Author Topic: Heathkit IT-1121 Curve Tracer Repair  (Read 5416 times)

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Offline jcrubin

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Heathkit IT-1121 Curve Tracer Repair
« on: February 15, 2017, 09:17:54 am »
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.....



« Last Edit: October 22, 2018, 11:20:22 am by jcrubin »
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Offline jcrubin

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Re: IT-1121 Curve Tracer Repair
« Reply #1 on: February 15, 2017, 10:15:32 am »
Logical connection that grounds PIN 2




Zoom out OPAMP IC3 PIN 6 showing correct step generation when PIN 2 is shorted to ground.      1V / step     83ms / 10 step cycle



Showing this view on the O10 just for fun




0-9 Step potentiometer 7.5OHM




Zoom in of step output [off of OPAMP IC3 PIN6]



Other possibilities besides off....  all within a short range in middle of adjustment pot



1 STEP




3 STEP




7 STEP




Collected notes

« Last Edit: February 16, 2017, 11:13:54 am by jcrubin »
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Online bson

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #2 on: February 15, 2017, 01:01:01 pm »
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...
 
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Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #3 on: February 15, 2017, 02:38:47 pm »
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.

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Offline technogeeky

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #4 on: February 15, 2017, 04:37:46 pm »
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.

Nope, not yet.
 

Offline Vgkid

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #5 on: February 15, 2017, 07:13:47 pm »
Pics still not working. :-BROKE
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #6 on: February 16, 2017, 12:23:34 am »
apparently i read google photos NO LONGER allows public linking to individual pictures.......  I will have to post them to a different host
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Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #7 on: February 16, 2017, 12:32:38 am »
My last attempt



can this picture be seen?

[photo deleted]

EDIT: problem resolved, account moved to IMGUR
« Last Edit: February 16, 2017, 11:16:30 am by jcrubin »
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Offline Bryan

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #8 on: February 16, 2017, 08:31:29 am »
Nope,  Using https://postimage.io/ seems to work fine on this forum
-=Bryan=-
 

Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #9 on: February 16, 2017, 11:17:06 am »
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 turned





TESTPOINT A - attenuated through the 4 resistors, previous post showed the opamp output of this step circuit at IC3



TESTPOINT 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


« Last Edit: February 16, 2017, 02:06:31 pm by jcrubin »
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Offline cnqhdszq

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #10 on: February 17, 2017, 12:12:32 am »
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 .
 

Online bson

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #11 on: February 17, 2017, 05:40:04 am »
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?
 

Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #12 on: February 17, 2017, 10:22:03 am »
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.
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Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #13 on: February 17, 2017, 02:15:11 pm »
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    .541
Q18        BE   .687    BC    .602

Given 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 manner






Q10 Q9 Q7             BE     ~ .360                  BC     ~8.4
Q8                         BE           0                    BC       16          DEAD SHORT

Well 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 down




Unfortunately now the step is distorted since a transistor is missing, so I need for my package to arrive to continue.






     



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Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #14 on: February 18, 2017, 11:56:55 am »
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 builder



Anyone 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





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Offline cnqhdszq

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #15 on: February 18, 2017, 05:34:14 pm »
congratulations!   that is too difficult to find the mistake  done by the manufacturer!
 

Offline SingedFingers

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #16 on: February 18, 2017, 09:32:43 pm »
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 :) )
 

Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #17 on: February 19, 2017, 12:54:21 am »
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    SW9
Disappointed 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.






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Offline cnqhdszq

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #18 on: February 21, 2017, 01:39:26 am »
Does the equipment been repaired?  if it is OK now !  Please take some photos when testing transistors, I am intensely  curious  with it .  thanks!
 

Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #19 on: February 21, 2017, 05:39:21 am »
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,
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Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #20 on: February 23, 2017, 10:20:49 am »
Here are a few photos of the operation


Transistor 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 lines



Control setup for following test



Test as displayed on the Hantek DSO



Test as displayed on the HeathkitO10   (traces in other direction)

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Offline cnqhdszq

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #21 on: February 23, 2017, 04:08:50 pm »
It looks much better with the DSO than CRT.  The curve on The CRT looks too bad,Thanks!
 

Offline SingedFingers

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #22 on: February 23, 2017, 05:37:15 pm »
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.
 

Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #23 on: February 27, 2017, 06:11:38 am »
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.
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Offline jcrubin

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Re: Heathkit IT-1121 Curve Tracer Repair
« Reply #24 on: April 03, 2017, 10:39:48 pm »
First movie posted


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