Heathkit IT-1121 Curve Tracer Repair - Page 1
Forum section: electronics / testgear

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 14, 2017, 10:17:54 pm
content: Im modifying this original post to include the 3 part series of the troubleshooting I went though to finally find the solution to this 40 year old issue. What an event indeed!!!Original content directly below the three videos...Ive been repairing a Heathkit IT-1121 Curve tracer.Anyone hear familiar with the IT-1121 inner workings? Notably the Step generator, step number control circuit?Step generation works fine, and if I clamp IC6 [decade counter SN7490] PIN 2 (reset) to ground I get all steps 0-9 at 1 volt at 83ms without issue. In normal operation the knob only works in a short range and goes from 0 to 4 to 8 (or 1,3,7 depending on how you want to read this from null).All resistors checked, 5v and 15 v checked. only thing that still bothers me is if I lift connection B away from Q17 I get a good voltage swing when I turn the step knob, when connected there is a considerable voltage drop. Im thinking I shouldn't expect a considerable voltage drop on an input of a transistor. Tested fine though using basic transistor diode test method.I ordered some NOS transistors anyway.Any thoughts.....

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 14, 2017, 11:15:32 pm
content: Logical connection that grounds PIN 2Zoom out OPAMP IC3 PIN 6 showing correct step generation when PIN 2 is shorted to ground. 1V / step 83ms / 10 step cycleShowing this view on the O10 just for fun0-9 Step potentiometer 7.5OHMZoom in of step output [off of OPAMP IC3 PIN6]Other possibilities besides off.... all within a short range in middle of adjustment pot1 STEP3 STEP7 STEPCollected notes

author: bson
author_rank: Unknown
timestamp: February 15, 2017, 02:01:01 am
content: googleusercontent.com redirects to a page that links to accounts.google.com/Login, so your images can't been seen unless someone is logged into google...

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 15, 2017, 03:38:47 am
content: Lovely, I wish the first 73 people let me know that, anyway, thank you, I reposted the links publicly. Please let me know if they are now visible.

author: technogeeky
author_rank: Frequent Contributor
timestamp: February 15, 2017, 05:37:46 am
content: Quote from: jcrubin on February 15, 2017, 03:38:47 amNope, not yet.

author: Vgkid
author_rank: Super Contributor
timestamp: February 15, 2017, 08:13:47 am
content: Pics still not working.

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 15, 2017, 01:23:34 pm
content: apparently i read google photos NO LONGER allows public linking to individual pictures....... I will have to post them to a different host

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 15, 2017, 01:32:38 pm
content: My last attemptcan this picture be seen?[photo deleted]EDIT: problem resolved, account moved to IMGUR

author: Bryan
author_rank: Frequent Contributor
timestamp: February 15, 2017, 09:31:29 pm
content: Nope, Usinghttps://postimage.io/seems to work fine on this forum

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 16, 2017, 12:17:06 am
content: OK! Thanks for the help, I had to go with IMGUR as it worked with the IPHONE. Now all of the pictures are up and running!!!Testpoints shown in the diagram . Testpoint C is DC but sees a voltage drop based on how far the knob is turnedTESTPOINT A - attenuated through the 4 resistors, previous post showed the opamp output of this step circuit at IC3TESTPOINT B - is the Base of Q17 ***Note that this is at 800ns and may be showing an artifact***TESTPOINT A and B together ***Note that this is at 800ns and may be showing an artifact***TESTPOINT A and B together zoomed out

author: cnqhdszq
author_rank: Regular Contributor
timestamp: February 16, 2017, 01:12:32 pm
content: The equipment looks so old ! but I like it . curve tracer is useful ,and I want to buy one . but it is more expensive than a oscilliscope now .hope it be repaired .

author: bson
author_rank: Unknown
timestamp: February 16, 2017, 06:40:04 pm
content: That's a cool instrument... I like the use of an R2R ladder driven off a counter! The pot at TPs A-C I assume is for bias adjustment?

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 16, 2017, 11:22:03 pm
content: No idea, I work on tube equipment, Its my understanding that the current of the base is measured against the current of the collector at Q17 and turns on the Emitter of Q17 like a switch if a threshold is exceeded.

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 17, 2017, 03:15:11 am
content: This evening, id decided that instead of accepting the readings of my fluke in diode mode I was going to test Q17 and Q18 as a matter of voltage drop from B-C and B-E. Well aware that this is all gouverned by a pot i cranked the dial to 9 and had a go.Q17 BE .593 BC .541Q18 BE .687 BC .602Given an expected drop of .7v it could be argued Q17 is a little low, however connected to the POT; expected.This is where the story begins........So I said to myself. Lemme double back and check the voltage drops across Q7,8,9,10 in the same mannerQ10 Q9 Q7 BE ~ .360 BC ~8.4Q8 BE 0 BC 16 DEAD SHORTWell thats something............ So I removed the transistor. Incidentally what I thought was a momentary pulse on the MCD pin on the IC was actually a square wave when it isnt pulled downUnfortunately now the step is distorted since a transistor is missing, so I need for my package to arrive to continue.

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 18, 2017, 12:56:55 am
content: After replacing the transistor I saw the same voltage drop , it wasnt the transistor, it had to be something else..............So I went looking trace by trace, component by component, two components out from each connection of that transistor.Notice the top 4 resistors, as originally placed by the builderAnyone see the problem I found!!!!!!!!!1.5KOHM resistor where 1.2Meg expected!!!! This was in the original build, no wonder the quality of this unit is so good, it never worked....... it was never used!The sweep now works from 0 to 9

author: cnqhdszq
author_rank: Regular Contributor
timestamp: February 18, 2017, 06:34:14 am
content: congratulations! that is too difficult to find the mistake done by the manufacturer!

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 18, 2017, 10:32:43 am
content: I've had a fair bit of Heathkit stuff over the years (and still have a ton of it) and there is literally always something the original builder screwed up. Interestingly the most common thing appears to be swapped resistors. I assume this happens when whoever assembled it was still working at 1AM, the beers had accumulated in the blood stream and the eyes had given out. Take a look around and see if there is a 1.2/1.5M swap on the board - it might save you some further debugging.About 20% of the stuff I've had was clearly never used because it never worked which is good as far as I'm concerned as it's always an easy fix (unless it was soldered together by a chimp)

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 18, 2017, 01:54:21 pm
content: Yeah I considered, that If one resistor was wrong there must be another.......Im going to do a separate effort and check through all of the components and make sure that they site in the correct locations.I was trying to get the baseline test going last night when I notices that the CURRENT/VOLTAGE switch only worked at the VOLTAGE setting SW9Disappointed that yet another thing was broken, I traced that signal all the way through and literally ended up at fuse F1 by Q11,12 in the STEP AMPLIFIER section which by all accounts looks good, but the sloblow spring disconnected due to age.

author: cnqhdszq
author_rank: Regular Contributor
timestamp: February 20, 2017, 02:39:26 pm
content: Does the equipment been repaired? if it is OK now ! Please take some photos when testing transistors, I am intensely curious with it . thanks!

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 20, 2017, 06:39:21 pm
content: Yes indeed, the system is now working and passed calibration. Ive started putting together the movies for editing to post to youtube but currently ive just started posting my movies for the RF signal generator, so it will be another 6 weeks before this appears on Youtube.... What I can do in the mean time is take a few pictures and post them here,

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 22, 2017, 11:20:49 pm
content: Here are a few photos of the operationTransistor set up for testing linearity and or displaying current over voltage [using auto persistence, cleaner dots]Same test but with .2sec persistence, less grainy, but fatter linesControl setup for following testTest as displayed on the Hantek DSOTest as displayed on the HeathkitO10 (traces in other direction)

author: cnqhdszq
author_rank: Regular Contributor
timestamp: February 23, 2017, 05:08:50 am
content: It looks much better with the DSO than CRT. The curve on The CRT looks too bad,Thanks!

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 23, 2017, 06:37:15 am
content: That's not a calibrated scope. Would look better of a calibrated one.I'm waiting for one of the later Heathkit scopes to come up on eBay. Those are pretty nice bits of kit.

author: jcrubin
author_rank: Frequent Contributor
timestamp: February 26, 2017, 07:11:38 pm
content: The old Heathkit scope was just for demonstration.... It is an antique and is AC coupled not DC coupled so you can see the lines of each trace loop back. On the DSO the lines are cleaner because it is DC coupled but doesn't have the persistence of a CRT so it is grainy. Ideally, it was designed for a non digital scope that is DC coupled. I have a tektronix, but it is not repaired right now.

author: jcrubin
author_rank: Frequent Contributor
timestamp: April 03, 2017, 12:39:48 pm
content: First movie posted

author: jcrubin
author_rank: Frequent Contributor
timestamp: April 10, 2017, 12:58:43 pm
content: Second Movie Posted

author: jcrubin
author_rank: Frequent Contributor
timestamp: April 17, 2017, 12:12:38 pm
content: Final movie postedTransistor test from moped ignition

author: tnjent98
author_rank: Newbie
timestamp: August 03, 2018, 08:12:03 pm
content: this is a great thread and shows many experts on this blog....so, my question is , since I do tube amp design, repair and upgrades, can this tracer by modified to test common audio tubes, like 12ax7, 6sj7, 6v6, 6l6 6bq5....looking forward to hearing the commentsthankstom

author: jcrubin
author_rank: Frequent Contributor
timestamp: August 04, 2018, 02:01:08 am
content: Quote from: tnjent98 on August 03, 2018, 08:12:03 pmModified the way a car CAN be modified into a boat, or a house. It would be a crazy modification that would outweigh the effort of building a uTracer.

author: xxx2fan
author_rank: Contributor
timestamp: October 21, 2018, 10:58:21 pm
content: Reading your post on curve Tracer RepairJust wondering if you found R61 to be the 1.2M resistor that was where the 1.5K was placed by the builder by mistake.Interesting thought.Thanks if you happen to see thisknow its an older post.That 1.2M in the kit had to end up somewhere LOL

author: jcrubin
author_rank: Frequent Contributor
timestamp: October 22, 2018, 12:14:29 am
content: A great question!After rejoicing in finding the mistake that someone decades ago spend countless hours banging their head against the wall and seeing it function, I did an inventory first for that resistor, and then every other. Turned out to be that one mistake and the subsequent burned out opamp, though, id imagine at the time the opamp was in working order.thanks for asking

author: WalterG
author_rank: Newbie
timestamp: October 22, 2020, 12:16:24 pm
content: I also have a IT-1121 in use. And made some modifications. I added the possibility to test enhance mode FET's. Originally this was not possible, only depletion mode.If interested go to my webpage. It's in Dutch but on top is a Google tanslate button, and you can enable it by hand.https://people.zeelandnet.nl/wgeeraert/drblan.htm#IT-1121

author: jcrubin
author_rank: Frequent Contributor
timestamp: November 07, 2020, 04:37:27 am
content: Thats cool.... I wonder, given 12VAC and enough DC supply of a 12AX7 could be traced on the unit with those mods.