11301 - basic analog scope troubleshooting help needed

[cncjerry]

I have the power supply out so I can't post a picture but the issue is easy to visualize.  If you have a square wave displayed and the vertical edge is aligned on the grid (anywhere) the top and bottom horizontal sections of the square wave overlap.  Looking at the signal, you can see excessive jitter in the horizontal (X) axis.  At first I thought it was related to focus but changed my mind. 

I've looked at as many Power supply voltages (+/-5; +/- 15; +50) as I can and don't see anything in terms of ripple that is excessive.  On all the voltages there is a very slight noise ramp up that seems to reset every two seconds.  The noise ramp up is in the low millivolts and maybe hits 10mv on +5 when it seems to reset.  All the caps look good and I plan to check the ESR while I have the supply out.

So basically I think there is a horizontal time base problem or it could be in the trigger. I would think that if it was the trigger that changing the trigger level and/or type would have some impact on the signal trace but it doesn't.  This unit has option 1T which adds a DOCXO and disconnecting it has no impact on the horizontal signal.  Without the schematics I think the DOCXO is used for the integrated counter.

As I mentioned in the earlier thread, the text is all pretty crisp and easily aligned so the basic scope display through the vertical and horizontal amps are probably ok.

The big issue with repairing this scope is that the schematics aren't available.  It's a board swap for FRUs.  I have the service manual as well as the calibration software running.  I've tried some of the automatic calibration options in the software without luck.  I've tried various vertical amps and they all check out OK in this unit as well as my DSA602.  I would really like to get this beast running to have a 400Mhz analog scope but so far, no luck.  The signal is visible and I can use if for some work, but with the trace being so wide, about .1 divisions, it is not very practical.

I've looked at some of the sync and other signals that are brought out the back and don't see much jitter in them, if any.  Other than the slight noise pulse across all voltages everything looks clean.

[cncjerry]

I checked the power supply to the extent possible and it all seems fine. 

You can see in the attached picture how the flat top of the square wave extends past the 4th graticule square and the bottom flat starts near the same graticule.

Hopeless without a schematic and a ton of specialized alignment gear?

I was able to scavenge the three vertical amps plus I can pull the Isotemp 10Mhz oscillator so this is some gain from a freebie.

I would like to see it work though.

thanks for looking.

Jerry

[TerraHertz]

What is the source of the displayed square wave?

Can you adjust the Horizontal Position knob, so the trigger point at the left is displayed on screen, turn the timebase to the next slower sweep speed, and take another picture please?

The trace you've posted, looks to me like the scope may be OK, and the source square wave has varying frequency or mark-space ratio. You are seeing the phosphor averaging of a lot of falling edges with slightly different timing. Can't see the actual vertical edges because they are fast, and the scope has decent bandwidth. This is quite unlike what you'll see with a digital scope display, which in line-draw mode will show the edges.

[cncjerry]

The source is the calibrator which looks fine on my other scopes (I have about 10 or so).  I put many other known signals into the scope with the same result.  A sine wave looks like it was drawn with a .1 division marker held horizontally as it traces the wave.  The jitter is all in the horizontal direction.

One interesting point is that if I zoom in on the signal so I have one very light trace at the trigger point, the trace doesn't look that bad for some reason.  I think it is because the lighter traces are washed out. 

I'll post others shortly.

[cncjerry]

Here's two more shots.  On the first I was trying to show the left edge where it triggers but it is tough to see as there is so much horizontal jitter there isn't enough trace to display an edge.  The second is just more of the same square wave showing more overlap.

There could still be a power problem I can't find but I've been all over the rails looking for ripple.  Other than that noise pulse I mention which could be related to something like the counter gating (i've turned it off without any luck) I think the power is fine.

[tautech]

Nah, it's not right and could need just a case of the good ol' follow the signal path detective work until you find where it all goes bad.
Is it full of unobtainium IC's ? 

[tautech]

Further thoughts.......
I've seen some funny stuff on displays when the sweep ramp sawtooth waveform is not nicely linear ..........reasonably easy to check with another scope.

[cncjerry]

the sweep ramp output which is exposed on a BNC looks fine, no jitter.

This scope is huge if you haven't seen one.  It is the size of a full-sized microwave, crammed with unobtainium custom ICs, discrete logic chips and no schematics.

[WastelandTek]

dude...you have invented time travel



ok, to be more helpful, is it some kind of severe geometry error, like an hourglass?  If you vert position it up and down does it change?

[cncjerry]

no, imagine the vertical sections of the square wave being spread over about .5 grat divisions and washed out because they aren't overlapping on the phosphor to create a solid vertical line.  The horizontal sections overlap and that is why they are bright.  Remember on an analog scope the phosphor retains the persistence so unless the phosphor is hit continuously it goes dim and you don't see a vertical line.  If there was a jitter problem in the vertical amplifier, the vertical lines would be bright and the horizontal lines would be so dim as to not be there.

So this is a timebase issue, I assume, or some kind of triggering problem.   When I can zoom in on the trigger point I can see a very dim line so if it was a trigger issue you would think I would see multiple line sections. I am going to try to eliminate the trigger as an issue by trying external triggering with a T connector on the same signal.

[TerraHertz]

http://w140.com/tekwiki/wiki/11301   for those wondering what it looks like. Sadly, no manuals there.

OK, that's interesting. Having a pretty clean vertical edge at the trigger point, offset 25% from the screen edge, suggests it can't be noise in the horizontal amplifiers or manual offset signal. I wonder if the clean sweep ramp you can see on the output connector, is the same as the one used to drive the H-amps? Or is there another amplifier stage in between?

Hmm... If you used another scope to look at the signal on the CRT H deflection plates, surely that amount of rate jitter should be visible. Follow it back from there?

The tek-wiki article says the scope is programmable. I wonder if that means the H-offset is programmable too? If so, is that summed together with the manual offset? Or what? And how does the sweep rate programability work?

Btw, are there on-screen readouts available? And are they stable when visible? Does enabling them make any difference to the signal display problem?

[cncjerry]

the text is pretty good with all the covers on but I get some wavy lines depending on how much is on the screen, brightness, etc and the covers are off.  The text adjustments have no impact on the trace but there seems to be a difference in vertical focus between the characters and trace which can be evened up with all the pots.  I mentioned in an earlier post that there are over 10 pots that effect the character display like the corners, horizontal and vertical centering, etc.

One thing I noticed last night was a signal where I check the voltage ripple that is clearly jumping arounds.  I don't know what the signal is though,

As far as looking at the deflection plates, I have a feeling this is a dual beam scope of some type because the trace seems so independent of the text display.  Or the scope is treating the horizontal as if it was a jittery signal so tracing back from there wouldn't buy me anything, not that it isn't a bad suggestion.  I'll post some pictures of the boards, these are a work of art.

I have a feeling there is something wrong in the timebase for the sweep.  The A14 board is listed as the timebase board and I was going to go poking around there today.

One other piece of info in this mystery is that the higher frequency signals have less jitter.   You would think a timebase problem to impact higher frequencies more, no? 

[WastelandTek]

...with all the covers on but ...

man Jerry, just like my comment in the other thread, when I had behaviors like this it ended up being RF in the shack, what are you using for lights in there?

[cncjerry]

Waste.  I can see some ripple from the lights with the covers off in the text but not in the trace.  I don't have any strong RF around it and had the same results in my garage as in the workshop.

Here's what I'm dealing with and no schematics.  The upper left set of pots help with the character focus.  The upper right side is the timebase area, mostly for horizontal.  I can shift the overall display left and right and tune it with the pots in that section but no specific impact on the trace.  The lower right of the board is the reference generator and counter logic.  The lower left is the DAC which sets all the voltages under program control.  This could be the issue as well - bad cal data but I can't find a specific test or adjustment in the calibration software that impacts the horizontal jitter.

I am also reposting a picture of a sine wave I took prior to understanding the trace width being a problem.

I've been probing in the upper right this morning trying to find some jitter but I see nothing specific.  I was going to hang two scope probes on the pins that are on the two white wires in the upper center of the board above the two wide connectors.  I should be able to get an XY on my other scope as those are going to the horizontal deflection amp (I assume as they are only around +40v).  the pot to the right of those wires sets the ripple for the text but not for the trace.

As I mentioned previously, I scavenged three vertical amps and get pull the DOCXO (don't need it because I have a GPSDO system).  The amps I used in my DSA603 (cool device as well).  But as I stated a few times, I would love to see this tuned up as I like having an analog scope around.  I have two 7603s but they are limited to 100Mhz.

thanks for all the input, keep the ideas coming.

Jerry

[StillTrying]

From the underexposed first half-peak of the sine it looks like it might be astigmatism.
Does the tube look like it has 2 beams.
Is it possible to turn the brightness well down and look at the shape of the dot, maybe in XY mode.

[cncjerry]

yes, it does look like astig but only on the trace, not the characters.

There is a focus and astig adjustment on the Display menu but it only adjusts the vertical component of astigmatism, not the horizontal.  I looked around in the calibration software and can't find a function to adjust it either.  Note in the sine wave trace that the characters aren't bad, just the trace.

You can't see it in the pictures but you can barely see the vertical lines tracing within a space of about .1 or so graticules. On some settings, some of the verticals you can see are slightly brighter than others meaning that the distribution of the vertical traces (that are out of alignment) is not equal.  So something is jumping all over the place.

I am going to try and XY trace to see what it looks like.

 

[StillTrying]

I've not found any evidence of a 2 beam CRT.
http://w140.com/tekwiki/wiki/11301
http://w140.com/tekwiki/wiki/11302

[cncjerry]

wrt: two beams, it's just sort of weird how the roughly 15 variable resistors, caps, etc impact only the text and not the trace with the exception of overall centering.

Playing around with the triggering, it looks like it is stable on the trigger edge and overlaps as the trace runs out.  I scoped the sweep output and it looks stable.  I am attaching a picture of the left edge when set to a negative slope.  You can see it looks better than the following edges.

I wonder if calling Tektronix would free up a set of schematics...

Thanks for the pointers again.

[tautech]

I am attaching a picture of the left edge when set to a negative slope.  You can see it looks better than the following edges.

The little steps in that falling edge don't look right to me.............. 

[cncjerry]

I've seen those steps on the calibrators on both my 11301 and DSA602.  I think they use a DAC to generate the signal.  There are several frequency options and on the DSA a DC option for the calibrator output.  I checked the output with my TDS3054b and it is the same.

I really like this scope.  It has a lot of features for an analog beast, windows, multiple delays, lots of measurements, integrated 750Mhz counter (with and DOCXO).  I've been looking for another one actually if I can't get this one running, maybe I can find a 11302a for cheap.

[cncjerry]

Let's go back to the timebase for the horizontal sweep.  The sweep ramp looks ok with no detectable jitter.  Shouldn't I be able to find a crystal someplace that is divided down to the horizontal timebase?  I found one where the DOCXO feeds into the counter but this doesn't seem to impact the sweep as I can connect/disconnect it and the only error I see is from the counter where it says the counter PLL isn't locked.  I've located the timebase board and can find anything that looks like a clock or timing generator.  Ideas?

[StillTrying]

I can't think of anything that explains all of what you see on the traces.
If you do try to get some 0s and 8s displayed in XY mode, it might help to eliminate some things such as the X ramp and triggering.

http://69.16.194.131/prod/MTAyMTg0MjE0/Tektronix-11301A-Digital-Oscilloscope-CRT-154088300-eBay

[tautech]

Let's go back to the timebase for the horizontal sweep.  The sweep ramp looks ok with no detectable jitter.  Shouldn't I be able to find a crystal someplace that is divided down to the horizontal timebase? 

Normally they are just an assortment of RC combinations but how they're generated in this Tek I'm not sure.

[cncjerry]

So it looks like a trigger problem which I don't understand.  I played with the various options and after careful tweaking I was able to get it to lock.  The pictures aren't perfect, but they are much, much better than prior traces.  For a few minutes I was thinking it was all operator problems, but I can't change the size of the vertical or horizontal and keep it triggered like this.    But it points me in a new direction at least.  So I am going to go back to the calibration software and explore the various trigger calibration options.

[TerraHertz]

Dammit, yesterday morning I wrote up a medium-long reply, then hitting Post got that blasted Gateway Error. Post gone, can't recover it. Then went out for a day bushwalk, during which I got a twig whiplash in the eye, so now will be off to the doctor.

Anyway, briefly: Put this scope in XY mode. Take the cal sig out from this scope, feed it to its Y input and a channel input of another analog scope. Trigger that other scope on the signal. Take that scope's sweep output ramp, and put that into this scope's X input.
Result after adjusting XY gains and offsets on this scope - it's acting just like a scope should, but you have eliminated this scope's triggering and timebase from the display process. Also you have a correct waveform displayed on that other scope, to compare against what you see on this one.