Tektronix 2201 Troubleshooting and Repair Video

[ManCave]

Hi Everyone,

Just wanted to share my troubleshooting and repair video for the Tek 2201 Oscilloscope. If you are interested, You can watch the first part (troubleshooting) here: http://youtu.be/FQM_jubUEX0

I will post link to the second part (repair) once I finish editing and uploading it or you can subscribe to my Youtube Channel and you get notified once it's there!

Any honest feedback WILL be appreciated. This is my first published video. I hope you enjoy it and that it helps someone!

All the best!
Filip @ManCave


EDIT: The second part of the video is here: http://youtu.be/Y-w08JOEvsw

[tggzzz]

Any honest feedback WILL be appreciated. This is my first published video. I hope you enjoy it and that it helps someone!

If we spent 40 minutes of our remaining life looking at this first video, in what way would we benefit?

A short definition of what you did and why it might be interesting to the audience would tempt us to watch.

[oldway]

You asked for "honest feedback" at your own risk...
For sure, it will not be appreciated 

I do not think you have neither the experience nor the knowledge to play the role of a professor teaching how to repair analog oscilloscopes.
40 minutes to diagnose a faulty TL494, that' s crazy. 

And it may take another 40 minutes to change it! 

[ManCave]

If we spent 40 minutes of our remaining life looking at this first video, in what way would we benefit?

A short definition of what you did and why it might be interesting to the audience would tempt us to watch.

Tggzzz, do mean to add a summary to the beginning of the video of what it is going to be about so that you can choose to watch it or skip to a section you are interested in? Or perhaps a few links in the comments to several sections of the video?

Thanks!

You asked for "honest feedback" at your own risk...
For sure, it will not be appreciated 

I do not think you have neither the experience nor the knowledge to play the role of a professor teaching how to repair analog oscilloscopes.
40 minutes to diagnose a faulty TL494, that' s crazy. 

And it may take another 40 minutes to change it! 

Oldway, I do appreciate ALL honest and constructive feedback!

It is my first released video and I am happy to learn from feedback and my own mistakes.

It is my first oscilloscope repair and I do not pretend anything. I am only sharing my knowledge, experience (however limited they might be) in hope to help others to either learn something or to be able to diagnose a problem like this.

I think you are being a bit unfair saying it took 40 minutes to diagnose the faulty TL494. You can see that it did not take anywhere near 40 minutes to diagnose. Most of the time I spent explaining how you would go about fault finding and why I was doing what I was doing.

[tggzzz]

If we spent 40 minutes of our remaining life looking at this first video, in what way would we benefit?

A short definition of what you did and why it might be interesting to the audience would tempt us to watch.

Tggzzz, do mean to add a summary to the beginning of the video of what it is going to be about so that you can choose to watch it or skip to a section you are interested in? Or perhaps a few links in the comments to several sections of the video?

Yes TIt is a cliche that the structure of any presentation should be "tell them what you are going to tell them", "tell them", "tell them what you've told them".

Most people can read faster than the spoken word.
Written words can be found by searching.
Written words can be mechanically translated.

Spoke words fail all those tests.

[oldway]

Oldway, I do appreciate ALL honest and constructive feedback!

It is my first released video and I am happy to learn from feedback and my own mistakes.

It is my first oscilloscope repair and I do not pretend anything. I am only sharing my knowledge, experience (however limited they might be) in hope to help others to either learn something or to be able to diagnose a problem like this.

I think you are being a bit unfair saying it took 40 minutes to diagnose the faulty TL494. You can see that it did not take anywhere near 40 minutes to diagnose. Most of the time I spent explaining how you would go about fault finding and why I was doing what I was doing.

I have repaired hundreds of analog oscilloscopes, I currently have 21 analog scopes at home.

That never pushed me to put videos online because I have the modesty and humility not to take me for Dave, I have neither his talent nor its competence.

You want to share your knowledge and experiences?
What knowledge and what experiences?
You tell yourself, this is your first analog oscilloscope repair, in addition, an 2201 is not a widespread model.
I'm not going to list here the wrong things you've said in this video, nor the important things you have forgotten to say.

[IanJ]

You asked for "honest feedback" at your own risk...
For sure, it will not be appreciated 

I do not think you have neither the experience nor the knowledge to play the role of a professor teaching how to repair analog oscilloscopes.
40 minutes to diagnose a faulty TL494, that' s crazy. 

And it may take another 40 minutes to change it! 

Cruel!

We should be engouraging EE newbies, not driving them away.

Ian.

[sunnyhighway]

The good things

 - Decent first video with a lot of attention to details.
 - Maybe too much details for most of us, but for the young players it seems suitable.
 - The quality of the images are more than adequate.


Opportunities for improvement

The following might seem very harsh and I honestly can't even blame you for feeling offended for me saying this.

You seem to have developed a habit in your speech you might or might not be aware of. There are three words which (to me at least) dominate your video: "soooo", "aaand" and the worst one of all "eeehm". I honestly think it needs some attention because it makes the video fairly hard to watch. You might want work on that in your next one or even experiment with narration afterwards from a script to prevent that problem.

[oldway]

Cruel!

We should be engouraging EE newbies, not driving them away.

Ian.

EE students would never fail a test because it's cruel to hear the professor say that you are denied.
Let's give a degree without examination to all young people to encourage them.
True is cruel sometime.
To learn to be humble and modest is also important.

[Shock]

You asked for "honest feedback" at your own risk...
For sure, it will not be appreciated 

I do not think you have neither the experience nor the knowledge to play the role of a professor teaching how to repair analog oscilloscopes.
40 minutes to diagnose a faulty TL494, that' s crazy. 

And it may take another 40 minutes to change it! 

EE students would never fail a test because it's cruel to hear the professor say that you are denied.
Let's give a degree without examination to all young people to encourage them.
True is cruel sometime.
To learn to be humble and modest is also important.

I have repaired hundreds of analog oscilloscopes, I currently have 21 analog scopes at home.

That never pushed me to put videos online because I have the modesty and humility not to take me for Dave, I have neither his talent nor its competence.

You want to share your knowledge and experiences?
What knowledge and what experiences?
You tell yourself, this is your first analog oscilloscope repair, in addition, an 2201 is not a widespread model.
I'm not going to list here the wrong things you've said in this video, nor the important things you have forgotten to say.

Calm down Mr 21 scopes, it's his first attempt, He didn't ask for life lessons He asked how He could improve his technique, and since you are now playing girly games refusing to even share, you're zero help in this thread.

[Shock]

Any honest feedback WILL be appreciated. This is my first published video. I hope you enjoy it and that it helps someone!

Pretty good for a first attempt, way better than others. I'd put in a disclaimer (and mention it) about measuring mains voltage and also observe good safety practice yourself.

I'm sure you will get better with practice. You should be able to find tips and tricks on how to avoid speech disfluency on the web. If your totally sure of something always say it as a firm statement, if not tell the viewer your going to try a certain method. It comes across as more sincere that way.

You wouldn't be the first to look up a datasheet and pretend you know exactly what your talking about though, I think this used to be called "on the job training".

[David Hess]

I have repaired hundreds of analog oscilloscopes, I currently have 21 analog scopes at home.

I am only up to about 11 oscilloscopes.

My proudest oscilloscope repair was for a Tektronix 2230 which suffered from the problem shown below.  It took more than a month to track it down if only because of its intermittent nature.  Anybody care to guess what the issue was based on the photos?

The noise shown on channel 2 was the only symptom and did not vary with vertical sensitivity.

That never pushed me to put videos online because I have the modesty and humility not to take me for Dave, I have neither his talent nor its competence.

He has to start somewhere.

You tell yourself, this is your first analog oscilloscope repair, in addition, an 2201 is not a widespread model.

It sure is not common.

I have always thought the 2201, 2205, 2210, 2211, and 2225 are part of the same series and the video seems to confirm it.  The 2210, 2211, and 2225 are notable for having 500uV/div vertical sensitivity.

Any honest feedback WILL be appreciated. This is my first published video. I hope you enjoy it and that it helps someone!

If you cannot find a 2201 service manual, then a 2225 service manual should be almost as good.  It has the same dual TL594 switching power supply design.

I liked your attention to safety and off-line ground fault issues however this power supply is galvanically isolated from the line by that big toroidal transformer which provides a nominal +57.6 volts (you measured about 66 volts) to the TL594 based switching preregulator.

There are two TL594 because the first one, U910, is the switching preregulator which steps +57.6 volts down to a floating 39.3 volts and implements current limiting.  The second TL594, U940, just drives the inverter and does not function as a regulator; it just provides non-overlapping drive to the inverter transistors.  Between the two is an NPN low dropout series regulator based around an LM358.

The second TL594's clock is disabled and run from the first TL594 so they operate synchronously.  The schematic says that the DC level at pin 5 should be 2.0 volts.

[IanJ]

Anybody care to guess what the issue was based on the photos?

Sounds like you are gearing up for a wierd one.......like mouse droppings/urine corroding tracks on the pcb or something.

 

Ian.

[JoeO]

David:
A bad memory chip?

[David Hess]

Anybody care to guess what the issue was based on the photos?

Sounds like you are gearing up for a wierd one.......like mouse droppings/urine corroding tracks on the pcb or something.

I fixed it several years ago but the problem the 2230 had was both unique and difficult to prove although I suspected it instantly when I saw the behavior shown in the photos on channel 2.

David:
A bad memory chip?

It sure looks like it, doesn't it?  I did consider this and probed the memory buses for stuck bits at one point because that was easier to check than what I suspected was the real problem.

What the photos show is popcorn noise (also known as burst noise) originating in a bipolar transistor inside of the integrated analog channel switch (a type of analog multiplexor) for the digitizer.  While it would be natural to suspect a digital problem because of the discrete levels, it is a purely analog phenomenon usually associated with contamination or semiconductor defects.

The photo below shows the area in question after the channel switch was replaced.  If you look carefully, you can see that I added a socket, some test points used to attach to ground, and that a couple of resistors now also have socket pins.  I had to resolder the ribbon cables to the connectors because I ended up working them so much that the stranded wires were breaking.  There are other changes outside the area of the photo related to improving the power supply decoupling to the IC.  The IC socket was verified not to affect the oscilloscopes 100 MHz performance although recalibration was necessary.  If I had to do it again, I would use low profile collet pins inserted directly into the board instead of a whole socket.

It was an especially vexing problem because it would subside for hours to days at a time so to actually verify it, I had to not only be making a finicky differential measurement in the 100s of microvolts to millivolt range, but also had to be making it while it was going on.  I did not have any at the time, but a Tektronix 7A22 differential amplifier would have been ideal with a 7A13 being a close second for tracking it down.  A Tektronix 2210, 2211, or 2225 with their 500uV/div vertical sensitivity would have worked as well which is why the current discussion about the related 2201 reminded me of this episode.  The signal I was looking for would be completely buried in the noise of most modern DSOs even with differential probing.

I have seen similarly difficult switching power supply problems though.

[ManCave]

Tggzzz, do mean to add a summary to the beginning of the video of what it is going to be about so that you can choose to watch it or skip to a section you are interested in? Or perhaps a few links in the comments to several sections of the video?

Yes TIt is a cliche that the structure of any presentation should be "tell them what you are going to tell them", "tell them", "tell them what you've told them".

Most people can read faster than the spoken word.
Written words can be found by searching.
Written words can be mechanically translated.

Spoke words fail all those tests.

Got it! Thanks!  I have added a longer description under the video.

Thanks for your feedback!

The good things

 - Decent first video with a lot of attention to details.
 - Maybe too much details for most of us, but for the young players it seems suitable.
 - The quality of the images are more than adequate.

Opportunities for improvement

The following might seem very harsh and I honestly can't even blame you for feeling offended for me saying this.

You seem to have developed a habit in your speech you might or might not be aware of. There are three words which (to me at least) dominate your video: "soooo", "aaand" and the worst one of all "eeehm". I honestly think it needs some attention because it makes the video fairly hard to watch. You might want work on that in your next one or even experiment with narration afterwards from a script to prevent that problem.

Thank you for the kind words Sunnyhighway! ...and especially thanks for being honest about my annoying speaking! I'm definitely not offended as I noticed that while I was editing the video. I will work on it. Although the second video (repair) has already been shot and I probably won't re-shoot it just for that. However, in all the new videos I will pay special attention to speech fluency. It's so bad that it annoyed me while watching myself

Calm down Mr 21 scopes, it's his first attempt, He didn't ask for life lessons He asked how He could improve his technique, and since you are now playing girly games refusing to even share, you're zero help in this thread.

I wouldn't say it better  ...Thanks!

Pretty good for a first attempt, way better than others. I'd put in a disclaimer (and mention it) about measuring mains voltage and also observe good safety practice yourself.

Thank you for Feedback Shock! I wrote a disclaimer at the end of the description. Also added a pointer to the disclaimer at the beginning of the video!

If your totally sure of something always say it as a firm statement, if not tell the viewer your going to try a certain method. It comes across as more sincere that way.

You wouldn't be the first to look up a datasheet and pretend you know exactly what your talking about though, I think this used to be called "on the job training".

Actually, I was quite familiar with the chip, didn't just learn it "on the job". I recently used SG3524 in my own design which is functionally pretty much identical to the TL494/594. I also didn't have the service manual for the scope so it was all done using my (limited ) knowledge and experience. But point taken and I will sure be honest about knowing/not knowing something when making future videos! Thanks again!

If you cannot find a 2201 service manual, then a 2225 service manual should be almost as good.  It has the same dual TL594 switching power supply design.

You are absolutely right. I have since bought the 2201 SM and the PSU schematic is identical to the 2225 (which is available to download for free). If anything, the 2225 SM has more test voltages on the schematic than the 2201 SM.

I liked your attention to safety and off-line ground fault issues however this power supply is galvanically isolated from the line by that big toroidal transformer which provides a nominal +57.6 volts (you measured about 66 volts) to the TL594 based switching preregulator.

There are two TL594 because the first one, U910, is the switching preregulator which steps +57.6 volts down to a floating 39.3 volts and implements current limiting.  The second TL594, U940, just drives the inverter and does not function as a regulator; it just provides non-overlapping drive to the inverter transistors.  Between the two is an NPN low dropout series regulator based around an LM358.

The second TL594's clock is disabled and run from the first TL594 so they operate synchronously.  The schematic says that the DC level at pin 5 should be 2.0 volts.

Nicely explained!

[David Hess]

If you cannot find a 2201 service manual, then a 2225 service manual should be almost as good.  It has the same dual TL594 switching power supply design.

You are absolutely right. I have since bought the 2201 SM and the PSU schematic is identical to the 2225 (which is available to download for free). If anything, the 2225 SM has more test voltages on the schematic than the 2201 SM.

It was not uncommon for Tektronix to include different or more test points and diagnostic data in different service manuals for the same design so it is often worthwhile to check.

[ManCave]

It was not uncommon for Tektronix to include different or more test points and diagnostic data in different service manuals for the same design so it is often worthwhile to check.

2201 SM also has an error (or errors) in the schematic of the PSU. The picture has LM358 regulator section - 2201 on the left and 2225 on the right. Identical circuits. Who can spot the difference (error)?

[tautech]

VR931 Zener reversed. 

[grumpydoc]

Emitter/collector of Q921 transposed.

[ManCave]

VR931 Zener reversed. 

YEP!

Emitter/collector of Q921 transposed.

...and YEP!! 


amazing how these things made their way into the service manual. Makes me wonder how come they were manufactured correctly with incorrect schematics.

...or were they?

[David Hess]

amazing how these things made their way into the service manual. Makes me wonder how come they were manufactured correctly with incorrect schematics.

...or were they?

I have run across similar mistakes in other Tektronix service manuals.  Later schematics of the 2465 series power supply for instance are missing the ground connections to the switching regulator controller.  I think the schematics were redrawn for the service manuals so do not represent the actual engineering schematics.

[ManCave]

Hey guys,

I have finally got to actually editing the repair video (Part 2). The video is here:

http://youtu.be/Y-w08JOEvsw

I have taken on board the feedback (although the video was shot before I received it) and edited out as many of my disfluencies as I reasonably could and I also made the video shorter so that it's easier to watch.

Thanks everyone who bothered to give me some constructive feedback!

Do you guys know where the issue might be with high voltages? I suspected the linear regulator section. The SM says that there should around 1.4V drop across the series pass transistor and I'm getting about 200mV. I swapped the op-amp and it did not make a difference.

My second suspect is the Pre-regulator tl594, although it seems to be working ok, it might be that the error amplifier section is defective...I guess. I will try and swap it for a new one tonight.

Does anyone have any other ideas?

Thanks!
Filip

[sunnyhighway]

I have taken on board the feedback (although the video was shot before I received it) and edited out as many of my disfluencies as I reasonably could and I also made the video shorter so that it's easier to watch.

You did a real good job there. 

[ManCave]

You did a real good job there. 

Thanks!