Heathkit IT-3121 — Vol 3: Servicing It, and Dave Erickson's Redesign

3.1 A note on what this volume is — and is not

It is worth being precise, because it is easy to get wrong. Dave Erickson’s write-up at djerickson.com/curve-tracer is titled “Curve Tracer: Heathkit IT3121/1121 Based design,” and that word design is load-bearing: Erickson is not restoring an original IT-3121. He is designing and building a modern DIY clone inspired by it. He opens by saying he had “been looking at the HeathKit IT-3121” because “this has decent specs, suitable for a DIY’er,” and then proceeds to reimplement the instrument’s architecture with contemporary parts — a microcontroller, DACs, and relays — rather than recapping and repairing a vintage chassis.

So this volume does two honest things. First (briefly) it covers the generic service a real 40-year-old IT-3121 needs, from community experience — no more than the sources support. Then it covers Erickson’s actual documented work: the specific design decisions and improvements he made relative to the original, attributed to him, because that is where his write-up is genuinely detailed and where the real engineering lessons live.

3.2 Servicing an original IT-3121 (the modest, generic part)

If you have a physical IT-3121 or IT-1121 on the bench, the wear items are exactly what you would expect of any all-analog instrument from the late 1970s, and the community guidance is consistent and short:

• Electrolytic capacitors. These units carry on the order of nine electrolytics, and after four decades they are the prime suspects — dried out, drifted, or leaky. A recap of the electrolytics is the standard first move before trusting the instrument’s readings.
• Switch contacts. The whole user interface is rotary switches and their multiple decks. Oxidized contacts cause intermittent ranges and noisy traces; a cleaning with contact cleaner is the usual cure.
• Bring it up gently. Because there is a mains transformer and a supply that reaches into the hundreds of volts, an unknown-history unit should be brought up slowly on a variac while watching current draw, exactly as you would treat any tube-era mains gear. (HV specifics are in Vol 4.)

That really is the extent of what the sources document for servicing a genuine unit: recap the electrolytics, clean the switches, power up carefully. There is no published teardown-and-repair log of a broken original to draw specifics from — which is precisely why the interesting, well-documented engineering here is Erickson’s redesign, below. Do not read the following section as things Erickson found wrong on a bench unit; read it as the design choices he made building a new one.

3.3 What Erickson kept from the original

Erickson’s redesign is a compliment to the original in the sense that he keeps the whole conceptual machine intact — the same three-part idea from Vol 2. He retains:

• a collector sweep supply with a front-panel voltage set and current sensing;
• a base/gate step generator producing a staircase of drive;
• X-Y output of voltage-versus-current.

He validated that core before committing to a board. His prototype demonstrated the collector-voltage generation and control, a working base-step circuit putting out 200 µA per step into a 1 kΩ resistor, and a first real trace — a D44C9 NPN power transistor showing a beta of about 50. His own verdict on those first plots: “OK, those are decent looking plots, it’s time to lay out a real PCB.” The architecture works; what he changes is how it is controlled and switched.

3.4 What Erickson changed, and why

This is the heart of his write-up. Each change targets a specific limitation of the 1970s design.

1. Multi-deck rotary switches → a microcontroller and relays. Erickson’s sharpest practical criticism of the original is cost and complexity of its front panel: those multi-deck rotary range switches are “quite expensive, $40 up to $130” today, and they demand a fussy hand-wired harness. His answer is to replace them with 15 DPDT relays at about $1.50 each, commanded by TPIC6C595 shift-register drivers, all under an ItsyBitsy M0 Express microcontroller. The ranges and the load-resistor selection that used to be knobs become software-controlled relay states.

2. A hardware step generator → dual DACs. The original’s counter-and-transistor-DAC staircase is replaced by an MCP4822 dual 12-bit DAC feeding a TL072 op-amp for the base drive. This does more than tidy the parts count — it makes the entire step program a matter of software. As Erickson notes, the fine base-current range selection (x1.0, x0.5, x0.2) “can be done … in software,” collapsing a bank of switches into a few lines of code.

3. JFET-only → true MOSFET support. The original, Erickson points out, “doesn’t do MOSFETs, just JFETs.” The reason is a polarity limitation: depletion JFETs need a negative gate voltage, while enhancement MOSFETs need a positive one, and the original’s gate drive doesn’t span both. His two-DAC scheme drives separate positive and negative amplifier inputs to inject an offset voltage, so the gate staircase can be shifted into either polarity — enabling MOSFET tracing the original could not do. This is arguably the most significant functional upgrade.

4. Manual-only → programmable and instrumented. Because a microcontroller is now in the loop, Erickson adds capabilities the analog original never had:

• Remote control over USB — “allows remote control over everything,” so the instrument can be scripted rather than knob-twiddled.
• On-board and PC display. He adds a 2.4-inch OLED for a self-contained curve readout, plus ADC readback of the measured points so the curves can be shown digitally on the OLED or streamed to a PC — the original had no display of its own at all.
• Room to grow the envelope. He observes the original’s ±200 V is “low for some devices” and designs so the voltage can be pushed higher later, and notes that AC testing, which the original lacks, “could be automated” in software.

Erickson’s original-versus-redesign, at a glance:

Table 1 — Erickson's original-versus-redesign, at a glance:

Aspect	Original IT-3121	Erickson’s redesign
Range/load selection	multi-deck rotary switches ($40–$130)	15 DPDT relays (~$1.50 ea) via shift registers
Control brain	none — all analog	ItsyBitsy M0 microcontroller, USB
Base/gate drive	counter + transistor DAC	MCP4822 dual DAC + TL072 op-amp
FET support	JFET only (single gate polarity)	JFET and MOSFET (±gate offset)
Display	external scope only	2.4” OLED + PC display + external scope
Measurement	analog only	ADC readback of curve points
AC test	no	planned, in software

3.5 The lesson the redesign teaches

Erickson’s project is the best kind of tribute: it proves the IT-3121’s analog core is sound — a good sweep supply and a clean step generator are still the right way to trace a device — while showing exactly which parts of a 1970s implementation are now the weak links. The expensive, hard-to-source rotary switches, the JFET-only gate drive, and the total dependence on an external scope are all artifacts of the parts and economics of 1978, not of the underlying method. Swap the switch banks for relays, the discrete DAC for an MCP4822, and the knobs for a microcontroller, and the same instrument becomes cheaper to build, able to trace MOSFETs, and controllable from a laptop. Vol 4 returns to the original hardware and puts it to work.