁ gurations in Figure 2-1.

2.3 Repackaging for Shipment

If the instrument is to be returned to IET Labs, contact the Service Department at the number or address, shown on the front cover of this manual, to obtain a “Returned Material Authorization” (RMA) number and any special shipping instructions or assistance. Proceed as follows:

1. Attach a tag with the following:

• Owner’s Name

• Model Number

• Serial Number

• RMA Number

2. Wrap the instrument in heavy paper or plastic.

3. Protect the front panel and any other protrusions with cardboard or foam padding.

4. Place instrument in original container or equally substantial heavy carton.

5. Use packing material around all sides of instrument.

6. Seal with strong tape or bands.

7. Mark shipping container “DELICATE INSTRUMENT,” “FRAGILE,” etc.

INSTALLATION

1864-1644 Positive Polarity Megohmmeter

2.4 Storage

If this instrument is to be stored for any lengthy period of time, it should be sealed in plastic and stored in a dry location. It should not be subjected to temperature extremes beyond the speciﬁ cations. Extended exposure to such temperatures can result in an irreversible change in resistance, and require recalibration.

2.5 Bench Setup

The bench (portable) model of the megohmmeter is cased in a Flip-Tilt cabinet. The cabinet opens by pushing down on the handle and tipping the instrument into the desired operating position (paragraph

1.2).

2.6 Rack Mounting

Consult IET Labs.

2.7 Power Connections

The 1864-1644 Megohmmeter can be operated from either a 100- to 125-V or a 200- to 250-V, 50-to 60Hz power line. Before connecting the 3-wire IEC power cord to the line, set the slide switch on the rear panel to the proper setting as indicated by the position of the white line on the slide switch. The slide can be actuated with a screwdriver blade. Verify that the correct size fuse for the input voltage selected has been installed (1/4 A slow blow for 100-125 V input or 1/8 A slow blow for 200-250 V input). If it is necessary to use a 3-wire adaptor plug, make certain that the third wire is connected to a good ground (water pipe or equivalent). If this is not possible, connect the panel of the 1864-1644 (uninsulated binding post) to a good ground. Plug the supplied IEC power cord into the instrument into a power receptacle. The power cord may of course be selected to match the available receptacle.

INSTALLATION

Chapter 3

OPERATION

1864-1644 Positive Polarity Megohmmeter

3.1 Measurement Setup

3.1.1 Ground-Link Connection

The grounding link connected to the uninsulated, grounded, binding post can be connected from this ground terminal to the GUARD (paragraph 4.6) or the -UNKNOWN terminal (Figure 31). The ground link should be connected to the GUARD terminal if the sample to be measured is a small, separate component, or if it is a component mounted in an enclosure that should be guarded (see paragraph 4.6). However, if one terminal of the unknown must be grounded, then the link should tie the -UNKNOWN terminal to the instrument case. See Figure 3.1.

3.1.3 Set ∞ Adjustments

To adjust the SET ∞ controls, proceed as follows:

1. Turn the instrument on.

2. Set the function switch to DISCHARGE.

3. Set the multiplier dial to any range.

4. Make certain that there nothing is connected to the UNKNOWN terminals.

5. Adjust the SET ∞ control for an ∞ reading on the meter.

6. Set the multiplier switch to 10-1T

7. Set the function switch to MEASURE.

8. Adjust the SET ∞ HIGHEST RANGE for an

∞ meter reading.

If this adjustment cannot be made electrically, turn the instrument off, and adjust the mechanical meter zero adjustment (the center screw on the meter) to give a meter reading of less than a line width beyond ∞. Repeat steps 1 through 7.

3.1.4 Connection of Unknown

Small components should be connected directly to the UNKNOWN terminals. Insulated leads can be connected to a nearby unknown; however, if the unknown resistance is high, leakage between the leads will cause a measurement error, and a change in capacitance to the high lead will cause a transient meter deﬂ ection. For such high resistance measurements, a shielded system is preferable (refer to paragraph 4.7).

Figure 3-1 Ground-link connection to GUARD terminal (top) and to -UNKNOWN terminal (bottom)

3.1.2 Test Voltage Selection

The TEST VOLTAGE switch(es) should be set to the desired measurement voltage. The 1864-1644 Megohmmeter has a selection of 10 to 109 Vdc in 1-V steps or 100 to 1090 Vdc in 10-V steps. On the 1864, the right-hand TEST VOLTAGE switch must be set to the V position for the low voltages and to the 0 V position for the high voltages.

OPERATION

1864-1644 Positive Polarity Megohmmeter

3.2 Measurement Procedure

3.2.1 General

Either of two measurement procedures may be used, depending on whether or not the correct resistancemultiplier range is known. If the range is not known, the search procedure (paragraph 3.2.2) should be followed. If repetitive measurements are to be made on a given range (i.e., if similar components are to be sorted) the sort procedure (paragraph 3.2.3) should be used.

3.2.2 Search Procedure

When the approximate resistance of the sample to be measured is not known, proceed as follows:

1. Set the multiplier switch to the lowest range.

2. Set the function switch to DISCHARGE.

3. Connect the unknown between the UN- KNOWN + and - terminals.

4. Set the function switch to MEASURE.

5. Rotate the multiplier switch cw until the meter gives a reading of less than 5.

6. The resistance of the unknown is the meter reading multiplied by the multiplier-switch indication.

7. Make measurements towards the low end of the meter scale whenever possible, for best accuracy and resolution.

3.2.3 Sort Procedure

When the approximate resistance of the unknown is known, proceed as follows:

1. Set the function switch to DISCHARGE.

2. Set the multiplier switch to the desired range.

3. Connect the unknown between the UN- KNOWN + and - terminals.

4. Set the function switch to MEASURE.

5. The resistance of the unknown is the meter reading multiplied by the multiplier-switch indication. For go-no-go checks, it can be useful to make high-and-low limit lines on the outside of the meter case with strips of masking tape.

6. Make measurements towards the low end of the meter scale whenever possible, for best accuracy and resolution.

3.2.4 Shock Hazard

Every precaution has been taken in the design of 1864 Megohmmeters to reduce the possibility of shock. However, high voltage must be present at the terminals to make measurements at the required voltage levels, and the operator should be aware of the dangers involved.

The current delivered by the megohmmeters under short-circuit conditions is approximately 5 mA. This 5-mA current is not lethal to most persons but might be lethal to those with poor hearts, and it is painful to all. The actual current that will ﬂ ow through a person depends on the resistance of the part of the body that makes contact with the terminals. This resistance can be as low as 300 Ω. Note that any of the three insulated binding posts can be at high voltage, depending on the position of the shorting link.

When capacitors are tested there is an especially dangerous condition because a charged capacitor easily can have enough energy to cause heart ﬁ brillation and death. The capacitor should always be shunted before connection to the megohmmeter, and the function switch should be set to DISCHARGE for a few seconds before the capacitor is disconnected.

We strongly recommend that additional precautions, such as rubber gloves and insulated benchtops, chairs and shoes should be used for anyone making repetitive measurements with the megohmmeter, particularly measurements on capacitors. These precautions should not take the place of careful discharge of the capacitors before and after measurement, but should be used as an additional safety measure.

OPERATION

3.3 Output Jack

The OUTPUT jack (J105) on the rear panel makes accessible a dc voltage that is directly proportional to the reciprocal of the meter reading, that is, the highest value is at the 0.5 scale reading and the lowest value is at ∞. The output voltage for a particular multiplierswitch setting can be calculated by

= 0.02 x V

OUT

where V

is the TEST VOLTAGE setting, R

TEST

is the lower value for a particular multiplier-dial setting (100k for the 1 M/100 k range) and R of the resistance being measured.

The output can be stored in a data ﬁ le for plotting, display, or analysis. It can also feed the user’s go/ no-go indicator. The full-scale voltage value for any test voltage can be calculated from the V using 0.5 times the measurement range as the R value. These values are available on the 1864 along with the other levels that can be set with the variable TEST VOLTAGE switches (see table 3-1).

RANGE

is the value

formula

out

1864-1644 Positive Polarity Megohmmeter

OPERATION

1864-1644 Positive Polarity Megohmmeter

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OPERATION

Chapter 4

APPLICATIONS

4.1 Insulation Testing

The insulation resistance of electrical machinery, transducers, etc, is one of several parameters that may indicate the condition of the insulation. Routine measurement of capacitance, dissipation factor, and leakage resistance provides useful data for monitoring the condition of the insulation and for guarding against incipient breakdown.

A routine test that has been widely adopted for insulation testing calls for the measurement of the apparent leakage resistance after a test voltage has been applied for one minute and again after the test voltage has been applied for 10 minutes. The ratio of the indicated resistances, sometimes referred to as the Polarization Index, can have some relation to the condition of the Insulation. The results of such a measurement are apt to be more dependent on the dielectric absorption of the insulator than on its true leakage resistance measured at equilibrium. A complete charge-current-vs-time plot will provide more useful information.

1864-1644 Positive Polarity Megohmmeter

The 1864-1644 Megohmmeters can be used for either true leakage measurements or for measurements at 1-or 10-minute intervals following the operating procedure described in Section 3.

MIL-STD-202C gives procedures for insulationresistance measurements of various components. On large machinery, one terminal must usually be grounded. The 1864-1644 Megohmmeter is designed so that the binding post grounding strap should be connected between the ground terminal and the

-UNKNOWN terminal.

To determine the charge current, divide the test voltage by the indicated resistance. At the start of a charge-current-vs-time plot, the meter will be off scale. The resistance in series with the insulator is the reading of the upper dial multiplier divided by

500. Table 4-1 lists dial readings and resistor values.

APPLICATIONS

1864-1644 Positive Polarity Megohmmeter

Figure 4-1. Electrode arrangement for resisitivity measurements

4.2 Test Sample Resistivity Measurements

The megohmmeter can be used for measuring the resistivity of test samples as described by ASTM Standard D257, which describes in detail the techniques for both surface-and volume-resistivity measurements. The most common electrode arrangement is that shown in Figure 4.1. In this conﬁ guration surface resistivity is measured with terminal 1 tied to the -UNKNOWN terminal, terminal 2 tied to the +UNKNOWN terminal and terminal 3 tied to GUARD. For volume resistivity measurements, terminal 1 is tied to the -UNKNOWN terminal, terminal 2 to the GUARD and terminal 3 to the +UNKNOWN terminal. The formulas required to convert from measured resistance to resistivity are given in the ASTM standard. Contact IET regarding the availability of resistivity test ﬁ xtures.

4.3 Capacitor Insulation Resistance

4.3.1 General

The insulation resistance, IR, of capacitors is measured by either the search or sort method (paragraph

3.2.2 and 3.2.3) used for resistors, except that some consideration must be given to the charge and discharge currents.

WARNING Capacitors being measured may be charged and may contain lethal energy. Always set the function switch to DISCHARGE before connecting or disconnecting the capacitor

under test.

4.3.2 Charging Time Constant

The time constant for charging a capacitor in the CHARGE position is determined by the value of the capacitor times the effective source impedance of the supply. The supply resistance is approximately,

MAX

 =

where E is the indicated test voltage in volts and I is the short-circuit current, which is approximately 5 mA. Therefore, the time constant is:

T = R

0Cx

where C R

is in F. As an example, on the 500 V range,

is approximately 100 kΩ so that the time constant

for charging of a 1 F capacitor is 0.1 s.

The time necessary for full charging depends on the type of capacitor and the leakage current that is to be measured. A capacitor with no dielectric absorption will have a charging current that decreases by a factor of 2.72 (the natural logarithm to the base e) for every time constant it is left in the CHARGE position.

Thus, the effective resistance at any moment is

(

)

R0 C

charged when this resistance is substantially higher

. The capacitor could be considered fully

than the true leakage resistance, even though the charging current theoretically never reaches zero. As an example a 1 F capacitor, with a leakage resistance of 10

Ω measured at 500 V, would have less than 1% error due to charging current, if measured after seventeen time constants, or 1.7 s.

0.005A

5000

 =

seconds

k

MAX

APPLICATIONS

1864-1644 Positive Polarity Megohmmeter

Dielectric absorption (dipole and interfacial polarization) is present in many capacitors and insulators, especially those with a laminated structure. When voltage is applied to such material, the charge slowly diffuses throughout the volume and several minutes, hours, or even days, are required for equilibrium in order to make the charging current small compared with the true leakage current. A measure of this effect, called the Polarization Index, is the ratio of the resistance measured after 10 minutes of charging to that measured after 1 minute of charging. Often, the measured resistance after 1 minute of charging is called the insulation resistance, even though charging current may be much larger than the true leakage current. (Some capacitor speciﬁ cations say less than 2 minutes).

4.3.3 Measurement Time Constant

When the function switch is set from the CHARGE position to the MEASURE position, the standard resistor is placed in series with the unknown capacitor. If the supply voltage is ﬁ xed, the capacitor must discharge by a voltage equal to that across the voltmeter at its ﬁ nal reading. The time constant for this discharge would be C output voltage is fed back to the supply, this time constant is reduced by a factor of 5. As a result, the time necessary for an indication, assuming an ideal capacitor, depends on this time constant or that of the meter movement, whichever is longer.

. Because 80% of the

XRs

4.3.4 Discharge Time

With the function switch set at DISCHARGE, the UNKNOWN terminals are connected through 470 Ω

and the discharge time is approximately

0.0005 x C s, where C is in F. The red DANGER light is turned off by the function switch, so that the capacitor might be charged even after the light is extinguished. However, the discharge time is so short that this is not a practical consideration, except for capacitors greater than 100 F.

Capacitors with high dielectric absorption (paragraph

4.3.2) can have a residual charge even after they are shunted and must be repeatedly shunted to be completely discharged. Usually this "voltage recovery" is only a few percent (i.e., 3%) of the original applied voltage and, therefore, not dangerous to the operator,

but it can cause damage to sensitive circuit elements.

4.3.5 Large Capacitors, Very High Resistance

Measuring insulation resistance of large capacitors that have very low leakage is difﬁ cult by any method. Considering the basic circuit of Figure 4.2, if R high, the R on the high resistance ranges if C

time constant can become very long

S CX

is large. If RS is

low, the voltmeter must be very sensitive for a given leakage resistance range and, therefore, the supply voltage (E) must be extremely stable to avoid large meter ﬂ uctuations. The design of the 1864 is a compromise between these factors. Measurements become difﬁ cult when the R

product is 106, even

SCX

under ideal conditions. This can be calculated as (C in F) x (R

4.1 contains values for R

in MΩ) or (CX in F) x (RS in Ω). Table

Figure 4.2 Basic megohmmeter circuit

APPLICATIONS

Measurements can be unsatisfactory even below this value for an R

product for several reasons:

SCX

1864-1644 Positive Polarity Megohmmeter

1. Dielectric absorbtion. (paragraph 4.3.2). This is the main cause of erroneous readings. Besides the difﬁ culty in deciding what charging period should be used, the previous history of the capacitor will greatly affect its indicated leakage. For example, if a paper capacitor is charged to its rated value, discharged for a short time, and then its leakage current is measured at some low value, it probably will give a reading beyond

∞. This is due to voltage recovery that is a

consequence of dielectric absorbtion. The voltage across the capacitor will increase above the test voltage causing current to ﬂ ow in the reverse direction.

2. Temperature coefﬁ cient. If the temperature on the unknown changes and it has an appreciable temperature coefﬁ cient, the voltage on the capacitor will change in the MEASURE position. If R

is large, the

charge (Q) of the capacitor is more-or-less constant, so if its capacitance changes, its voltage must change (Q=CV). A temperature-controlled environment is recommended.

3. Test voltage changes. The test voltage can have rapid ﬂ uctuations due to large line-voltage tran- sients even though good regulation is provided in the instrument, because when R

is large, the test

SCX

voltage ﬂ uctuations are transmitted to the voltmeter unattenuated. This difﬁ culty can be reduced if the line voltage is regulated. Slow drift of the test voltage can cause erroneous readings if R

. is large,

SCX

because even a slow drift rate can be fast compared to the R

time constant. A decreasing test voltage

SCX

can cause a reading beyond. Sufﬁ cient warm-up time (30 minutes) will allow the temperature inside the megohmmeter to stabilize and result in a more constant voltage at the UNKNOWN terminals.

4.5 Measurement of Voltage Coefﬁ cient

The 1864 Megohmmeter may be used to measure voltage coefﬁ cient as long as its accuracy is adequate. The voltage coefﬁ cient of resistance is deﬁ ned as:

R1-R

R2(V1-V2)

x 100%

where V1 > V2 R1 is the resistance at V1, the higher voltage R2 is the resistance at V2

For example, if V1 = 500 V and V2 = 100 V

Voltage coefﬁ cient =

500V-R100V

(400)R

ΔR

100V

x 100%

100V

This voltage coefﬁ cient is usually negative (except for reversed semiconductor junctions).

4.4 Resistance Measurements

The recommended test voltage is 100 V for ﬁ xed composition resistors, ﬁ lm resistors, and wire-wound resistors above 100 kΩ. (Refer to EIA Standards RS172, RS196, and REC 229.) These resistors can be measured easily on the megohmmeter as long as the accuracy of the instrument is adequate. If the resistors are separate, we suggest that they be measured ungrounded (with the grounding link connected to the GUARD terminal).

Figure 4-3. Guarded measurement of a three-terminal resistor

APPLICATIONS

1864-1644 Positive Polarity Megohmmeter

4.6 Guarded 3-Terminal Measurements

In many cases it is necessary to measure the resistance between two points in the presence of resistance from each of these points to a third point. This third point can often be guarded to avoid error caused by the extraneous resistances.

This situation can be shown diagrammatically as a three-terminal resistor (Figure 4-3). Here, R quantity to be measured in the presence of R R

. If the junction of RA and RB is tied to a guard, RA

is placed across the power supply and has no effect if it is greater than 500 kΩ. R

shunts RS and causes

a much smaller error than that which would be present if no guard were used. The error is approximately

-R

x 100%, where RS equals the value shown in

S/RB

Table 4-1 for the various ranges. If a choice is possible, the higher of the two stray resistances should be connected as R

The guard terminal can be used whether the GUARD or the - UNKNOWN terminal is grounded, but note that if the -UNKNOWN terminal is grounded, the GUARD terminal will be a high positive voltage level. Often the terminal to be guarded is a large chassis and it is, therefore, safer to ground the GUARD terminal. If this third terminal is true ground then the

GUARD terminal must be grounded.

is the

and

4.8 Measurements Under Humid Conditions

The 1864 Megohmmeter has been designed to operate under conditions of high humidity but, nevertheless, a few simple precautions should be taken to ensure accurate measurements. These precautions are:

1. Allow several minutes warmup (internal heat will reduce humidity inside the instrument).

2. Clean the binding-post insulation with a dry, clean cloth.

3. Use ungrounded operation (tie the GUARD terminal to the panel ground).

To determine the presence of errors due to humidity, measure the resistance between the binding posts with no external connections. Note that with the

-UNKNOWN terminal grounded, breathing on the terminals will cause a meter deﬂ ection because leak- age from the insulator of the +UNKNOWN terminal to the panel is measured.

Actually, this problem is somewhat academic because the unknown to be measured is usually much more severely affected by humidity than is the megohmmeter.

4.7 Remote Shielded Measurements

Measurements can be made on components that are some distance from the instrument if care is used to prevent leakage between the connecting leads and to avoid the shock hazard. A convenient way to do this is to use a shielded cable. If the unknown can be measured ungrounded, make the connection to the +UNKNOWN terminal with the shielded lead, tie the shield to the GUARD terminal, and connect the GUARD terminal to the panel ground with the connecting link. If one side of the unknown must be grounded, connect the grounding link to the +UNKNOWN terminal, shield the +UNKNOWN terminal, and tie the shield to the GUARD terminal. In this instance, the shield is not at ground potential and should be insulated.

APPLICATIONS

1864-1644 Positive Polarity Megohmmeter

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APPLICATIONS

Chapter 5

THEORY

1864-1644 Positive Polarity Megohmmeter

5.1 General

The 1864 Megohmmeter basically consists of a regulated dc power supply, a set of precision resistors, and a FET-input voltmeter (Figure 5.1). Switch S

is closed in the DISCHARGE position of the function switch and open in the CHARGE and MEASURE positions, while S

is open only in the MEASURE

position.

The regulated voltage, E, is controlled by a resistance R

. A meter sensitivity resistor, RB , is ganged to the

voltage control resistor, R

to make the meter read-

ing independent of applied voltage, (assuming that the unknown has no voltage coefﬁ cient). An inverse scale is used on a reversed meter to give a reading proportional to R

(and not its reciprocal) and yet

have a scale that increases from left to right (0 to ).

Metal-ﬁ lm standard resistors are used on all ranges. The top range of each instrument uses feedback to effectively multiply the value of the previous standard resistor by a factor of ten. In the 1864 the 2 GΩ resistor is multiplied to 20 GΩ. The speciﬁ cations are again broadened to allow for the tolerance variations of this multiplication.

The voltmeter uses a FET-input, four-stage, unity-gain ampliﬁ er (AMP, Figure 5.2) to obtain high stability and low drift. The SET is a voltage balance control, while the SET

∞ control on both instruments

∞ HIGH-

EST RANGE control compensates for the FET gate current on the highest ranges.

5.2 Circuit Description

5.2.1 General

5.2.2 Type 1863 Megohmmeter (Figure 7.6)

The voltage supply section (RECT.) of the 1863 consists of ﬁ ve different circuits, three dc and two ac. One ac circuit is a voltage source for the three pilot lamps used, two to indicate the measurement range (P101, P102) and the third to light the DANGER indicator (P103). The second supplies ﬁ lament volt- age to the vacuum tube V101.

The ﬁ rst dc supply is a half-wave rectiﬁ er circuit with a 24-V Zener diode (CR 111) that supplies voltages to the ampliﬁ er (AMP) circuit. A second dc supply is a voltage doubler (CR101-CR104, C101C102) that supplies the plate voltage to V101. The voltage to the plate is the same for the 50- to 250-V ranges but R109 is eliminated from the circuit for the 500 V range. The third dc supply is a half-wave rectiﬁ er with a 20-V Zener diode (CR211) to supply voltage levels to run the unity-gain ampliﬁ er (+1).

Tube V101 is a series regulator that is controlled by the 5.6 V Zener diode (CR112, REF) and the setting of R140.The voltage picked off R 140 is fed into one side (Q102) of the differential ampliﬁ er (Q102, Q103) while part of the output voltage is fed into the other side (Q103). The output of the ampliﬁ er is fed to the base of Q101 (AMP) and then to the grid of V101 for controlling the output voltage.

The output selection resistors are R124 through R133. These resistors determine the TEST VOLTAGE level. Resistors R211 through R219 are the standard resistors (R ment range. The output from this circuit is fed through the SET (R241) to the FET ampliﬁ er.

) that determine the measure-

∞ HIGHEST RANGE control

The following paragraphs will relate speciﬁ c com- ponents from the schematic diagrams of the 1863 (Figure 7.6) and 1864 (Figure 7.9) to the general components shown in Figure 5.1.

THEORY

1864-1644 Positive Polarity Megohmmeter

Figure 5-1 Megohmmeter block diagram

A unity-gain FET-input ampliﬁ er (+1) follows the standard resistors in the circuit conﬁ guration. R210 and C203 comprise a low-pass ﬁ lter input to FET Q204. The ampliﬁ er components include a differ- ential ampliﬁ er (Q202, Q203), a coarse

 control

(R244), the SET  control (R242) and an output transistor (Q201). The signal then enters the series combination of R135 and R134 back to the GUARD terminal.

Resistors R221 through R223 (R

) are meter-sensi-

tivity resistors that are ganged to the voltage resistors R124 through R127 (R

). R222 is used for both the

50 V and 500 V ranges, while the 200 V range uses the circuit resistance and has no added resistor. The remaining two resistors, R221 and R223, are used for the 250 and 100V ranges, respectively. Potentiometer R243 is an adjustable control in the meter sensitivity circuit.

5.2.3 Type 1864 Megohmmeter (Figure 7.9)

The circuit of the 1864 Megohmmeter is basically the same as that of the 1863 (paragraph 5.2.2). The exceptions are explained in the following paragraphs.

In the 1864 the second dc power supply is a quadrupler. This supply establishes the plate voltage of V101 with the use of resistors R109 through R114.

The regulator circuit has a slightly different input when the TEST VOLTAGE switch is switched from V (1) to 0 V (10). Resistors R124 and R125 are switched out of the circuit in the 0V (10) position.

Voltage-selection resistors for the 1864 are R126 through R133 and the meter sensitivity resistors are R221 through R228. An additional range resistor, R220, is in the 1864.

THEORY

1864-1644 Positive Polarity Megohmmeter

Chapter 6

SERVICE AND MAINTENANCE

WARNING

Dangerous voltages are present inside this case. When troubleshooting, a ground strap should be connected between GUARD and GROUND on panel to keep the subpanel (Guard) at ground potential. Refer all servic-

ing to qualiﬁ ed service

personnel.

6.1 Service

The warranty attests the quality of materials and workmanship in our products. When difﬁ culties do occur, our service engineers will assist in any way possible. If the difﬁ culty cannot be eliminated by use of the following service instructions, please write or phone our Service Department (see last page of manual), giving full information of the trouble and of steps taken to remedy it. Be sure to mention the type, ID, and serial numbers of the instrument.

Before returning an instrument to IET for service, please phone or e-mail to our Service Department, requesting a “Returned Material Authorization Number.” and instructions for return.

6.2 Minimum Performance Standards

The following checks are provided for verifying the performance of the 1863 and 1864 Megohmmeters. The test equipment necessary to perform these checks is listed in Table 6.1. To check an instrument, proceed as follows:

(1-13: meter tracking check) (14-19 voltage accuracy check)

1. Connect the case to the GUARD terminal with the shorting link.

2. Set the decade resistor to 0500000 (500 k).

3. Set the TEST VOLTAGE switch to to 1-0-0V.

4. Set the multiplier switch to 1 M.

5. Set the POWER/OFF switch to POWER.

6. Adjust the two SET ∞ controls as described in Section 3.

7. Connect the Decade Resistor to the UNKNOWN terminals with the dual banana plug patch cord.

8.Set the function switch to MEASURE.

9. Read the panel meter.

The reading should be 0.5 ±3%, that is, ±2 (1 + meter reading)% or 2 (1 + 0.5) = 3%.

10. Set the decade resistor to 1,000,000 (1 M).

The meter should read 1 ±4%.

11. Set the decade resistor to 5000000 (5 M)

The meter should read 5 ± 12%.

12. Set the TEST VOLTAGE switch to 10 V.

13. Set decades to 5,000,000 (5 M) and the MULTIPLIER to 10M.

The meter should read 0.5 ± 3%.

14. Increase the voltage to 20 V.

The meter reading should remain at

0.5 ±3%.

15. Continue to increase the voltage settings and observe that the meter reading remains

at 0.5 ±3%.

NOTE When the light under the 1 M on the multiplier switch goes out, the switch must be rotated so that the 1 M on the adjacent scale is lighted.

SERVICE AND MAINTENANCE

1864-1644 Positive Polarity Megohmmeter

16. Set the POWER/OFF switch to OFF and disconnect the decade resistor.

17. Connect the Digital Multimeter between the GUARD and -UNKNOWN terminals with two single banana plug patch cords.

18. Connect the two ground terminals together with a third patch cord (Figure 6.1).

The megohmmeter shorting link should remain attached only to the ground terminal.

19. Set the multiplier switch in the full ccw position (1M, 100k) and the function switch to MEASURE.

20. Measure the various standard resistors of the megohmmeter with the DMM according to the settings and tolerances of Table

6.2. Use the IET LOM-510A megohmmeter for resistance values beyond the range of the DMM.

Figure 6-1. Connections for measuring standard resistors with a digital multimeter

SERVICE AND MAINTENANCE

6.3 Cabinet Removal

To remove the instrument from the cabinet, remove the two screws on the rear of the instrument cabinet and pull the instrument out of the cabinet.

Warning

Be careful when troubleshooting the instrument when it is out of its cabinet and connected to the power line. Dangerous voltages are present, particularly at the transformer terminals. Connect the shorting link between the GUARD and ground terminals to keep the voltmeter circuitry near ground potential.

1864-1644 Positive Polarity Megohmmeter

† This value only appears as a ﬁ xed resistor in the 1864. Since the value is determined by feedback multiplication of the 200-M Ω resistor in the 1863, no measurement should be made with the mogohm bridge. *This range only appears on the 1864. Its range value is determined from the feedback multiplication of the 2-G Ω resistor, therefore, no mesurement should be made with the megohm bridge.

* Voltages are dc and the values are typical. Set TEST VOLTAGE switch to 200, function switch to CHARGE, connect the shorting link between the ground terminal and GUARD, and set the mulitplier switch to 1 M. Measurements made with 1863 line voltage set at 115 Vac.

6.4 Troubleshooting

6.4.1 General

The following information is designed to assist in troubleshooting the 1863 and 1864 Megohmmeters. An understanding of the theory involved in these instruments (Section 5) makes the instrument easy to analyze because the problem can usually be located quickly in either the voltage regulator or in the meter circuit.

SERVICE AND MAINTENANCE

1864-1644 Positive Polarity Megohmmeter

If the instrument is completely inoperative, be sure to check the power-line connection and the fuse (located in the IEC power input assembly on the rear panel).

6.4.2 Test Voltages

Tables 6.3 and 6.4 list a number of typical test voltages to assist in trouble analysis. Figures 6.2 through 6.5 and the diagrams of Section 7 will assist in locating components for testing purposes.

6.5 Calibration Procedure

6.5.1 General

The accuracy of the 1863 and 1864 depends on the accuracy of the range resistors, the accuracy of the applied voltages and the meter tracking accuracy. The over-all accuracy can be checked most easily by checking each one of these contributing quantities separately, for to check all points on all ranges at all voltages would require a tremendous number of measurements.

6.5.2 Meter Tracking

The scale tracking can be easily checked using a decade resistance decade box with 100-kΩ and 1-MΩ steps. Steps a through m of paragraph 6.2 should be performed to check the tracking. If all readings are corrected by the amount of the error found at a reading of 0.5 they should be better than the speciﬁ cation.

6.5.3 Voltage Accuracy

While the voltage can be checked to be within its speciﬁ cation, a more important check is to see that the voltage and meter sensitivity track to give a correct resistance reading. Such a check is generally adequate for it would be an unusual coincidence if both the voltage-control and meter-sensitivity resistors were in error, such that a good reading is obtained. To check this tracking, perform steps n through s of paragraph 6.2. If a reading is incorrect, the voltages should be checked with a DMM connected between the +UNKNOWN and -UNKNOWN terminals. The function switch can be set to either the CHARGE or MEASURE positions.

SERVICE AND MAINTENANCE

1864-1644 Positive Polarity Megohmmeter

If all the voltages are out of tolerance in the same direction, they can be set within the tolerance by adjusting R140 located on etched-circuit board P/N 1864-2701 (common to both the 1863 and 1864 Megohmmeters and shown in both Figures 6.2 and 6.4). The adjustment can be made as soon as the instrument is removed from the cabinet (paragraph 6.3). It is not necessary to move either of the printed-circuit boards, since the adjustment is on the top printedcircuit board. This adjustment affects all voltages by the same amount, but adjustment at 200 V minimizes possible errors due to resistance tolerances.

If all the voltages are correct but all meter readings are in error in the same direction, the meter sensitivity can be reset. Adjust R243 (Figures 6-2 and 6-4), located on the same etched-circuit board as R 140, to correct the meter readings. This adjustment affects ail measurements but on the 1863 is most sensitive at 200-V and 250-V and least sensitive at 100 V. In the 1864, it is most sensitive at the lower settings of the ﬁ rst digit of the test voltage adjustment, i.e. 100 V, 200 V, etc.

6.5.4 Range-Resistor Accuracy

The range resistors can be checked by performing procedure steps in of section 6.2.

6.5.5 Coarse  Adjustment

If it is impossible to set the inﬁ nity controls on the front panel, set both controls at their center positions and adjust R244 (Figures 6-2 and 6-4), located on the etched-circuit board with R140, for a reading as close to with the front-panel controls.

 as possible. Make the ﬁ nal adjustments

SERVICE AND MAINTENANCE

1864-1644 Positive Polarity Megohmmeter

6.6 Knob Removal

If it should be necessary to remove the knob on a front-panel control, either to replace one that has been damaged or to replace the associated control, proceed as follows:

1. 1. Grasp the knob ﬁ rmly with the ﬁ ngers, close into the panel (or the indicator dial, if applicable), and pull the knob straight away

from the panel.

CAUTION

Do not pull on the dial to remove a dial/knob assembly. Always remove the knob ﬁ rst. To avoid damage to the knob and other parts of the control, do not pry the knob loose with a screwdriver or similar ﬂ at tool, and do not attempt to twist the Knob from the dial.

Figure 6-2. Top interior view of 1864 Megohmmeter

2. Observe the position of the setscrew in the bushing, with respect to any panel markings (or at the full ccw position of a continuous control).

3. Release the setscrew and pull the bushing off the shaft.

4. Remove and retain the black nylon thrust washer,

NOTE

To separate the bushing from the knob, if for any reason they should be combined off the instrument, drive a machine tap a turn or two into the bushing for a sufﬁ cient grip for easy separation.

Figure 6-3. Bottom interior view of 1864 Megohmmeter

* or any commercial equivalent

SERVICE AND MAINTENANCE

1864-1644 Positive Polarity Megohmmeter

6.7 Knob Installation

To install a knob assembly on the control shaft:

1. Place the black nylon thrust washer over the control shaft, if appropriate.

2. Mount the bushing on the shaft, using a small slotted piece of wrapping paper as a shim for adequate panel clearance.

3. Orient the setscrew on the bushing with respect to the panel-marking index and lock the setscrew with the appropriate hex-socket

key wrench.

NOTE

Make sure that the end of the shaft does not protrude through the bushing or the knob won’t bottom properly.

4. 4. Place the knob on the bushing with the retention spring opposite the setscrew.

5. 5. Push the knob in until it bottoms and pull it slightly to check that the retention spring is seated in the groove in the bushing.

NOTE

If the retention spring in the knob comes loose, reinstall it in the interior notch that has the thin slit in the side wall. It will not mount in the other notch.

6.8 Meter Cover Care

It is treated inside and out in manufacturing with a special non-abrasive anti-static solution, Statnul*, which normally should preclude any interference in meter operation caused by electrostatic effects. The problem is evidenced by the inability of the meter movement to return promptly to a zero reading, once it is deenergized. As supplied by IET, the meter should return to zero reading within 30 seconds, immediately following the placement of a static charge, as by rubbing the outside surface. This meets the requirements of ANSI standard C39.1-1972.

If static-charge problems occur, possibly as the result of frequent cleaning the window should be carefully polished with a soft dry cloth, such as cheesecloth or nylon chiffon. Then, a coating of Statnul should be applied with the polishing cloth.

CAUTION

Do not use any kind of solvent. Tissues or paper towels can scratch the window surface.

If it should be necessary to place limit marks on the meter window, paper-based masking tape is recommended, rather than any kind of marking pen, which could be abrasive or react chemically with the acrylic.

NOTE

Electrical parts information in this section is presented in such a way that all the data for a part-numbered sub-assembly are visible together in the manual. Thus, the parts list appears on left-hand pages, while the part-location diagram (on the apron) and the schematic diagram (tip out) are on right-hand pages.

The clear acrylic meter cover can become susceptible to electrostatic-charge buildup, and can be scratched if improperly cleaned.

SERVICE AND MAINTENANCE

REFERENCE DESIGNATOR ABBREVIATIONS

B = Motor P = Plug BT = Battery Q = Transistor C = Capacitor R = Resistor CR = Diode S = Switch DS = Lamp T = Transformer F = Fuse U = Integrated Circuit J = Jack VR = Diode, Zener K = Relay X = Socket for Plug-In KL = Relay Coil Y = Crystal KS = Relay Switch Z = Network L = Inductor M = Meter MK = Microphone