Heathkit IT-3121 — Vol 4: Using It — Procedure, Reading Curves & Safety

4.1 Safety first — this is mains gear with a real high-voltage supply

Read this before you plug anything in. The IT-3121 has no CRT, so there is no picture-tube high-voltage supply to fear — but do not let that lull you. Its collector sweep supply reaches ±200 V and can deliver up to 1 A, and that is genuinely hazardous energy sitting on the device-under-test terminals and inside the case. Treat it with the same respect as any tube-era mains instrument.

• The DUT terminals are live. When the collector voltage is dialed up, the socket contacts and the device leads can carry up to 200 V. Never change a device, touch a lead, or probe the socket with the collector voltage advanced — turn the collector voltage control fully down to zero first. Build the habit: zero the voltage, then touch anything.
• Filter capacitors hold charge. After switch-off, the supply’s electrolytics can stay charged. Before opening the case, unplug it, wait, and confirm the reservoir caps are discharged (verify the bleeder actually bleeds) before reaching in.
• Bring an unknown unit up on a variac. For a vintage or recently recapped unit, ramp the mains slowly on a variac while watching current draw, as in Vol 3.
• One hand behind your back for any live internal work, and keep the metal cabinet properly grounded.

None of this is exotic — it is the standard discipline for mains-powered vintage test gear — but the ±200 V / 1 A supply is easy to underestimate precisely because there is no scary-looking CRT.

4.2 Setting up the scope

The instrument is useless until it has a display, so start there:

1. Connect the IT-3121’s X output to your oscilloscope’s horizontal/X input and its Y output to the vertical/Y input, using the banana jacks.
2. Put the scope in X-Y mode (channel 1 drives horizontal, channel 2 vertical; timebase off). An analog scope is the traditional choice, but a digital scope’s X-Y mode works.
3. Set both scope channels to DC coupling and center the trace. With no device installed and the collector voltage at zero, you should have a single dot at the origin.

From here on you read values against the scope’s graticule: horizontal divisions are volts-per-division and vertical divisions are milliamps-per-division, both set by the IT-3121’s sensitivity switches, not the scope’s own volts/div.

4.3 General operating procedure

A safe, repeatable sequence for any device:

1. Start cold and small. Collector voltage to zero, choose the lower 40 V / 1 A range unless you know you need 200 V, and set a large load resistor — this limits current while you find your footing.
2. Set polarity for the device: NPN or N-channel one way, PNP or P-channel the other. The whole picture reflects into the matching quadrant.
3. Insert the device with the voltage at zero into the A (or B) socket, minding the E-B-C or S-G-D pin assignment.
4. Pick the step drive. For a BJT, choose a base current per step (start modest, e.g. tenths of a mA) and a step count of about 5. For a FET, switch to gate voltage steps. Set the number of steps to 5–10.
5. Set the axis sensitivities roughly for the device class — a small-signal part wants mA/div and a few V/div; a power part wants larger current/div.
6. Advance the collector voltage slowly and watch the family bloom onto the screen. Trim the sensitivity switches so the curves fill the graticule without clipping. Keep an eye on how hard the device is being driven — back off if a curve is heading for a current or dissipation you don’t want.
7. Read it (next section). Then zero the collector voltage before removing the device.

4.4 Reading transistor curves

For a bipolar transistor the family carries several numbers at once (refer to the figure above):

• DC current gain (beta). The vertical spacing between adjacent curves is the collector-current change for one base-current step. Beta = ΔI_C / ΔI_B. Read ΔI_C from the graticule, divide by the base step you dialed in. Even spacing means constant gain — a healthy, linear device.
• Saturation voltage, V_CE(sat). The knee at the far left, where the curve turns from its steep rise into the flat active region, is where the transistor is fully on. The V_CE at the knee is the saturation voltage — smaller is better for a switch.
• Output resistance / Early effect. In the active region the curves are not perfectly flat; the slight upward slope is finite output resistance. Curves that rise steeply (low output resistance) or fan out unevenly flag a weaker or leaky device.
• Breakdown voltage. Push V_CE toward the range maximum and the top curve eventually bends sharply upward — that is avalanche breakdown, read directly off the horizontal axis.

4.5 Reading diodes and FETs

Diodes need only a two-terminal trace (no step drive). Forward, you see the exponential turn-on and can read the forward drop at a given current; reverse, you can push toward the zener or breakdown voltage and watch the sharp reverse knee — this is how you confirm a zener’s voltage or find a rectifier’s reverse rating. The model’s documentation notes it handles zener and tunnel diodes as well as ordinary rectifiers; a tunnel diode’s negative-resistance region shows as the curve’s characteristic backward fold.

FETs use the voltage step generator. For a JFET you sweep drain-source voltage while stepping gate voltage negative, reading the drain-current family, the pinch-off behavior, and transconductance from the curve spacing. Remember the original’s limitation from Vol 1 and Vol 3: the IT-3121 handles JFETs, not enhancement MOSFETs, because its gate staircase only reaches the JFET’s negative-gate polarity. (Enabling MOSFETs is exactly the gap Erickson’s redesign closes with a ± gate offset.)

4.6 Device matching

One of the most valuable everyday jobs is picking matched pairs — two transistors with nearly identical curves for a differential input stage, or a complementary NPN/PNP pair for a push-pull output. The A/B device select makes this practical: trace device A, then flip to device B, and compare the families on the same graticule. Parts whose curves overlie each other — same spacing (beta), same knee, same slope — are matched. For audio work this is the difference between a balanced output stage and one that quietly runs hot on one side. It is also, per the community, a common real-world use: pulling a suspect transistor from a piece of gear and confirming against a known-good part before committing to a repair.

4.7 Putting it together

The IT-3121’s whole value is that it turns invisible device physics into a picture you can read in seconds: gain from the spacing, saturation from the knee, output resistance from the slope, breakdown from the bend, and a match from an overlay. Wire it to the scope you already own, keep the collector voltage at zero whenever your hands are near the socket, and it earns its bench space every time you need to know not just whether a device works but how well. For the story of how it draws that picture, see Vol 2; for the modern relay-and-microcontroller take on the same machine, see the VBA Curve Tracer and Dave Erickson’s redesign in Vol 3.