MANUFACTURER'S SPECIFICATIONS
D.C. voltmeter: 5 ranges-200 mV, 2V, 20V, 200V, and 1000 V. Accuracy: ±0.2%± 1 digit with furnished calibrator; +0.1%-i1 digit with lab calibration. Overload protected. Ohmmeter: 6 ranges--200 ohms with 1 mA test current; 2000 with 100 µA; 20,000 with 10 uA; 200k with 10 µA; 2.0 megohms with 1 µA; 200 megohms with 100 nano amps. A.c. voltmeter: 5 ranges (same as d.c. voltmeter). Accuracy: 0.75% ± 1 digit with calibrator from 40 Hz to 10 kHz; increasing to maximum of 1.5% +/- 1 digit on 1000 V range. Overload protected. D.C. ammeter: 5 ranges--200 µA, 2 mA, 20 mA, 200 mA, and 2A. Accuracy: ±0.5%± 1 digit for 2-amp range, better on other ranges.
Overload protected to 3 amps. A.C. ammeter: 5 ranges (same as d.c. ammeter) Accuracy: better than 1.5%± 1 digit on all ranges.
Display: Maximum count, 1999, with over-range indication automatic beyond 1999; polarity indication, automatic "+" or "-" (on d.c. ranges). Numeric display by side-viewing neon glow tubes with integral decimal points.
Dimensions: 3 in. H x 7 in. W x 7.9 in. D.
Weight: 8 lbs.
Price: $229.95.
Anyone who has ever done any servicing or experimenting with either linear or digital IC's will certainly welcome this instrument. Having made hundreds of measurements with the older Heathkit IM-18 vtvm, the writer has been annoyed by the need for switching from "+" to "-" every time the probe was moved from one point to another (almost). The automatic polarity indicator eliminates this requirement and thus speeds up the measurement procedure appreciably. In comparison with the older vtvm-which has given many years of reliable service-the increased accuracy of reading the instrument is noticeable, and especially with solid-state devices, the minimum range of 200 mV is extremely useful. This observer never could see the reason for having a digital instrument until finally putting one into daily use-and now would not be without one. The IM-102 is not inexpensive compared to the IM-18 which is priced at $29.95, but it is a worthwhile investment in any case, and does not require occasional replacement of the ohmmeter battery.
[1. J. Eargle: "A Summary of Recording Studio Monitoring Problems" given at the 44th AES Convention in Rotterdam, February 1973.]
Principle of Operation
The IM-102-and most other digital multimeters, for that matter--combines analog and digital techniques. The basic input range--either 200 mV or 2 V-is the voltage to which all inputs are converted by the range switch. This basic voltage-protected by a pair of diode-connected bipolar transistors--is fed. to a linear IC and its output charges a capacitor, which is then discharged at a constant rate. The time required to discharge the capacitor is measured by the number of cycles of the 40,000-Hz clock oscillator that it takes to discharge it. This action takes place every 200 milliseconds, and the number of cycles of the clock that pass during the discharge period are counted, decoded, and displayed. Measurement of resistance is done by passing a specific current through the unknown and measuring the voltage drop across the unknown, so the instrument is still a voltmeter. For a.c. measurements, the input is converted to d.c. by an average sensing, rms-calibrated converter. Thus the instrument is always a voltmeter, regardless of the quantity it is measuring.
This is, of course, true of any vacuum-tube multimeter, and with the except of measuring shunts across the meter movement, this also applies to any multimeter. Even then, one could say that the meter movement itself is actually a voltmeter that draws a finite amount of current when making any measurement.
It should be noted here that the input resistance of the IM-102 is considerably higher than in the average vtvm on the two lowest ranges. On the 200 mV range, the input resistance is greater than 100 megohms, and on the 2-volt range it is greater than 1000 megohms, whereas most vtvms have a constant input resistance of 11 megohms. On the three highest ranges, input resistance is 10 megohms, which is approximately the same as with vtvms. Similarly, the test current ranges from 1 mA on the 200-ohm range to only 100 nanoamps on the 20-megohm range. On the a.c. voltage ranges, the input impedance is constant at 1 megohm shunted by 150 pF, and the voltage in the current-measuring modes is only 0.2 volts for both a.c. and d.c.
The main differences between the digital instrument and its counterparts is in the method of displaying the results. In an analog instrument, the display is always on the scale of a meter-usually with a movement sensitivity of 50 microamps for a 20,000-ohms-per-volt instrument or of 200 µA for a 5000-ohms-per-volt model. In the digital instrument, the actual voltage measurement is done by analog techniques, and the readout is supplied by display elements-in this case, by neon tubes of the "Nixie" type. Three such tubes are employed, each with full display of numerals from 0 to 9, which accounts for a maximum display of 999. The fourth digit-required to provide a display of 1999--is simply an ordinary neon tube with elements of the same length as the height of the numerals in the display tubes.
Assuming the determination of the number of cycles of the 40-kHz oscillator that pass during the measurement of the discharge time of the integrating capacitor, and repeating this measurement every 200 milliseconds (that is, five times per second), there are 8000 cycles to work with during each measurement period. The actual number of cycles elapsed are counted, fed into buffer-storage units, and then fed to decoder-drivers which actuate the three numerical display tubes.
Separate dual flip-flops actuate bi-polar transistors which cause the fourth digit (the "1") to be illuminated, the "over" lamp to light up, or to display the "+" or "-" lamps for d.c. polarity.
The normal display range is 1999, which could be either 199.9 µA, or mV, 1.999 mA or volts, 19.99 mA or volts, 199.9 mA or volts. The maximum d.c. voltage range is 1000; on a.c. it is 500; and for the current measuring positions it is 2 amperes--1.999 A plus the "over" light. Operation is quite simple, since there are only two knobs-one controls the function, with positions marked mA and volts/ohms in the d.c. section and V and mA in the a.c. section. The other knob, with twelve positions, is labeled in volts and mA from 1000 to 2, then 200 mV or µA, with six positions labeled from 200 to 20M for ohms. The initial position is the power-off condition. Three banana jacks are mounted on the front panel-red for volts and ohms, black for common, and white for mA. Note that the instrument measures both direct and alternating voltages and currents, with the conversion from a.c. to d.c. being done by the a.c. converter circuit board on which are mounted one linear IC, one bi-polar transistor, and one FET, four diodes, and two adjustable controls for calibration.
The power supply, integral with the main circuit board, consists of the transformer (mounted on the rear panel) which feeds 54 volts center tapped to a full-wave rectifier, 12.7 volts center tapped to another full-wave rectifier, and 102 volts to a half-wave rectifier circuit to supply the operating voltage to the display tubes and the neon indicator tubes. The 22 volts d.c. following the rectifiers feeds the collector of a power transistor which is controlled by another transistor which is referenced by two Zener diodes to furnish 2, 12 volts to the linear IC's and to most of the transistors in the unit.
The 6.75 volts d.c. following the low-voltage rectifiers is fed to a Darlington-connected transistor (all in one package) and referenced by another Zener diode to supply 3.5 volts to the digital IC's. The 102-volt winding is rectified by a single diode to furnish 95 volts to the display tubes. This winding also supplies either a.c. or d.c. at an adjustable 9.0 volts for use in the calibrating procedure. In all, the circuit employs a total of 19 IC's (three of them linear), 18 transistors, six Zener diodes, and 19 diodes of varying characteristics to actuate the three digital display tubes and the four neon lamps which indicate polarity, over-range, and the "1" of the digital display.
Construction
The layout of the instrument is interesting in its simplicity.
The range switch consists of an eleven-wafer switch which is designed for printed-circuit board use, and which is first disassembled to "straddle" the a.c. converter circuit board, then installed on the main circuit board and its 25 contacts soldered in place. Several other connections are made to the switch by wires and by components. This arrangement puts the a.c. converter circuit board some two inches to the rear of the front panel, with a number of other connections making a sturdy mounting. The four-wafer function switch has 24 contacts which solder similarly to the circuit board, and which is entirely in front of the converter board.
The circuit boards are glass epoxy with foil connections on both sides, and with appropriate "solder-through" points. The DIP IC's mount on Molex connectors which are first soldered in place in strips of seven or eight and then the connecting backbone is broken off, using a "tool" furnished for the purpose. The three linear IC's are housed in the familiar TO 99 case, and they mount in a series of female connectors which accept the pins of the IC in the same manner as a tube socket. Most of the bi-polar transistors simply solder to the foil, in the usual way for installing transistors. Two dual transistors, each with six pins, mount in the connectors as used for the linear IC's. The three digital display tubes mount in sockets which are soldered to the circuit board, and the neon single tubes for the "1" and the over-range indicator are also soldered to the circuit board, being positioned by a plastic light shield which holds them in the proper locations. The + and - indicator lights also mount in the plastic shield.
The various indicators are visible through the smoky plastic front panel window with the designations visible as illuminated to indicate the +,-, and "over." A pressure sensitive light shield is furnished to provide a black background behind the display tubes.
On the whole, the instrument goes together with usual Heathkit ease. The rear panel is screened with a space for the Heath label which is put on to cover the lettering for the line voltage for which the instrument is not wired, leaving the line voltage for which it is wired visible. Fuses mount oil the side panel to protect the line voltage supply (1/4 amp) and for the voltage-measuring circuits (Vs amp) and the current measuring circuit (3 amps).
The a.c. line cord is a three-wire assembly with the usual three-terminal plug on one end and a female receptacle on the other which plugs into the male connector on the rear panel. The housing consists of a top cover and a bottom cover which are held in place by the side trim strips. The handle, with its plastic grip, is detented to hold it in any of three positions-for carrying or for a tilted position on the workbench.
Calibration
While the instrument may be calibrated using laboratory equipment, the average constructor will use the d.c. calibrator furnished with the instrument. This device is a small circuit board on which are mounted a 1.35-volt mercury cell, three fixed resistors, and two adjustable ones (one is factory set). This device provides an accurate source of 0.2 volts which is used to calibrate the 200 mV range of the unit. Connecting one lead from the calibrator to the 3.5-volt test point on the instrument permits the adjustment of the variable resistor to provide an accurate 2.0 volts which is used to calibrate the 2-volt full-scale position.
The a.c. calibrator circuit is built directly into the instrument and permits accurate adjustment of the a.c. circuits, after first making an adjustment to provide an indicated 9.0 volts d.c. with the calibrator switch in the d.c. position. Possibly hard to explain but simple to do.
We assembled the instrument in about 10 hours with no problems attributable to Heath, and only one attributable to ourselves-we connected the black "common" lead to where the "hot" red lead should be and vice versa, resulting in some odd indications until we found the trouble and corrected it.
After that, the unit worked just as expected. While the specifications seem to indicate that the a.c. response is somewhat limited in frequency response, we compared the voltages indicated with those indicated on a Heathkit a.c. VTVM, model IM-38. Practically the same indications were observed at any frequency up to 100,000 Hz on both instruments, and both coincided with the output indications on the IM-72 audio generator. It would probably be desirable to make a complete check on any individual instrument to make sure that this performance is duplicated, but it was a pleasant surprise to find such an agreement throughout the audio spectrum in spite of the specifications. Maybe we were just lucky, but that is what we found.
The digital multimeter is a real joy to use when working on circuits which employ both positive and negative polarities as is common with IC circuits, and being able to read voltage to as low as 1 mV is most helpful in almost any transistor circuits. The IM-102 is an exceptionally useful instrument, and after living with one for a few days, the user would never give it up for the older analog instruments. The addition of current-measuring capability to the usual voltage-measuring qualities of the average multimeter makes it an important instrument for any workshop or laboratory. Try it--you'll like it!
-C.G. McProud
(Audio magazine, Nov 1973)
Also see:
Leader Model LBO-301 Oscilloscope (The Workbench) (May 1973)
Nakamichi Model T-100 Audio Analyzer (Equip. Profile, Nov. 1978)
Realistic Sound Level Meter (Nov. 1978)
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