TEKTRONIX 2215 Issue

[Kidon]

Hi,

This is my very first post on this forum, so forgive me if this as been asked before, or if this is not the right place to ask these questions.

I picked up 2 Tektronix osciloscopes yesterday for a very fair price.  A 2335 and a 2215 (not 2215A).
Not the main issue is with the 2215.
Everything seems to work fine, the display is fine and bright, no dead spots or anything (on both).
These are the first osciloscopes that I ever owned.  But I’ve used many.
That being said, I notice that the 500mV/1Khz calibration square wave On the 2215 is not showing a neat square wave at all.
The « walls » are not straight.  The waveform looks slightly like a pyramid.  Meaning that the if you take the positive portion, it is narrower than negative at the top.  Not my much, but it’s clearly visible.  I’ve tried everything I know to correct it, but it just won’t let me.
Now I don’t know if this is the generated signal that is « defective », or something else.
I should be receiving my new frequency generator soon, and compare.
Of course I have calibrated the probes.

Any ideas as to what this could be due to?  Is there a inside trim pot to adjust it maybe?
I also was thinking of replacing all the electrolytic caps in the power supply.  I noticed a 2x70Mfd 450V can capacitor.
I measured the voltages across, and I have about 320-ish V.  I have 2 100Mfd 400V caps.  Unless there is a spike when powering up, I assume it’ll be safe to use them, correct?

PS:  Does Someone know where I can find schematics for these 2 units?

Thank you for your help.

[David Hess]

The « walls » are not straight.  The waveform looks slightly like a pyramid.  Meaning that the if you take the positive portion, it is narrower than negative at the top.  Not my much, but it’s clearly visible.  I’ve tried everything I know to correct it, but it just won’t let me.

I have never noticed such a distortion in the calibration output signal however you might be seeing significant geometry distortion in the CRT which indicates a problem.  Your frequency generator should allow a determination of which is occurring.

PS:  Does Someone know where I can find schematics for these 2 units?

The 2213 is the same oscilloscope as the 2215 but without the dual delayed timebase so its manual can be used in most cases.

The 2215 manual at Tekwiki may be pretty good:

http://w140.com/tekwiki/wiki/2215

Beware that the power supply of the 2213 and 2215 underwent a significant upgrade which is described in the later service manuals.  My guess is that you have a later 2215.

[Kidon]

I have never noticed such a distortion in the calibration output signal however you might be seeing significant geometry distortion in the CRT which indicates a problem.  Your frequency generator should allow a determination of which is occurring.

I hope it’s not the CRT...
If it is, is it fixable?  Iris there a way to compensate?

The 2213 is the same oscilloscope as the 2215 but without the dual delayed timebase so its manual can be used in most cases.

The 2215 manual at Tekwiki may be pretty good:

http://w140.com/tekwiki/wiki/2215

Beware that the power supply of the 2213 and 2215 underwent a significant upgrade which is described in the later service manuals.  My guess is that you have a later 2215.

I may have the later 2215...
I’m not knowledgeable enough to differentiate between iterations yet...  I can’t really tell.
All I know is that mine have
I have the manuals. I was more looking for the actual schematics...

[Kidon]

Some pics...

[PA0PBZ]

Why not use the calibration signal from the 2335 to test the 2215?

[David Hess]

I have never noticed such a distortion in the calibration output signal however you might be seeing significant geometry distortion in the CRT which indicates a problem.  Your frequency generator should allow a determination of which is occurring.

I hope it’s not the CRT...
If it is, is it fixable?  Iris there a way to compensate?

First determine where the problem is.

If there is a geometry problem then the shape should change as the position control is adjusted.

The calibration signal is made using a 741 so it should have a discernible slope but not as much as shown.  My 2232 has effectively the same circuit (1) and I measured the slope at 0.06 volts per microsecond which is consistent with the performance of a 741 when the output is divided down 10 times as they did.

Are the 0.5 volt and 1 kHz amplitude and frequency correct?

(1) The only difference is a 5.1 volt zener diode across the output which to me indicates that at some point ESD damaged someone's calibrator output.
 

[Kidon]

Why not use the calibration signal from the 2335 to test the 2215?

Why didn’t I think of that?!!?  Thank you!

I just did, and it is indeed the internal signal generator that seems to be faulty for some reason.

Thanks.

[Kidon]

I have never noticed such a distortion in the calibration output signal however you might be seeing significant geometry distortion in the CRT which indicates a problem.  Your frequency generator should allow a determination of which is occurring.

I hope it’s not the CRT...
If it is, is it fixable?  Iris there a way to compensate?

First determine where the problem is.

If there is a geometry problem then the shape should change as the position control is adjusted.

The calibration signal is made using a 741 so it should have a discernible slope but not as much as shown.  My 2232 has effectively the same circuit (1) and I measured the slope at 0.06 volts per microsecond which is consistent with the performance of a 741 when the output is divided down 10 times as they did.

Are the 0.5 volt and 1 kHz amplitude and frequency correct?

(1) The only difference is a 5.1 volt zener diode across the output which to me indicates that at some point ESD damaged someone's calibrator output.

It is in fact the signal generator that is defective.
I have no clue whatsoever what you’re talking about.  I’m not an expert on these at all.
What’s a 741?  Can I just replace it?  Do you know the part number so I can order it?

Thanks.

PS:  For good measure, I just recapped the entire Power Supply Section, and it works like a charm.

[mawyatt]

The test signal on a Tek 2465 I got off eBay was hosed up, this was traced to a bad zener diode, as a custom marked Motorola device. A little circuit analysis and replace with a 1N4733A zener diode.

Don't know if the 2465 test waveform circuit is similar to the 2213 or 2215, but here's what the defective waveform looked like.

Best,

[shakalnokturn]

The "741" or μA741 is an old op-amp.
http://www.ti.com/lit/pdf/slos094/

Check it's decoupling capacitors before replacing.

[Kidon]

The "741" or μA741 is an old op-amp.
http://www.ti.com/lit/pdf/slos094/

Check it's decoupling capacitors before replacing.

Hummm....  I don’t see that chip anywhere on the board...  And I’ve looked everywhere.

I would really like to fix this issue...

[jdragoset]

U 535, Front Panel Board

[Kidon]

U 535, Front Panel Board

Thanks.
 I thought so, but couldn’t clearly read the prints.
I see 2 zener diodes beneath it.

That board seems like a b...  to remove...
Could you tell me which ones are it’s coupling caps?

Thanks

[Per Hansson]

C531?

[shakalnokturn]

No, C531 is part of the oscillators timing.
I'm assuming there would be decoupling although it does not figure on that extract of schematic.
Often decoupling capacitors are all piled together in one corner of a schematic and referenced to whatever they decouple.
If they exist they'll be physically close to the 741a'd easy to find as they link to its supply pins.

[Kidon]

No, C531 is part of the oscillators timing.
I'm assuming there would be decoupling although it does not figure on that extract of schematic.
Often decoupling capacitors are all piled together in one corner of a schematic and referenced to whatever they decouple.
If they exist they'll be physically close to the 741a'd easy to find as they link to its supply pins.

That would be pin 4 and 7.  I found C726 directly connected to pin 7. 0.1uF 20% 50v
I’m just gonna change this one for now, until I find where the other one is...

[Kidon]

I changed C726 to pin 7 and no change....

[Per Hansson]

What about C725?

[Kidon]

What about C725?

Yeah, it’s an electrolytic cap.
It’s already been replaced as part of the recap job I did yesterday.

But thanks anyway.

[Kidon]

Oddly enough, there is not coupling cap I can find on pin 4 -Vcc....

I’m at a dead end here.
Gonna grab a UA741 tomorrow at the store and change it.
Could it be it?  Would it even work at all if it was bad?
Since I’m gonna change it, do you guys know of a direct replacement « upgrade »?

Thanks

[David Hess]

Gonna grab a UA741 tomorrow at the store and change it.
Could it be it?  Would it even work at all if it was bad?

It could be the 741.

Since I’m gonna change it, do you guys know of a direct replacement « upgrade »?

There are lots of alternatives however none are going to provide better performance in this application and the precision of the output voltage depends somewhat on the output saturation characteristics of the 741.

Other parts I might consider are the TL071, TL081, LF351, and similar JFET parts because they have similar output characteristics and a wide differential input voltage range.  These will provide much faster edges.

Oh, a clever upgrade would be the old LM301A since it uses external compensation and no compensation is required in this application making it a performance upgrade.  I wonder why Tektronix did not use it.

[Kidon]

Gonna grab a UA741 tomorrow at the store and change it.
Could it be it?  Would it even work at all if it was bad?

It could be the 741.

Since I’m gonna change it, do you guys know of a direct replacement « upgrade »?

There are lots of alternatives however none are going to provide better performance in this application and the precision of the output voltage depends somewhat on the output saturation characteristics of the 741.

Other parts I might consider are the TL071, TL081, LF351, and similar JFET parts because they have similar output characteristics and a wide differential input voltage range.  These will provide much faster edges.

Oh, a clever upgrade would be the old LM301A since it uses external compensation and no compensation is required in this application making it a performance upgrade.  I wonder why Tektronix did not use it.

Thank you.  I’m gonna install a socket and try a couple.
It’s gonna be a drag since I’m doing this without removing the board.
I’m not even sure I’m gonna be able to do it.

[David Hess]

Since I’m gonna change it, do you guys know of a direct replacement « upgrade »?

There are lots of alternatives however none are going to provide better performance in this application and the precision of the output voltage depends somewhat on the output saturation characteristics of the 741.

Other parts I might consider are the TL071, TL081, LF351, and similar JFET parts because they have similar output characteristics and a wide differential input voltage range.  These will provide much faster edges.

Oh, a clever upgrade would be the old LM301A since it uses external compensation and no compensation is required in this application making it a performance upgrade.  I wonder why Tektronix did not use it.

Thank you.  I’m gonna install a socket and try a couple.

That is what I would do for my own edification.

It’s gonna be a drag since I’m doing this without removing the board.
I’m not even sure I’m gonna be able to do it.

I have worked on the 2213/2215 before but I never had to deal with the front board so I am not sure what is involved.  It could be nasty.

[Kidon]

I have worked on the 2213/2215 before but I never had to deal with the front board so I am not sure what is involved.  It could be nasty.

Yep, it seems to be.  I’m not gonna remove it.  It would be too much of a hassle.
This my first time ever working on an osciloscope.  I’m learning as I go, and discovering more small issues.
For example I noticed a problem with the horizontal section.  Meaning that the wave forms are getting narrower As you look further and further at the right side of the display.  I didn’t notice that before.  Or maybe I wasn’t paying attention.  Could it be due to the electrolytic cap change on that board, and it now needs to be re-calibrated?
Any ideas that would save me from going through pages and pages of Service Manual to find the answer (If there is even one)?

[tautech]

For example I noticed a problem with the horizontal section.  Meaning that the wave forms are getting narrower As you look further and further at the right side of the display. 

Sounds like sweep non-linearity.
You might need another scope to examine the sweep ramp waveform.

Study the sweep generation section in the manual.