Date
21 - 40 of 78
2235 Horizontal Calibration Issue
Stephen
On Mon, Jan 18, 2021 at 12:09 AM, keantoken wrote:
Will do, and report back. Thanks 😊
Check the voltages on the horizontal amp schematic on page 193. If those are
okay, then move to whatever is before the horizontal amp and check those
voltages. If you have another scope you can check the waveforms at the input
and output of the sweep amplifier to see if the distortion occurs in the
amplifier or before it.
Stephen
On Mon, Jan 18, 2021 at 03:31 AM, teamlarryohio wrote:
The only other CRT that would potentially fit is the one in my 2215. I haven’t checked whether they’re the same parts, but it’s the only I have that could fit in there.
I’ve never changed or swapped a CRT before. And as far as I know, I would also have to recalibrate many things after that... So I hope it’s not that...
I’ve thought about that actually.
Don't rule out the possibility of a slightly deformed plate. The test is of
course to swap out the jug and see if it changes.
-ls-
The only other CRT that would potentially fit is the one in my 2215. I haven’t checked whether they’re the same parts, but it’s the only I have that could fit in there.
I’ve never changed or swapped a CRT before. And as far as I know, I would also have to recalibrate many things after that... So I hope it’s not that...
Just wanted to thank all who are replying to this thread. I don't want you to think you're speaking into a void.
Guys like me are soaking this stuff up. I'm loving the references provided. Looking at the schematic my eyes would have glazed over a bit. But hearing such simple observations as "working around the PNP performance" is one of those insights that goes a long way toward understanding why. I'm loving reading through the other references too. The Gray & Meyer paper is going to be fun to read. I'm curious to see how much I can follow still.
Dave
Guys like me are soaking this stuff up. I'm loving the references provided. Looking at the schematic my eyes would have glazed over a bit. But hearing such simple observations as "working around the PNP performance" is one of those insights that goes a long way toward understanding why. I'm loving reading through the other references too. The Gray & Meyer paper is going to be fun to read. I'm curious to see how much I can follow still.
Dave
I'm seeing various efforts and recommendations to get rid of the non-linearity.
I'm not seeing a clear awareness that the non-linearity as visible on the images is (just) within the specifications of the 'scope, which it is.
Of course, the amount of nonlinearity as seen may be caused by component deterioration but it is quite possible that the 'scope left the factory having exactly the same linearity performance. In any case, trying to find a fault is at least interesting and educating, though the reward, in the form of correcting the fault may prove elusive, because no fault exists. That does not preclude the possibility that some tinkering might improve things.
In my experience, the double-glass-seal caps in Tek's timing circuits are very time- and temperature- stable. What's more, no simple drift in capacitance or leakage would explain the kind of nonlinearity observed (at least one inflection point, if I'm using the right English word here).
It would be quite simple to connect various DC voltages directly to the horizontal plates and via the horizontal input to pinpoint a/the main contributing factor(s).
Raymond
I'm not seeing a clear awareness that the non-linearity as visible on the images is (just) within the specifications of the 'scope, which it is.
Of course, the amount of nonlinearity as seen may be caused by component deterioration but it is quite possible that the 'scope left the factory having exactly the same linearity performance. In any case, trying to find a fault is at least interesting and educating, though the reward, in the form of correcting the fault may prove elusive, because no fault exists. That does not preclude the possibility that some tinkering might improve things.
In my experience, the double-glass-seal caps in Tek's timing circuits are very time- and temperature- stable. What's more, no simple drift in capacitance or leakage would explain the kind of nonlinearity observed (at least one inflection point, if I'm using the right English word here).
It would be quite simple to connect various DC voltages directly to the horizontal plates and via the horizontal input to pinpoint a/the main contributing factor(s).
Raymond
On Mon, Jan 18, 2021 at 06:14 PM, Jean-Paul wrote:
Raymond
AFAIR, the OP has already tried optimization by adjustment.
The CRT geometry, focus and astigmatism can interact.
Hor linearity is normally a CRT effect, as the electronics designed for
Tektronix are quite good.
I would find the geometry adjustment and tweak that first
Raymond
Dave Wise
?Tek has optimized the horizontal amp for sweep since 1954. Type 535 has cathode followers to handle the ramp, and a "gated current booster" to pull down the left-hand deflection plate on retrace.?
Dave Wise
________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Tom Lee via groups.io <tomlee@...>
Sent: Sunday, January 17, 2021 8:24 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 2235 Horizontal Calibration Issue
A horizontal amplifier has very different requirements from those for a
vertical amplifier.
<snip>
The answer
is that the output stage runs largely Class A at low speeds and
transitions to Class AB only for fast edges, where you need strong push
and strong pull to provide the necessary slew rates without burning huge
static power when it's not needed. It has nothing to do with worries
about CM oscillation. There is negligible CM feedback here, so CM
oscillation is a non-issue.
-- Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu<http://www-smirc.stanford.edu>
Dave Wise
________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Tom Lee via groups.io <tomlee@...>
Sent: Sunday, January 17, 2021 8:24 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 2235 Horizontal Calibration Issue
A horizontal amplifier has very different requirements from those for a
vertical amplifier.
<snip>
The answer
is that the output stage runs largely Class A at low speeds and
transitions to Class AB only for fast edges, where you need strong push
and strong pull to provide the necessary slew rates without burning huge
static power when it's not needed. It has nothing to do with worries
about CM oscillation. There is negligible CM feedback here, so CM
oscillation is a non-issue.
-- Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu<http://www-smirc.stanford.edu>
Bert Haskins
On 1/17/2021 8:21 PM, Ozan wrote:
I have been following this thread while feeling quite certain that if any of my 22xx scopes had a horizontal calibration or linearity problem I would certainly have noticed it.
So I tested my favorite 2232 and FG over a wide span of ranges and the results were as perfect as my vision could discern.
Seems quite amazing to me that after something like thirty years the scope could easily hit the original specifications.
I don't have a working 2235 to test but I think that the curcuity is quite similar.
-- Bert
It is unlikely A and B sweep interaction is causing the sweep nonlinearity but if B looks more linear then you can focus on the sweep generator. A and B sweeps have different paths until they hit U760. Again, this could just be just be within spec and nothing to fix. I don't own this particular scope so I don't know what is normal.You made me look!
If both A and B sweeps behave the same I would focus on the horizontal amplifier (sheet 6 in the SM), assuming it is not normal and it bothers you enough to debug it. In your pictures sweep speed is slightly faster (horizontal gain is slightly higher) at mid point (zero volts differential) compared to end points. This is common for most differential amplifiers, some kind of local or global feedback is used to improve linearity. I would check DC operating points marked on the schematic, especially after U760 to see if any voltages drifted. For example if voltage at bases of Q780/Q770 are off and/or voltage at top of VR764 zener is off diodes CR770/CR780 may be slightly conducting and compressing the signal. Note that 100V is present at this stage so safety is important.
Q770/Q780 diff amp is an interesting one. It is made of NPN and PNP with diodes at the emitters. Does anyone know the source of this circuit (patent, paper, etc)?
Ozan
I have been following this thread while feeling quite certain that if any of my 22xx scopes had a horizontal calibration or linearity problem I would certainly have noticed it.
So I tested my favorite 2232 and FG over a wide span of ranges and the results were as perfect as my vision could discern.
Seems quite amazing to me that after something like thirty years the scope could easily hit the original specifications.
I don't have a working 2235 to test but I think that the curcuity is quite similar.
-- Bert
Bert Haskins
On 1/18/2021 9:31 AM, teamlarryohio wrote:
A much easier test is ti set the scope up for XY operation and feed the same signal into both X and Y.
Then adjust to get a diagonal line across the screen.
This line should be perfectly straight even while positioning at different parts of the screen .
I cannot stress this strong enough.
This test if one of the most useful tools for diagnosing problems in oscilloscopes.
I have a 2221A where this test is perfect except for the extreme upper left portion of the screen where it is distorted.
The fault is in the CRT.
-- Bert
Don't rule out the possibility of a slightly deformed plate. The test is of course to swap out the jug and see if it changes.The following assumes that the scope can be set up for XY mode.
-ls-
A much easier test is ti set the scope up for XY operation and feed the same signal into both X and Y.
Then adjust to get a diagonal line across the screen.
This line should be perfectly straight even while positioning at different parts of the screen .
I cannot stress this strong enough.
This test if one of the most useful tools for diagnosing problems in oscilloscopes.
I have a 2221A where this test is perfect except for the extreme upper left portion of the screen where it is distorted.
The fault is in the CRT.
-- Bert
Ozan
Hi Stephen,
Before I derailed the thread (Tom opened a new one for those of us who can talk about two transistors for hours) I gave some suggestions to debug in #176717
https://groups.io/g/TekScopes/message/176717
Take a look at B sweep to rule out nonlinearity in A sweep generator.
When I suggested to check the voltages I meant to check the voltages shown in the schematic sheet <6> (Probe Adjust, XY Amplifier, Horizontal Output). In particular voltages at the base of Q780 and Q770, and voltage at the cathode of VR764. You need to put the scope in a setup shown in "Control Settings" section prior to the schematic. Voltages could be slightly off but pay special attention to the difference, i.e. if everyone is up/down by 100mV could be OK. Report back here if any voltages are off.
If voltages are correct and you still want to find the root cause, this is what I would do (I haven't tried it, take this as food for thought):
You could put he scope in X/Y mode, apply GND to both inputs and center the dot. Keep dot intensity to minimum and turn off while not looking at the screen to prevent screen burn.
Now set X input to DC and apply a DC voltage to move the dot to the 1st graticule. Write down input voltage, differential voltage at test points 34/35 and 32/33 (be careful on this sheet there is 100V). Now increase DC to move to next graticule point, write down all three voltages. Continue until the last one.
Now you have a table of input vs output for few critical points. You can figure out where the nonlinearity is. If test points 32/33 are linear then CRT could be the source.
Everyone's experience is different so let me know if this is too little or too much detail.
Raymond's point is also well taken this is within spec but groups common experience is our Tek scopes are more linear.
Ozan
Before I derailed the thread (Tom opened a new one for those of us who can talk about two transistors for hours) I gave some suggestions to debug in #176717
https://groups.io/g/TekScopes/message/176717
Take a look at B sweep to rule out nonlinearity in A sweep generator.
When I suggested to check the voltages I meant to check the voltages shown in the schematic sheet <6> (Probe Adjust, XY Amplifier, Horizontal Output). In particular voltages at the base of Q780 and Q770, and voltage at the cathode of VR764. You need to put the scope in a setup shown in "Control Settings" section prior to the schematic. Voltages could be slightly off but pay special attention to the difference, i.e. if everyone is up/down by 100mV could be OK. Report back here if any voltages are off.
If voltages are correct and you still want to find the root cause, this is what I would do (I haven't tried it, take this as food for thought):
You could put he scope in X/Y mode, apply GND to both inputs and center the dot. Keep dot intensity to minimum and turn off while not looking at the screen to prevent screen burn.
Now set X input to DC and apply a DC voltage to move the dot to the 1st graticule. Write down input voltage, differential voltage at test points 34/35 and 32/33 (be careful on this sheet there is 100V). Now increase DC to move to next graticule point, write down all three voltages. Continue until the last one.
Now you have a table of input vs output for few critical points. You can figure out where the nonlinearity is. If test points 32/33 are linear then CRT could be the source.
Everyone's experience is different so let me know if this is too little or too much detail.
Raymond's point is also well taken this is within spec but groups common experience is our Tek scopes are more linear.
Ozan
Stephen
Thank you al for everything. Your expertise is invaluable.
I didn’t have much time with work to follow through yet. But I will shortly and report back.
And yes, as Raymond mentioned, I have already tried all the known adjustments (geometry, astig, and the likes), to no avail.
Yes Ozan, this a bit too much details for me at this point, but Ill try to do my best to do all that.
Thank you so very much all.
PS: I know most scopes, although the specs mention a +/- tolerance, are very linear. None of my other 10+ scopes behave that way. This is the only one that has this issue.
I didn’t have much time with work to follow through yet. But I will shortly and report back.
And yes, as Raymond mentioned, I have already tried all the known adjustments (geometry, astig, and the likes), to no avail.
Yes Ozan, this a bit too much details for me at this point, but Ill try to do my best to do all that.
Thank you so very much all.
PS: I know most scopes, although the specs mention a +/- tolerance, are very linear. None of my other 10+ scopes behave that way. This is the only one that has this issue.
Stephen
Hi all,
A little follow up.
Although I’ve been very busy with work, and still haven’t had time to check all what you guys told me to (I will shortly, by the end of the week, though), I’ve just noticed something.
If you take a look at the last 2 pictures I just took, you’ll see that the trace rotation is right on the money.
https://groups.io/g/TekScopes/photo/259498/1?p=Created,,,20,2,0,0
However, if you look at this 5V p-p signal, you’ll notice that the right traces get shorter. Particularly at the bottom. Not by a lot, but they do.
https://groups.io/g/TekScopes/photo/259498/0?p=Created,,,20,2,0,0
This might be an indication that the CRT is somewhat a tiny bit crooked, right?
It’s not overall very important. I mean, the scope works flawlessly otherwise, but... you know...
PS: Just in case you’re wondering why this 2235 (not AN/USM-488) has scale illumination, that because I installed it. It’s always on because I didn’t want to cosmetically alter the scope. The tiny 5k pot is hot glued inside.
Thanks all for bearing with me.
A little follow up.
Although I’ve been very busy with work, and still haven’t had time to check all what you guys told me to (I will shortly, by the end of the week, though), I’ve just noticed something.
If you take a look at the last 2 pictures I just took, you’ll see that the trace rotation is right on the money.
https://groups.io/g/TekScopes/photo/259498/1?p=Created,,,20,2,0,0
However, if you look at this 5V p-p signal, you’ll notice that the right traces get shorter. Particularly at the bottom. Not by a lot, but they do.
https://groups.io/g/TekScopes/photo/259498/0?p=Created,,,20,2,0,0
This might be an indication that the CRT is somewhat a tiny bit crooked, right?
It’s not overall very important. I mean, the scope works flawlessly otherwise, but... you know...
PS: Just in case you’re wondering why this 2235 (not AN/USM-488) has scale illumination, that because I installed it. It’s always on because I didn’t want to cosmetically alter the scope. The tiny 5k pot is hot glued inside.
Thanks all for bearing with me.
Stephen
On Thu, Jan 21, 2021 at 10:43 AM, fiftythreebuick wrote:
Of course I have. I’ve just installed it a few days ago. This scope has always had that issue since I have it.
Hi Stephen-
Have you checked for the horizontal non-linearity with the scale illumination
switched off? Just a thought....
Good luck!
Tom
Stephen
On Mon, Jan 18, 2021 at 10:51 AM, Ozan wrote:
Base of Q770 reads +6.090 VDC, while the base of Q780 reads +6.081 VDC
However, the cathode of VR764 reads high a +5.135 VDC....
Hi,
Hi Stephen,
Before I derailed the thread (Tom opened a new one for those of us who can
talk about two transistors for hours) I gave some suggestions to debug in
#176717
https://groups.io/g/TekScopes/message/176717
Take a look at B sweep to rule out nonlinearity in A sweep generator.
When I suggested to check the voltages I meant to check the voltages shown in
the schematic sheet <6> (Probe Adjust, XY Amplifier, Horizontal Output). In
particular voltages at the base of Q780 and Q770, and voltage at the cathode
of VR764. You need to put the scope in a setup shown in "Control Settings"
section prior to the schematic. Voltages could be slightly off but pay special
attention to the difference, i.e. if everyone is up/down by 100mV could be OK.
Report back here if any voltages are off.
Base of Q770 reads +6.090 VDC, while the base of Q780 reads +6.081 VDC
However, the cathode of VR764 reads high a +5.135 VDC....
Ozan
VR764 zener is spec’ed at 12.6V +/-5%, with its variation +5.135V at cathode of VR764 is not out of range. Beam find diodes are still reverse biased with some margin. You may want to double check cathode of CR764 (beam find) is at -8.6V, and top of R764 is +8.6V.
Base of Q770 reads +6.090 VDC, while the base of Q780 reads +6.081 VDC
However, the cathode of VR764 reads high a +5.135 VDC....
[You can skip this section, this is mostly for explaining why I would look at the left side of the beam find diodes. The compression in sweep is symmetric around mid point. Assuming it is coming from the circuits, not the CRT, it should be in a block that is symmetric around 0V (mid point of deflection voltage). Right of R780 and R770 is not symmetric around 0V so it should either be in this area with the diodes CR780/CR770 or in U760 which has symmetric diff pairs. Inside of U760 can be seen in Tek 155-0124-00 document]
I recommend looking at the voltages around U760. Its supplies +5V (pin 15) and -5V (pin 4) are generated locally, that should be the first place. Voltages at pins 1,2,5,7,8 can give clues about whether operating point of U760 is good. Pin 3 & 6 voltages not marked in schematic but looking at inside of U760 they should be about pin 14 voltage minus 0.7V (probably -0.7 to +0.3V).
If all are OK measuring X-input to horizontal deflection path linearity I described in #176793 is what I would do if I was debugging.
Ozan