Heathkit IM-25 (1967–1970s)
The IM-25 was Heathkit's solid-state VOM. It was a bench instrument, not hi-fi gear, though it turns up in the same catalogs and on the same workbenches as the company's amplifiers and tuners. Heath described it as combining the functions of a vacuum-tube voltmeter and a volt-ohm-milliammeter in one cabinet. It could run from a 120 V AC line or from an internal battery, selected by a single switch. That meant you could take it into the field or use it on a live chassis without a cord trailing behind you.
The front panel carried a 6-inch meter with three separately colored scale sets: red for AC volts, black for DC volts, green for ohms. Heath skipped the single shared range selector and gave each function its own switch - Volts, Ohms, and Milliamperes - so each could have its own number of ranges. A single test probe replaced the usual tangle of test leads. A dual-winding power transformer let the builder wire the unit for either 120 or 240 V AC. The ohmmeter section was deliberately low-voltage, intended for in-circuit resistance checks in transistor circuits without forward-biasing junctions. Construction used a circuit board and cable assembly, which Heath pitched as "easy and clean."
The meter movement itself was rated to ±2% of full scale. Add the ±1% divider resistors on DC and you get ±3% of full scale. On AC the rectifier circuit pushed it to ±5%. Ohms accuracy depended on the multipliers (1%) and the movement (2%), but Heath noted the nonlinear ohms scale made a simple percentage figure meaningless. Input impedance was about 10 megohms at low frequencies. It fell off at higher frequencies as input capacitance came into play, to roughly 100 kΩ of capacitive reactance at 10 kHz, which Heath warned could load a circuit under test. Max safe AC measurement was 1500 volts. The manual also includes a procedure for checking meter coil continuity using a series limiting resistor of at least 10,000 Ω. It carries a pointed warning that checking the coil directly with an ordinary ohmmeter would overload and probably destroy it.
Pricing comes from a period source rather than a Heath catalog. The adjustment procedure for a Crown organ lists the IM-25 at $80.00 in kit form and the factory-wired IMW-25 at $115.00, and cites it as an acceptable substitute for a Simpson 715 VTVM and a Simpson 260-5 VOM together. So it was cheaper than the two separate instruments it could replace, and judged good enough for professional service work.
Surviving documentation is decent for a kit instrument. The assembly manual runs to 64 pages with a full parts list, calibration and troubleshooting sections, chassis photographs, a circuit board X-ray view, and a fold-out schematic. Period press coverage is thin. The IM-25 doesn't appear to have drawn much review attention beyond Heath's own advertising, which ran under the tagline "When you think meters... think Heath!" and "Best Value Meters Come From Heath!" in the late 1960s. Those ads are manufacturer claims, not independent evaluations.
- Type: Solid-state VOM (volt-ohm-milliammeter / VTVM substitute)
- Power: 120 or 240 V AC (dual-winding transformer) or internal battery
- Display: 6-inch meter, three color-coded scale sets (AC red, DC black, ohms green)
- Functions: Volts, Ohms, Milliamperes — separate range switch per function
- Accuracy: ±3% FS DC, ±5% FS AC, ±2% meter movement
- Max AC input: 1500 V
- Input impedance: ~10 MΩ at low frequency
- Price: $80 kit / $115 factory-wired (per 1960s Crown service documentation)
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