Heathkit IM-17

A battery-powered FET multimeter that sold for $21.95 and put service-grade input impedance within reach of anyone with a soldering iron.

Overview

Heathkit pitched the IM-17 as "everybody's VOM" - a battery-powered volt-ohm-milliammeter simple enough for a homeowner to assemble and accurate enough for a working serviceman to trust. The hook is the input circuit. A field-effect transistor front end gives the IM-17 an 11-megohm input resistance on every DC range, the same figure a service-grade VTVM delivers. For a kit that sold for $21.95, that's a lot. Ordinary moving-coil VOMs load the circuit under test and drag the reading down. The FET input lets the meter look at a high-impedance node without disturbing it.

The ranges come in decade steps. There are four DC voltage ranges at 0-1, 10, 100, and 1000 volts full scale, four AC ranges at the same spans, and four resistance ranges running from R×1 up to R×1 megohm with 10 ohms at center scale. Accuracy is specified at ±5% of full scale on both the DC and AC sections. The AC section holds its frequency response to ±1 dB from 10 Hz to 1 MHz. Its input resistance is 1 megohm on all ranges, lower than the DC figure, as you'd expect.

Electronics Illustrated (May 1969, p.34) put the meter through its paces and came away impressed with the front end specifically, noting that the FET's 11-megohm input impedance matches a service-grade VTVM and that zero-set stability beat higher-priced commercial instruments — once zeroed, the meter stayed put across range changes. The same review flagged the decade-only range switching as the IM-17's real compromise: measuring 1.5 volts means sitting near the bottom of the 10-volt scale, and checking 117 VAC puts the needle way down on the 1000-volt range. Service-grade meters typically fill in with 3- or 5-volt steps to avoid exactly that. The reviewer also clocked an eight-hour build.

The meter movement is a 4½-inch, 200-µA, 100-degree unit with four separate scales: one for resistance, one for DC, one for AC above the lowest range, and a dedicated scale for the 1.2-volt AC position. The circuit is all solid-state, with one FET, four silicon transistors, and a silicon diode. Heathkit's catalog copy emphasizes the practical conveniences: zero and ohms-adjust controls, a DC polarity-reversing switch, three permanently attached test leads, and a jack for an accessory probe. The leads don't have to be moved between jacks for different measurements, since a single selector switch handles range and function. When the power switch is off, the movement is damped to protect it in transit. The case is a rugged, portable enclosure with a storage compartment for the probe, sized at 5½ by 9½ inches.

Power comes from two batteries that Heathkit did not include: a single 1.5-volt C cell and an 8.4-volt mercury cell. The mercury battery is the maintenance headache. Mercury cells have been off the consumer market for decades, so a working IM-17 today either runs on a substituted power solution or doesn't run at all. Calibration, at least, is straightforward. Electronics Illustrated (May 1969, p.34) described connecting the meter across a 1.5-volt battery and trimming the DC calibration control, then across the AC line for the AC control, after which all ranges are calibrated.

Heathkit positioned the IM-17 as a beginner-friendly kit. The catalog copy claims a three-to-five-hour assembly and calls it an ideal first project, and the company recommended it alongside its Continuing Education Series of electronics courses. It turns up in estate sales and workshop clearouts with surprising frequency. It's not a collector's piece in the way a tube-era VTVM is, and nobody is paying a premium for one.