Heathkit IM-18 (1968–1976)

A $9.95 kit meter that taught a generation of hobbyists what a volt really is.

Overview

Heath billed the IM-18 as "the basic tester EVERYONE can use," and at $9.95 for the kit, they weren't exaggerating the price. This was the VTVM that came in the mail in a box of resistors and a punched chassis, got assembled on the kitchen table over a weekend, and then spent the next twenty years on a workbench measuring everything from TV B+ rails to appliance continuity. The company's own catalog copy called it "our most popular VTVM," which for a kit house moving thousands of units a month is saying something.

The heart of the thing is a balanced-bridge circuit built around a twin triode, driving a 4½-inch 200 µA meter movement. That bridge design matters — it's why the meter reads zero at dead center and swings either direction, and it's why the input resistance on every DC range sits at 11 megohms, with 1 megohm of that living out in the probe itself. For anyone coming off a cheap VOM, that figure was a revelation: you could measure high-impedance points in tube gear without the meter itself loading the circuit down and lying to you. On AC ranges the input drops to 1 megohm shunted by 35 pF, which is fine for audio work and the occasional RF sniff.

Seven ranges cover the practical waterfront. DC and AC RMS run 0–1.5, 15, 50, 150, 500, and 1500 volts full scale, with AC peak-to-peak scales at 0.4, 14, 40, 140, 400, and 4000 volts. The separate 1.5- and 5-volt scales are the detail that separates a usable meter from an annoying one — low-voltage readings on solid-state gear land near full scale instead of squinting at the bottom 5% of a 15-volt arc. Resistance runs from 10 ohms center scale on the ×1 range out to the ×1 meg range, and the meter will read from 1 ohm to 100 megohms on its internal 1.5-volt C cell (not supplied — Heath never supplied batteries).

Accuracy is honest rather than spectacular: ±3% of full scale on DC, ±5% on AC. Frequency response holds to ±1 dB from 25 Hz to 1 MHz into a 600-ohm source, which is wider than most owners will ever need. The 1% precision network resistors in the divider are the reason the numbers hold up as well as they do. The whole thing runs on 120 or 240 VAC at 50/60 Hz and measures 7⅜ inches high by 4¾ wide by 4¼ deep — small enough to live on a crowded bench, and the single-probe setup (one test lead makes every measurement, including ohms) keeps the cable clutter down.

Assembly is a printed-circuit board job, which by the late '60s was Heath's standard and a genuine improvement over the point-to-point harnesses of the previous decade. Buyers who didn't want to solder could order the factory-assembled SM-20A for $5.00 — which tells you how much Heath valued the educational experience versus the labor. For the hobbyist who wanted to understand what he was measuring, the kit was the point.

There's a small footnote worth flagging for anyone shopping today: Heath sold the IMA-18-1 solid-state conversion, a pair of plug-in modules that replaced the 6AL5 and 12AU7 tubes and eliminated pointer drift on warm-up. At $17.95 it cost more than the meter kit itself, which is either a testament to how cheap the IM-18 was or a warning about how much the tube front end drifted. Both readings are probably fair.

Production ran from 1968 until 1976, according to *DB Magazine* (June 1969, p.34), which is a respectable eight-year run for a kit instrument and puts it squarely in the era when every TV repair shop and ham shack in America had at least one. The magazine's October 1969 issue (p.41) ran Heath's "Think Best Value! Choose Heath!" advertising alongside conference coverage of audio aids to medicine, per DB Magazine (October 1969, p.41) — a reminder that these meters were sold into every corner of the electronics world, not just hi-fi. The company's own advertising billed the IM-18 as the tester for "audio, TV, appliances, instruments, etc.," and that breadth is exactly why so many survived.

Collectors note that the IM-18 turns up regularly and cheaply, usually with a yellowed meter face and a probe that's lost its 1-megohm resistor to time and flexing. The 200 µA movement is the fragile part — a hard knock can peg it, and replacements are scarce. The C-cell holder corrodes if a battery was left in, and the calibration pots drift after fifty years. But as a working instrument, a properly restored IM-18 still does what it did in 1970: reads DC volts without loading the circuit, checks resistance, and tells you whether that capacitor is really leaky or just old. For the price of a decent dinner, it's the cheapest way to own a piece of the kit era that still earns its bench space.