Heathkit IT-3121 Curve Tracer
Overview
The Heathkit IT-3121 (and its identical-but-for-color-and-price twin, the IT-1121) is Heathkit's late-1970s semiconductor curve tracer — a bench instrument that draws the I-V characteristic curves of transistors, diodes, FETs, and other solid-state devices. It has no CRT of its own: it outputs X (collector voltage) and Y (collector current) on banana jacks to an external oscilloscope in X-Y mode, which is exactly the trade that made it the affordable hobby-and-service counterpart to the Tektronix 575/576 transistor tracers. This dive covers what the IT-3121 is and its place in the Heathkit test line, how a semiconductor curve tracer works (stepped base drive, swept collector supply, X-Y display), and — following Dave Erickson's well-documented IT-3121/1121-based redesign at djerickson.com/curve-tracer, a modern DIY clone rather than a restoration of an original — how the architecture works and how a contemporary build modernizes it.
Context
The IT-3121 is the vintage semiconductor anchor of this category — a 1970s Heathkit that put transistor curve tracing on the hobby and small-shop bench when a Tektronix 576 was out of reach. It pairs naturally with the modern DIY VBA Curve Tracer (same job, current parts) and sits opposite the tube tracers (uTracer6, eTracer, uTracer NXT). Dave Erickson's write-up is a modern IT-3121/1121-based DIY redesign (microcontroller + DAC-driven), not a repair log of an original — which makes it the ideal source for understanding the architecture and how a contemporary build modernizes it. See the category primer at Curve Tracers — Overview.
