Need help with a partially distorted Tektronix 2246 scope CRT

[Skylineview]

ey everyone,

I'm new to the forum but have been keeping up with Dave on YouTube for quite some time.   I'm relatively new to the electronics hobby and I recently purchased my first analog scope. It's a Tek 2246 100 MHz 4 channel I found on e-Bay and the listing said it "Wasn't working".  Everything is functioning on it as you'd expect it to, except for one slight issue.  The lower left 1/4 of the screen seems to be distorted.   All of the menus, lower part of the trace and lower portion of the cursors appear to be affected. It doesn't appear (to me) to be the CRT as it is clear and bright throughout its entirety.  As I've said I'm relatively new to the hobby, but have repaired a few things in the past, so i'm not afraid to break it open and find the fault, I just don't know where to even begin troubleshooting such an obscure issue.

Traces are clean, except the lower-left portion mentioned above, display does not have any jitter, all pots/selectors and buttons work properly. I have pictures of the issue, so you can see for yourself.

Any help on where to begin to fix this issue is greatly appreciated.

Also, I've tried posting about this several times, but couldn't seem to get it to post, sorry if it posts several times. 

Clayton
Sealy, TX USA

[Ian.M]

Hook it up to a signal generator or a low voltage bell transformer or similar to get a sine wave trace on the screen safely.  Adjust Volts/div and timebase to fill 90% of the screen vertically and get two cycles horizontally and take a photo and post it here so we can see the distortion.   If you've got a signal generator repeat with a triangle wave.

[Skylineview]

Here are pics of the builtin in 1kHz square wave

[Skylineview]

You can see from the pictures, its not just the waveform that's affected.  The onscreen display of the cursors and volts/div, the menu selections are affected as well.

thanks for your help.

(I tried to post pics with the original post, but they were too big, all fixed now)

[Ian.M]

That's an odd one.  Either there's some sort of unwanted complex cross-coupling between the X and Y amplifiers driving the tube, possibly via their power supplies, or something has got magnetised near the tube    Hopefully one of our Tek experts will chip in with an opinion.

[Skylineview]

Here's a triangle wave and sine, just for reference.

Thanks for your help Ian, my first thought was something magnetized as well, especially since its only that one corner that's affected.

[StillTrying]

It looks magnetic to me! Have you tried it in a quite different position on the desk, and without other test equipment around it.

[Skylineview]

Yeah, it's been on the other end of the desk, the desk across the room, on the floor and even the dining room with zero change.

[MarkL]

Barring any magnetic interference, it looks like mechanical damage to one or more of the elements inside the CRT.  Usually from the scope getting dropped.

[Skylineview]

Could that damage happen without any visible damage to the case? I guess I'll start tracking down a replacement CRT for it.

[MarkL]

Could that damage happen without any visible damage to the case? I guess I'll start tracking down a replacement CRT for it.

Sure, anything is possible.  The distortion is not severe, so it could have dislodged something and it might not even be obvious looking at the tube.

One thing to try before replacing it is to flip either the horizontal or the vertical plate connections on the CRT neck to make sure it's not some weird electrical problem.  The traces will then be mirrored or upside-down, but if the problem stays in the lower left, it's probably a damaged CRT.

You can also trying poking around with a compass to see if anything is magnetized, but I think that seems a less likely cause.

[james_s]

Swapping deflection wires is a great idea, that should make it obvious. If it does turn out that the tube is bad, it may be possible to correct it with strategic placement of one or more small magnets around the tube.

[duak]

Something magnetized wouldn't be my first guess but with the preponderance of supermagnets these days, it wouldn't have taken much to magnetize either some hardware in the scope or the mu-metal CRT shield.

If something in the scope is magnetized, the best way to deal with it is to demagnetize it.  Look up "color tv degaussing coil".  This is a big coil of wire that is powered from the AC mains and is waved around the TV to remove any residual magnetism. In the 60's & 70's every TV serviceman had one, but later on TVs incorporated their own.  If you can get a scrapped color TV, you should be able to remove the coil.  They aren't designed for continuous use and will get hot.  A 25W incandescant light bulb in series might be a good current limiter.

Cheers,

[tggzzz]

My bet would be on a distorted mesh inside the CRT (assuming that CRT does have a mesh). If that's the case you have two options: replace the CRT or use other parts of the display.

To see how fragile the mesh is, have a look at the two mesh photos: https://entertaininghacks.wordpress.com/2016/03/09/rescuing-a-broken-tektronix-465-crt/

Having said that, I did have a CRT that suddenly developed a small (~2mm) distortion for no apparent reason. Gently shaking in the appropriate directions moved the distortion, so I presume that was a flake of something that had found its way onto the mesh.

[Cnoob]

The two Tek scopes I bought from ebay, turned up with something rattling inside. One was a smd cap which I replaced with new part and the other has a chassis screw which I couldn't find where it went.
 
I would be worth taking a look inside to see if anything has been dislodged.

I've had a look at a parts diagram of a tektronix's  scope and can see the defection coil but no ring for correction magnets which you get on monitors. (If it was a monitor that would have been my suggestion. A loose magnet.)

[StillTrying]

If it's the crt, I can't help wondering if the crt could be used upside down so that the distortion ends up in the lesser used top right corner.

[MarkL]

...
I've had a look at a parts diagram of a tektronix's  scope and can see the defection coil but no ring for correction magnets which you get on monitors. (If it was a monitor that would have been my suggestion. A loose magnet.)

There are sometimes magnets.  You can see three of them (out of four) on this CRT from a 2465.  They are the small metallic dots under the tape wrapped around the neck.  They are not symmetrically placed and they are very weak and seem to vary in intensity.  They were probably hand-selected at manufacture time to straighten up any remaining asymmetry near the edges of the screen.

Take a look for them on the 2246, but I don't think a missing or misplaced one is the cause.  I think tggzzz has a good idea.

If it's the crt, I can't help wondering if the crt could be used upside down so that the distortion ends up in the lesser used top right corner.

I think there's too many connections sticking out at random places on the neck which then protrude through specific holes in the shield to make that practical.  Plus all your etched graticule numbers would be upside down too.


The CRTs can sometimes be a real pain to remove.  If you're going through the trouble to remove it, I would replace it.  Horizontal and vertical gain/centering plus other beam adjustments (like focus and grid bias) would need to be done for a different tube, but you can probably get away without doing a full calibration.

Be very careful with the pins sticking out of the neck.  They bend easily and won't take a lot of back-and-forth.  The wires may look like they're soldered onto the pins but they aren't.  They are single-pin sockets and you can pull them off with a needle nose pliers (gently!).

EDIT: Added photo of what I mean about the pin sockets.

[Skylineview]

Thanks everyone.   I'm going to crack it open tomorrow, since I'm off from work and see what I can find.   I'll update again if I'm able to find anything.

[David Hess]

My bet would be on a distorted mesh inside the CRT (assuming that CRT does have a mesh). If that's the case you have two options: replace the CRT or use other parts of the display.

That is my guess as well.  The unit got dropped and something broke loose inside the CRT and dented the scan expansion mesh.  A problem with magnetism would affect a much larger area.

A video showing the behavior of traces moving through that area would remove all doubt.

Replacement CRTs for the 2246 are plentiful but I have never changed the CRT in the 2246 series so I do not know what is involved.  Calibration of a new CRT is relatively easy in this series of oscilloscopes because it does not rely on digital calibration constants.  But given the small area affect, I would consider the oscilloscope still usable.

[Ian.M]

One could pretty much map the distortion with a rectangular grid with no extra test equipment.

There's four individually positionable traces, which with their inputs grounded should be horizontal lines, so you could set up the four traces at 1 div vertical intervals on the lower part of the screen, against the undistorted right edge to check the distortion.   There's also a delay timebase with a trace separation control , so in at least one of the delay modes in the  right delay mode, you coudl use the trace separation control to put four more traces on the top half of the screen, or to get more lines on the bottom half, for a total of eight horizontal lines.  If you cant figure that out,  simply take another photo with the traces set up as horizontal lines on the top half. 

Similarly for vertical lines, use the calibration signal on a single channel, adjusted to just go off the screen at the top and bottom, and adjust the variable timebase and x position to get a line on each division checked along the undistorted top edge.

Overlay the two photos and you've got a good map of the distortion.

[Armadillo]

The mesh theory sounded plausible except the mesh is located a distance away from the phosphor screen.
The spacing of the mesh is large in comparison to the resolution hence a bloat there could distort visibly larger area on the screen.

Display a very high frequency sine wave that completely filled and bloat the whole screen, adjust timebase so that the sines waves are closely knit together but still discernible, so that the extent of the distortion could be visualized.

James_s proposal should be tried and I am eagerly waiting for the outcome.

[StillTrying]

The mesh theory sounded plausible except the mesh is located a distance away from the phosphor screen.
The spacing of the mesh is large in comparison to the resolution hence a bloat there could distort visibly larger area on the screen.

And with the distortion being in a corner, the kink in the mesh would have to be right near it's edge, while leaving the rest of it undistorted, so I'm still not 100% convinced it's the mesh either.

[David Hess]

The mesh theory sounded plausible except the mesh is located a distance away from the phosphor screen.
The spacing of the mesh is large in comparison to the resolution hence a bloat there could distort visibly larger area on the screen.

The area of distortion in this case is completely consistent with a damaged scan expansion mesh.  I have seen many examples over the years and some of them had even smaller areas.