Instrument Gallery
Basic Specifications
Front Panel Controls
The IM-28 utilizes a rugged, utilitarian layout designed for bench-top precision:
- Function Selector: Rotary switch for AC, DC-, DC+, and Ohms.
- Range Selector: Step-switch covering 1.5V up to 1500V and resistance multipliers.
- Zero Adjust: Mechanical screw-knob for zeroing the needle.
- Ohms Adjust: Potentiometer for calibrating the infinity (∞) point.
- Calibration Trimmers: Three rear/front accessible pots (DC CAL, AC CAL, AC BAL) for periodic circuit tuning.
Accessories Included
- Standard Probe Set: Heavy-duty insulated leads with braided grounding line for standard AC/DC and resistance measurements.
- RF Probe (Model 309-CU): A specialized detector probe with a silver barrel and red cap. Essential for measuring high-frequency RF signals (radio, TV, transmitters) which standard probes cannot read. Rare Original
Green: Ohms • Red: AC Volts (1.5V/5V Only) • Black: AC/DC RMS & Decibels (DB)
Legacy & Collectors' Commentary
In the 1960s, owning a professional-grade vacuum tube voltmeter was a substantial financial barrier for the average hobbyist, technician, or ham radio operator. The Heath Company solved this by delivering the IM-28 in a cardboard box filled with hundreds of discrete components, a pre-drilled chassis, and a thick manual of schematic diagrams and assembly instructions.
For the builder, constructing the IM-28 was a 10- to 15-hour rite of passage. It required careful soldering, meticulous wire routing, and proper tube seating. The reward was an instrument that matched—and in some cases exceeded—the accuracy and input impedance (11 Megohms) of commercial units costing multiples of the kit price.
The RF Probe: A Window into the Radio Age
The distinctive 309-CU Radio Frequency Probe shown in the gallery is a crucial piece of this collection. In the pre-digital oscilloscope era, this probe was the only way to trace AM/FM signals through a receiver. It contains a miniature germanium diode rectifier that converts high-frequency AC signals into a steady DC voltage, allowing the VTVM's analog meter to display the signal strength.
Today, a complete Heathkit IM-28 with its original 309-CU probe is considered a highly desirable collectible. Vintage audio restorers and tube-amp enthusiasts prize them not only for their reliability in servicing legacy equipment but also for their distinct, warm analog aesthetic.
âš ï¸ Service Note: As a mains-powered instrument using over 50-year-old electrolytic capacitors, any original IM-28 should be fully restored (capacitors replaced, tubes tested) before being plugged into a modern workshop wall outlet.