GUIDE to VOMs and VTVMs--ANSWERS to QUESTIONS

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SECTION 1

1. 0.637 x 150 = 95.6 volts, approximately.

2. Zero.

3. 440 X 0.707 = 311 volts, approximately.

4. Electrons are negative charges of electricity. They are repelled by other negative charges and are attracted to or toward positive electrical charges.

5. A movement of electrons.

6. 1 ampere equals 1000 milliamperes, 1 milliampere equals 0.001 ampere; 1 ampere equals 1,000,000 microamperes, 1 microampere equals 0.00000 1 ampere.

7. 50 milliamperes times 0.001 equals 0.050 ampere; 0.00005 ampere times 1,000,000 equals 50 microamperes.

8. Voltage.

9. Battery and electric generator.

10. Electrons move from the negative terminal of the battery, through the electric circuit, to the positive terminal of the battery.

11. Ac voltage also causes electrons to move from negative to positive, but the ac polarity reverses at a regular rate. At first, one of the two ac terminals is negative and the other terminal is positive. After a half cycle of ac voltage has been completed, the first terminal becomes positive and the second one negative. The process repeats at regular intervals. Thus, the ac current moves first in one direction and then in the other.

12. Hertz, abbreviated Hz.

13. 60 Hz.

14. Resistance; it is measured in ohms.

15. Ohm's law expresses the relationship of current, voltage, and resistance in an electrical circuit. The equation for Ohm's law is E = I X R, in which E is voltage in volts, I is current in amperes, and R is resistance in ohms.

E E 16. ( a)I = R:;(b ) R = y . E 20 20 17. R = T = 80 X 0.001=0.08= 250 ohms.

E 100 18. I= "it = 25 = 4 amperes.

19. E = I X R = 3 X 60 = 180 volts.

20. P = E X I = 50 X 4 = 200 watts.

21. Peak = rms X 1.414; 150 X 1.414 = 212 volts, approximately.

22. 2 X 125 = 250 volts.

SECTION 2

1. The d' Arsonval movement.

2. The function switch in the VOM is used for setting the instrument to measure ac or dc voltage or current, or resistance.

3. The main parts are a permanent magnet and a coil to which a pointer is attached; the coil rotates in the field of the magnet when current passes through the coil. Also included are the scale plate, or faceplate; a fixed iron core around which the coil is wound; magnet pole pieces; etc.

4. The meter movement should be returned to the manufacturer for replacement if still in warranty, or for repair if out of warranty. First obtain the manufacturer's authorization or direction for returning the movement.

5. The moving coil.

6. Turn the eccentric screw on the outside front of the meter case. This adjustment is provided for in most meter movements-see the instruction manual for the instrument being used.

7. The one requiring 50 microamperes.

8. 20 microamperes (half the full-scale current).

9. The coil probably will be burned out or the meter otherwise damaged.

10. A resistor can be added in parallel with the meter. (A resistor used in this way is called a shunt. )

11. The shunt should have a value 1/9 that of the resistance of the meter movement, or 1/9 X 500 = 55.5 ohms. Then 1/10 of 10 mA, or 1 mA, will pass through the meter movement, and 9/10 will pass through the shunt.

12. One or more shunts are provided, and the required shunt is switched into the circuit by means of the range-selector switch.

13. Multiplier resistors are used in series with the meter movement.

14. The total circuit resistance must be 1000 -;- 0.001 = 1,000,000 ohms.

The resistance of the multiplier is then 1,000,000 - 1000 = 999,000 ohms.

15. By including a rectifier in the meter circuit for changing the ac to dc.

16. Any one of the circuits of Fig. 2-9, preferably B or C.

17. Because the resistance of the rectifier and the nature of the rectified current must be taken into account in the ac voltage-measuring function.

18. The pointer will deflect to the half-scale point.

19. Shunt type and series type.

20. Shunt ohmmeter (Fig. 2-10C).

21. For the measurement of output power, generally in audio circuits.

22. Sensitivity depends on the amount of current required for full-scale deflection; less current required indicates higher sensitivity. Sensitivity is specified in terms of ohms per volt; a large number of ohms per volt means that a small current is needed to indicate a given voltage, and therefore indicates a sensitive meter.

23 The sensitivity is 20000 ohms per volt.

24. Usually the sensitivity rating is lower; a VOM that has a rating of 20,000 ohms per volt on the dc ranges might have a sensitivity of 10,000 ohms per volt or less on the ac ranges.

25. Loading effect is the change in circuit conditions caused when a measuring device is connected to the circuit. Loading effect of a VOM is more noticeable in circuits of high impedance, and in these circuits VOM's having higher sensitivity should be used for most accurate measurements.

SECTION 3

1. Test-lead wire should be flexible, have good insulation, and should not be susceptible to tangling and snarling.

2. A high-voltage test probe consists of a series multiplier resistor built into the plastic insulating probe. The multiplier resistor permits measurement of voltages higher than those for which a VOM is normally designed.

3. The cause of the damage (usually a mistake made in using the VOM) should be determined so that the replacement will not be damaged also. An exact-replacement resistor should be obtained from the manufacturer if possible. Most VOM's are designed with shunts and multipliers of precise values, of a particular material, or having other characteristics which must be maintained if the VOM is to retain its versatility and accuracy.

4. A typical tolerance rating is 1%.

5. R24.

6. Error in reading the scale value caused by being at an angle from the meter.

7. A mirror sometimes is included on the faceplate behind the pointer so that the pointer and its mirror image will coincide when the eye of the observer is directly in front of the pointer.

8. A shunt which some manufacturers make available for use with their particular VOM's for measurement of current values in excess of the highest built-in current range of the VOM.

9. Test leads should be inspected regularly for good electrical connections at the probes and tips, for loose strands of wire that could cause a shock or a short circuit (or an inaccurate reading ) , and for breaks in the test-lead insulation.

10. The flange helps to prevent the user from placing his hand too close to the high-voltage point being tested, or too close to the high-voltage multiplier resistor contained near the tip end of the probe.

SECTION 4

1. Its loading effect when used for measuring current.

2. This means that when the VOM is used for measuring current and when it is deflecting full-scale, it reduces the voltage to the load by 100 millivolts. This reduction is due to the voltage drop across the terminals of the instrument.

3. In low-voltage circuits.

4. Between 2 and 5% for dc ranges, and 2 and 10% for ac ranges.

5. At or near the upper, or full-deflection end of the scale.

6. We could say that resistance measurements are accurate within ±5% of the arc length, or ±5 degrees.

7. The value of the resistance being measured is 150 X 100, or 15,000 ohms.

8. By noticing the midscale value of the resistance scale and then selecting a range that will give a reading closest to this midscale point for the particular resistor to be measured.

9. When measuring a given amount of voltage, the instrument is accurate within 1 dB at any frequency between 50 Hz and 100 kHz as compared to its reading at 400 Hz.

10. The ability to repeat the same reading for successive measurements of the same quantity.

11. The ability of the VOM to indicate accurately at every paint on its scale.

12. Most VOM's are designed with scales marked in rms ac values, but meters are deflected in proportion to the average value of the ac signal being measured. If the ac voltage is not a sine wave, the reading from the rms scale may not accurately indicate the true RMS, or effective, value.

13. Place the instrument in the position in which it is to be used. If the pointer does not rest on zero, adjust the zero-set screw (usually located at the lower center of the front of the meter ) for zero indication of the pointer.

14. Plug the red test lead into the jack marked PLUS. Plug the black test lead into the MINUS jack. Set the function switch to dc voltage. Be sure the range switch is set for a range with a full-scale value that exceeds the value of voltage to be measured. Connect the test prods across the two points between which the voltage is to be measured, the red test lead at the more-positive point and the black test lead at the more-negative point.

15. Plug the red test lead into the positive jack, and plug the black test lead into the negative jack. Set the function switch for current. Set the range switch to a current range which will include the value of current to be measured. Be sure the circuit is turned off and any capacitors are discharged. Open the circuit at the point where the current measurement is to be made. Connect the positive test lead to the more-positive circuit point, and the negative test lead to the more negative circuit test point. Stand clear of the VOM. Turn the circuit on and observe the reading on the meter.

SECTION 5

1. Use a high-resistance range, for example the R X 10,000 range. The capacitor should be uncharged; if in doubt, short the leads of the capacitor together. Connect the ohmmeter test leads across the capacitor. The pointer should deflect in the direction of zero resistance and then return toward the infinite end of the resistance scale. If the capacitor is shorted, the pointer will go to zero ohms and remain there. A reading near infinity indicates that the leakage resistance of the capacitor is high.

2. Typically about 20 megohms.

3. Electrolytic capacitors have more leakage, or measure lower values of leakage resistance, than do paper or mica capacitors. Also, with the test leads in the correct polarity position, the leakage resistance is much higher than it is with the opposite polarity for the test leads.

4. Use the ohmmeter function of the VOM. Connect the test leads across the terminals of the diode and note the resistance reading. Then reverse the test leads and again note the resistance reading. The reading should be high in one direction and low in the other. The high reading might be about 10 times the low reading.

5. With the circuit off and discharged, remove the fuse. Its resistance should be very close to zero ohms if the fuse is good, and infinite ohms if the fuse is open, or bad. If the circuit is live, measure the voltage across the fuse. With a closed live circuit, if the fuse is blown, the voltage across the fuse will be the full voltage of the source. If the fuse is good, the voltage across the fuse will be zero.

6. In a series-string circuit, when one tube develops an open filament, all the tubes in the circuit go out. The full line voltage is then across the terminals of the open filament. The VOM can be used as a voltmeter to measure the voltage across each of the tubes in the string. The tube across whose terminals the full voltage is measured is the one with the open filament.

7. The condition of a battery is best tested when the battery is operating under its normal load. Connect the battery to its load (for example, a transistor radio or amplifier ). Turn the switch on and measure the voltage across the terminals of the battery. If the voltage measures 75% or less of its rated value, the battery probably should be replaced (or recharged if its the rechargeable type ).

8. By using a resistor in series with the "hot" test lead of the VOM. The value of the resistance should be selected so that it is high enough to eliminate the "upsetting" effect, but low enough so that a usable reading on the VOM can be obtained. The reading with the resistor in series with the test lead will not necessarily be an absolute indication of the actual voltage. (See Table 5-2. )

9. When possible, work with one hand behind you. Be alert to the possibility that faulty operation of the equipment or device being tested can make the device more dangerous than usual to work on. Do not stand on damp or wet surfaces. Keep clear of the circuit. Be sure the power is off when making resistance measurements. When one of the test leads is connected to the live circuit, do not touch the tip of the other test lead.

10. Set the VOM function and range switches for measurement of line voltage. Connect one of the test leads to ground. Connect the other test leading to the chassis of the equipment. If a substantial reading is obtained, the chassis is hot.

11. If the pointer does not deflect for any of the functions or ranges, and if the test leads are in good condition, there is a chance that the meter movement is burned out or open. Do not take the meter movement apart; it can be repaired only by the manufacturer. If you know the resistance of the meter movement, and if you are careful, you can set up a test circuit to check the movement as follows.

Determine the resistance of the meter movement and its rated full-scale current. Connect a 1.5-volt battery and a series resistor to the meter movement. The combined resistance of the series resistor and the meter movement should cause only 2/3 full-scale deflection of the pointer. See Fig. 5-10 and related text.

12. Either the function switch or the rectifier is defective.

13. Only an exact replacement from the manufacturer or other source should be used; otherwise the instrument will not be accurate on the ac ranges.

14. Try not to damage adjacent parts with heat from the hot soldering iron. Beyond this and the use of other ordinary care, special effort should be made not to overheat precision resistors, since this could cause a change in their value.

SECTION 6

1. The VTVM generally has a higher input resistance. Also, the VTVM includes an electronic amplifier which increases its sensitivity.

2. The VTVM is less stable and requires warm-up time. It has more-complex circuitry and must be calibrated more frequently. An external source of power is required for the VTVM.

3. (See Fig. 6-5. )

4. The meter movement is "bridged" between the plates of two identical vacuum-tube circuits. When no voltage is being measured, the currents through the tubes are equal and no voltage appears across the meter to cause deflection of the pointer. When the voltages to the grids are not equal, current through the tubes is unbalanced, resulting in current through the meter.

5. It includes a switch and a I-megohm resistor in series with the test lead. The resistor is switched into the circuit for dc measurements, and is switched out for ac and resistance measurements.

6. It isolates the circuit being checked and reduces the effect of the in put capacitance of the VTVM.

7. An rf probe.

8. 1089 megohms is a typical value.

9. By a factor of about 100.

10. The rf probe contains a diode. Rectification of the rf signal takes place in the probe so that the rf signal does not have to travel through the coaxial cable, which would otherwise attenuate the signal.

SECTION 7

1. An additional stage of amplification is included.

2. Most recent instruments use selenium or silicon rectifiers ; earlier VTVM's used vacuum-tube rectifiers.

3. When the switch is in the "transit" position, a short is placed across the meter terminals. This "damps" (limits or reduces ) the movement of the coil and pointer, and prevents damage to the movement and pointer when the instrument is not in use.

4. To prevent accidental changes in the control settings.

5. With the VTVM off, the pointer is set to zero with the zero-set screw usually located on the meter face just below the bottom of the pointer ). During operation, the panel ZERO ADJUST control is used in accordance with the operating instructions for the instrument.

6. At least 5 minutes, but 20 to 30 minutes is a better figure. (Consult the manufacturer's instructions for the particular instrument in use.)

7. It is a good practice to install a fresh battery. The output of a weak battery may drop during the course of the calibration procedure and cause an inaccurate calibration.

8. The new tube should be aged for 30 to 50 hours before the VTVM is recalibrated. The instrument can be used during this time, although measurements may not be within rated accuracy.

9. This scale is useful in the alignment of fm-receiver discriminators.

10. The usual reference is 1 milliwatt in 600 ohms. If measurements are made across a different impedance, a correction factor usually must be applied to decibel values read from a scale or chart.

11. The same as the VOM: Calibrate it properly before using; replace batteries when they become weak or leaky; observe safety precautions; keep test leads in good condition; store the VOM in a safe place, away from machinery, dirty or dusty places, or areas of high temperature or humidity; use exact replacement parts for repair; do not attempt to repair the meter movement yourself-it should be repaired only by the manufacturer.

12. Negligent or accidental misuse of the instrument, failure of component or tube, defect in line cord or ac plug, blown fuse, faulty on-off switch, etc.

13. Recalibrate the VTVM in accordance with the manufacturer's directions.

14. A defective resistor in the multiplier circuit might cause this symptom.

(Other possible causes are a weak battery and a weak tube. )

15. By use of an additional external battery and series resistor. (See Fig. 7-9.)

16. The plastic cover over the meter face might have accumulated a static charge as a result of cleaning or polishing of the plastic. The charge can be removed by using a commercially available antistatic solution, or a solution of a good liquid detergent and water. Dampen a clean cloth in the solution and wipe the plastic cover.

SECTION 8

1. They are small, lightweight, compact, battery operated, portable, versatile, and they require no warm up.

2. Field-effect transistor.

3. High input impedance.

4. Basically, the functions of the gate, source, and drain correspond to those of the grid', cathode, and plate, respectively, of a triode vacuum tube.

5. See Fig. 8-2.

6. For checking the condition of the batteries that power the instrument.

7. 10 to 15 megohms.

8. It provides a high input impedance so that the circuit under test is not loaded.

9. Roughly six months to a year. Battery life can be extended by turning the instrument off when it is not used and by not storing it in a warm location.

10. From 0 deg. F downward, battery capacity decreases, but batteries become completely inoperative at -20 deg. F.

SECTION 9

1. No parallax error; fast, accurate readings; repeatability; no eye strain; easier to read; no sticky pointer; automatic ranging and over-ranging on some models; automatic polarity on some models; and less skill required.

2. Changing values harder to interpret, less rugged, does not usually make decibel measurements, less convenient due to size and usual operation from power line, harder to read in high surrounding brightness, limited usefulness for semiconductor testing, and greater cost.

3. Signal conditioner, analog-to-digital converter, display.

4. LED, gas discharge, incandescent, and Nixie.

5. Less susceptible to noise, etc.

6. Resistance-measuring function.

7. Percentage of reading plus or minus one digit.

8. 1200 volts.

9. The one farthest to the left.

10. To control the brightness of the display.


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