Tektronix 576 Curve Tracer - Restoration/Repair Questions

[fmashockie]

Hey y'all! Just acquired a Tektronix 576 Curve Tracer off eBay (listed for parts/not-working).  The original thread is here https://www.eevblog.com/forum/testgear/tektronix-576-cruve-tracer/ but I wanted to create this thread for any repair questions I might have.  I've never used or worked on one of these before so I'm just learning things as I go.  I apologize if I am making too many threads for the same topic (if I am I'll just consolidate to one thread).

My first question is related to the 'Max Peak Volts' knob.  So on this unit, the plastic ring that has indicators for setting 'Max Peak Power/Watts' (from 0.1, 0.5, 2.2, 10, 50, and 220) has seperated from the upper push/pull knob.  So I'm not sure the correct orientation for the indicator ring.  My question is (for those that own one of these) when the Max Peak Power knob is turned all the way clockwise, what setting is that in Watts and Series resistance?  Is it 220W and .3ohms like in my photos attached? Or is it different?  Your help would be greatly appreciated! Thanks!!!

[fmashockie]

Okay I think I already answered my own question - 220W is when it is set all the way counterclockwise.  But if someone could confirm that for me I would greatly appreciate it!!!

[jonpaul]

Great machine, had one since 1992

Suggest W140/Bama manual archive for 576.

Get OP and SERV manual.

Also for plugins you have

Jon

[fmashockie]

Thanks John,

Yea I was able to find the same service manual that was available on the Bama website.  Very helpful!

So far I've found a bad resistor on the terminal selector switch (R376) and I replaced that.  Also disabled the HV interlock with a mod to the switch since mine did not come with the plastic protection cover and the switch was broken.

I thought I was in the clear because I measured a resistor and diode and got the correct IV curves.  But no steps for a transistor.  I think there's a problem with the step generator and/or step amplifier.

Also, all the power supply voltages (Low-mid voltage) are low and have high ripple (well outside spec listed in the manual).  So it looks like I'm going to have to replace those big caps that are buried underneath the CRT.  The collector supply voltages are bang on.  Can produce smooth DC, full rectifed, and sine-wave voltages for 15V, 75V, and 350V.  Don't have the capabilities to test the 1500V here at home lol.

Should be a lot of fun!

[fmashockie]

Here's a video I put up on my YouTube channel documenting the repair so far (see below).  Not included was tonight's work which I have some photos of below as well.  Removed the Low Voltage Regulator and Low Voltage Rectifier boards.  I literally labeled every wire that connects to these boards with a piece of tape/notation so I didn't mess anything up 

I was able to get to those large caps that filter the low voltage rails.  All of them tested fine in-circuit except C707.  It is a 200uF cap but it measured ~500nF and 20kohm ESR.  This cap is across the -75V rail which has the tightest accuracy and ripple specs according to the manual so it must be important  Also explains why adjusting the trimmer cap for it didn't do anything to help.

Was able to get to the caps without removing the CRT by removing the HV power supply board.

I'll keep documenting this restoration on here and YT as I go! 

[Shock]

A while back someone was doing TO-3 package adapter pcbs so you could retrofit modern replacements if you decide to service the power supply fully. The thermal grease and mica insulators otherwise at some point.

[fmashockie]

Thanks! The test fixture adapters are the last thing I'm worried about right now.  This isn't near operational yet.  Everything regarding the low voltage supply appears good except this C707 cap.  Trying to see where I can find a suitable alternative.  Not really looking for new old stock.  Just maybe something that at least looks similar?  Like a an electrolytic with solder lugs (>= 250VDC, 200uF).  But having trouble finding anything like that.  Any help would be apprecciated!

[jonpaul]

See the Groups.io Tekscopes and Tekscopes2 for Tek specialists on 576.

https://groups.io/g/TekScopes2/topics
https://groups.io/g/TekScopes/topics

Be sure the service manula is for your 576 particular serial number.

Bon Chance,

Jon

[Shock]

Thanks! The test fixture adapters are the last thing I'm worried about right now.  This isn't near operational yet.  Everything regarding the low voltage supply appears good except this C707 cap.  Trying to see where I can find a suitable alternative.  Not really looking for new old stock.  Just maybe something that at least looks similar?  Like a an electrolytic with solder lugs (>= 250VDC, 200uF).  But having trouble finding anything like that.  Any help would be appreciated!

I was meaning adapters for the transistors on the rear, I read several people having similar faults with them. But it may have been date related. From a quick search it appears the Tek 576 were sold until the last stock from 1990 was used up. So it appears you have one of the later ones.

[Shock]

To keep aesthetics on old caps if there is no alternative, radio/audio restoration guys gut the old can package and mount a modern cap inside. You should be able to find videos on this on youtube.

[fmashockie]

I was meaning adapters for the transistors on the rear, I read several people having similar faults with them. But it may have been date related. From a quick search it appears the Tek 576 were sold until the last stock from 1990 was used up. So it appears you have one of the later ones.

Oh okay sorry for the confusion! That makes sense about them being made until 1990.  Because the HV interlock on mine seemed a bit newer looking than all the other components.  It said copyright 1985 on the little PCB which I thought was odd since everything else says copyright 1968.

Yea I guess I can give guting the original cap a try and sticking the alternative in there.  I don't really care too much about the aesthetic.  I just want to make sure whatever I get fits nicely in that insert somewhat.  It would be nice if the replacement cap had solder lugs, too.  But I can't really find any for this particular value/voltage rating that do.

[Hamelec]

for old style caps you should have a look to:
https://www.die-wuestens.de/dindex.htm?/k1.htm
and there "KON5100".
but i dont know if they will ship to the US..

[fmashockie]

for old style caps you should have a look to:
https://www.die-wuestens.de/dindex.htm?/k1.htm
and there "KON5100".
but i dont know if they will ship to the US..

Wow this site is awesome!  Thanks a lot for sharing this.  So I ended up gutting the original cap and sticking a 250VDC, 220uF snap-in inside it  I always thought this was kind of silly to do to keep the vintage look, but I wanted to be able to utilize the solder lugs, and the inserts/mounts so I could still mount it on the chassis.  It worked out great!  I also ordered some capacitor adapters from Oshpark (https://oshpark.com/shared_projects/hpUFRhHC) - I found these on one of the Tekscope forums.  I just didn't have the patience for them to arrive (a virtue I don't have  )

Here's part 2 of the restortation which includes gutting the cap and fixing the power supply! All rails are within spec for accuracy and ripple.  The trace on the CRT also looks like it has improved.  Now on to the step generator/amplifier and a few of the gang switches.  Really appreciate everyone's help so far!

[fmashockie]

Part 3 of my video series on attempting to repair this Tektronix 576 curve tracer.  Unfortunately, still have not been able to repair the step generator amplifier. But I did verify that the step generator itself is working!  I cannot seem to find a bad transistor, diode, or cap within the entire circuit.  There is however, a 8.2V zener diode (D147) with leads that seem a bit darkened/charred almost.  It still seems to measure fine in-circuit.

All of the waveforms embedded within the schematic diagrams (pages 2 and 3 for Step Generator and Step Amplifier) I am able to repeat with my own measurements.  Except of course for the Steps Out to connector J360.  Which indicates the problem is not at the standard test fixture.

I am also wondering if some of the ICs that make up the step generator polarity logic (such as U72 and U33) are suspect.  I will check the truth table in the service manual for those next.

But I'm starting to run out of ideas so if anyone has any experience repairing these and has seen a fault like ethis I would greatly appreciate their input! I've joined the Tekscopes group on groups.io and asked for help there as well.  Thanks everyone!

[jonpaul]

Rebonjour:

1/ Clean and lube, exercised the step gen switches, controls?

Be sure to check # steps and step V or I switches.

2/ Check harmonica connectors for corrosion, loosness

3/ Get schematic and theory of op for step gen of your 576 SN

4/ Follow the theory and troubleshoot logically start as the power and grounds, the follow signal flow.

Bon Chance


Jon

[fmashockie]

Thanks Jon! I really appreciate your help so far by directing me to the Tekscopes group and a few other things.  I have done most of what you suggest:

1) I have cleaned/exercised all the step gen switches with Deoxit.  But still haven't repeated the performance check after doing that.  So I will give that a try once I put that together.  I know from experience that these types of gang switches can cause all sorts of issues when they get dirty.  Fortunately they are very easy to penetrate with the cleaner since they have holes in the back and top.

2) I have done a visual inspection of the harmonica connector J360 (I believe that is what you are referring to).  The gold pins look nice and clean.  I'm fairly certain that the issue isn't due to a faulty connection here.  Because I loose the step generator signal right after the Current Sensing Resistors (R196-R216).  This comes before the J360-1 connection.  Also, the current and voltage sensing resistors on the amplitude switch board all test fine.  But I admit that connector still deserves a closer look.

3) This will be the most difficult part for me.  The step generator/step amplifier circuit is complex.  Especially for someone like me who is more of a novice.  But you're absolutely right.  If I could get a better grasp of how the circuit works, I'd be much more likely to find the fault.  I guess that is what I am asking for help on more so than anything else.

4) I'll continue trying to follow the flow of the signal.  As I mentioned, I've been able to match all the waveforms embedded within the step generator/step amplifier schematics.  Except the step out waveform right after the current sensing resistors (no signal there). 

Haven't given up yet!  This is what I bought this thing for (at least I tell myself that)! 

Thanks again!

[jonpaul]

fmashockie (your name?)

Reading all your posts and suffering your  long video, I conclude:

1/ Step gen works fine , 576 is very likely working.

2/ NO DUT! Attach a transistor to the test fixture (banana plugs and alligator to any old power BJT) to see the curves.


3/ The manual performance tests, and CAL are NOT what you need! You may do more damage than improvement.


4/please find,  read and understand:

a/ 576 user manual, 176 and other test fixtures manual

b/ Curve tracer theory and app notes.

See TEKwkiw, W140, BAMA Archive
lots of great info here https://w140.com/tekwiki/index.php/576
fmashockie (your name?)



c/ Buy a bound printed copy of op/serv orig for your SN or Qservice or Artech (?) copy
The PDF , are not good for troubleshooting  in general.

4/ DO NOT mess with the harmonica connectors unless you understand the decades old plastic can easily break.

5/ Many old TEK and 576 bugs are due to transistors or DIP ICs in sockets that have oxidized contacts.

6/ The Tekscopes and Tekscopes2 (different) at groups io may be a better resource.

Bon chance,

Jon

[Shock]

Catching up now, thanks for taking the time to video it.

[fmashockie]

Catching up now, thanks for taking the time to video it.

Well Shock you are just in time for the conclusion!  I think this is a wrap on the Tektronix 576 - it's fixed!  As much as I hate to admit, it looks like a loose connector J360 which mates with the Standard Test Fixture may have been the cause all along for the nonfunctioning Step Generator Amplifier.  There's a slight chance that Q226 could also have been bad, but doubtful.  So in part 4 of my video series, I go into detail in redoing the performance checks and calibration procedure.  Sorry it is another long video (I gotta do better to shorten these videos  ).  But hopefully someone finds them helpful.  I try to add detailed timestamps so viewers can scroll thru to what they really need/want to see. 

Thank you to everyone who helped me out! Especially Jon Paul who told me to look at the harmonica connectors! I might do another video on some component testing, but I need to get a better grasp on how to use it first!

[abell118]

Any idea how I can find replacement knobs for my 576?

Thanks
abell118

[BlownUpCapacitor]

Ebay, tektronix knob lots, or 3D printing.

3D printing seems to be the most popular method of getting knob replacements. You mustn't toss away the aluminum inner base of the knobs though. You can't 3D print that. You are merely 3D printing the outer plastic shell.

It's a purely plastic knob though, just print the plastic. easy.

[Wallace Gasiewicz]

Sphere has lots of Tek Knobs, don't know if yours is there, have a look:   

https://www.sphere.bc.ca/oldsite/test/tek-parts/tek-knobs.html

[Shock]

Put a wanted post in the buy/sell/wanted section.
Be specific on the part numbers, which knobs you want and show an image to make it easier for the reader.
You can also go on other forums/groups and ask the same.