Heathkit it-1121 / 3121 clone curve tracer suggestions ? - Page 1
Forum section: electronics / projects

author: MathWizard
author_rank: Super Contributor
timestamp: March 10, 2023, 07:43:17 pm
content: I've built versions of curve tracers before, on a breadboard. And now I want to make the real thing, an analog version, with some salvaged transformers. (I gutted a stereo, and left the multi-tap trans, in the chassis, so I can just swap in/out different devices, like a PSU or curve tracer) . It has suitable rails up over 100V, but I don't need that much anyways.So I looked up some Heathkit models and found the IT-1121/3121 schematics (maybe about the same)https://www.pestingers.net/pages-images/heathkit/test-equipment/it3121/it3121.htmhttps://www.rsp-italy.it/Electronics/Kits/_contents/Heathkit/Kits/Heathkit%20IT-3121%20semicond%20curve%20tracer%20schem.pdfhttp://tubularelectronics.com/Heath_Manual_Collection/Heath_Manuals_IT-NE/IT-1121/IT1121_man.pdfIt uses a binary ripple counter, triggered from mains, and uses a R2R type ladder to creates the base current or voltage steps. And some other op-amps make a variable PWM from that...And I made a full working model in LTspice. I would replace the zener based pass regulators, with TL431's, and TIP31/32It uses the following old BJT's and chips on page 103 in the 1121-manual. There's probably nothing too critical, in a perfect world.MPSA20 - Is/B/Va similar to high beta bc547/8MPSA42 - 2k beta, I might need something like themN5741V - I think an old 741 version with offset. I have u741, and they are probably the only adjustable opamps I have.LM301 - I think a low slew rate op-ampSN7490 - BJT version of decade/binary ripple counter, so in LTSpice I used a CD4024 binary ripple counter, I own some too.I have jelly bean op-amps like LT1013, LM358, u741, TL082, and not much else. So I'd just make it and try them all. I'm not used to dealing with offset voltages and leakage currents, or the finer details of op-amps.They use a lot of Ferrite beads or larger, IDK what kind of those I'd need.I'm sure I'd have to change some values to get it calibrated. But they just use a lot of 5% and a few 1% resistors, some high power 1-10W's also, so I'd have to order them. And a few large variable resistors. And a few switches, and even if I don't get multi-stack sw's, that's ok.Anyone make one similar to these before or have any tips ?

author: MathWizard
author_rank: Super Contributor
timestamp: March 14, 2023, 11:58:38 am
content: Whats a good way to calculate saturation currents? For a reset control BJT, grounded common emitter, with a 2.7k Rc, I would use the basic 0.2V VCE saturation model, calculate IC then IB~3.4uA , or a bit more if I used 50mVCE for the sim.But LTSpice uses more than Nf, IS, Bf, VA that I use so far. And they have IB ~48uA in saturation w/~50mVCE. And it matters a lot for my calculations since there's Mega ohm resistors 1 BJT away. I could use the sim's number's, but I want to calculate something closer by hand.https://en.wikipedia.org/wiki/Saturation_currentI'm getting used to hearing about that stuff from lectures, but not in practice. But so do I use Ise, and some of those other param's, and the eber's-moll, reverse currents, that I usually ignore?I've remade the PSU section to match my transformer, with TL431 and LM723. The whole circuit works great in the sim. But of course now the 1V steps would be 0.68V steps. So I'm calculating a pile of stuff for the reference currents and a few op-amps.Ok so those base currents won't even matter, and just plan for lot's of saturation, and use some rule of thumb like 20, 50, 100x your calculated base current, or upto your Beta even. Same for the current ref BJT's., as long as the caps have enough charge still, they are fine too.

author: MathWizard
author_rank: Super Contributor
timestamp: March 17, 2023, 11:10:02 am
content: Well I got the modified heart of it running on a breadboard with 100uA steps, so long as the lower rail is stable and set to to right voltage, each Rref will have 10.9V across it. I would like to use JFET's now tho, if I was to keep the capacitivly driven gates/bases. And then the base drive would be a lot more constant.In LTspice Vref out from the op-amp, total drop after 16steps of 0.5V, is only 50mV, so 7.45V instead of 7.5V max. My breadboard version is pretty close.The bottom 4 pics, are a better version, each step is a lot more stable. But with more overshoot on the raising steps. I used Pch JFETs, J176, and TL082. Now I could probably drive the transistors just about as good this way with DC coupling, but anyways.Now when I solder it up, I'll just and 4 little 10-turn trimmers, and I'm going to use dual LM723's for the voltage rails. In the sim it seems the best out of anything I have, although I'll try just zener's too.It overshoots almost to the next step tho, green the 0.5V step

author: MathWizard
author_rank: Super Contributor
timestamp: March 20, 2023, 04:27:45 am
content: Well it will work, now it's just to test it with the actual transformer circuits, solder it up and get some big switches/etc, and make a little case.Here's the step-gen output in Blue, and Green is the collector steps, for a BC547B, connected to 10.5V though a 2k resistor. It's stepping down about 0.5V at a time, so 250uA Ic. The base steps should be about 1uA, on paper as well as the sim, and ignoring the Early effect, that would give an effective beta of about 250.

author: DenCraw
author_rank: Contributor
timestamp: January 30, 2025, 05:49:52 pm
content: how was the outcome of this project? succeded? thank you for posting it.

author: djerickson
author_rank: Regular Contributor
timestamp: October 17, 2025 12:39:52 pm
content: I too like the Heathkit Curve tracers. I started a different project, a functional copy of the Heathkit IT-1121/3121. Instead of lots of rotary and manual switches, I use relays and a simple Arduino with OLED display, encoders and buttons for control. Project web page is athttps://djerickson.com/curve-tracer/The PCB and parts are almost ready to order. The base step generator hardware and firmware are working on a breadboard. I plan to complete it this winter. I will probably do some Youtubes on the project. If you are interested, let me know.Thanks, Dave Erickson

author: MathWizard
author_rank: Super Contributor
timestamp: October 17, 2025 07:03:48 pm
content: Well I never finished mine yet, so I'll have a look at your's too, your website looks nice. I've learned a lot more about op-amps since I started on my Heathkit clone, so now I need to fix/adjust some things on what I made so far.I did buy a bunch of the hardware tho, like the multi-deck rotary switches. This winter I should get right back at it. And now I have some better metal working tools. I plan to use an old TV cable or modem box, or DVD player, for the case. I don't really have a proper power supply transformer yet either, I think I was going to use one from a stereo, but they are all so over powered, then it's the linear vs switching PSU issues.Adding some relays would be nice too, and using some MCU and digital display is within my talents now too. I want to try some more bare-metal coding.

author: djerickson
author_rank: Regular Contributor
timestamp: «Reply #7 on:Todayat 01:19:48 pm »
content: Thanks. You found multi-deck rotary switches? Can I ask where?Thanks, Dave

author: Martinn
author_rank: Frequent Contributor
timestamp: «Reply #8 on:Todayat 06:35:45 pm »
content: Just had a look at Ali:2x11https://de.aliexpress.com/item/1005006157667208.html2x12https://de.aliexpress.com/item/1005001670858655.html2x24!https://de.aliexpress.com/item/1005005968832658.html4x11https://de.aliexpress.com/item/1005007728089968.html8x11!https://de.aliexpress.com/item/1005005887873370.htmland with resistorshttps://de.aliexpress.com/item/32806520025.htmlUnfortunately I could not find any for PCB mount, all for free wiring.Thanks for sharing!Martin