Crazy scope

[spuffock]

Yesterday my scope was working fine. It went out in the car boot to do a job, and was working fine then. I turn it on today, and first I suspect that I'm too near some big magnets. Both traces are bent into strange shapes.
Not magnets. I look at the timebase with the other scope. Linear.
The distortion looks like something in the tube, the effect itself seems fixed with respect to the face of the tube. On a slow scan, both traces can be seen to wobble across the screen AT DIFFERENT SPEEDS, and at one point to actually STOP. (that,s one point each, not at the same time)
I'll go and take some photos, (and another couple of those little green tablets).

 

[spuffock]

A few pictures of it...
Now,the question is, what the hell happened to it? I don't remember hitting a bus......

By the way, that's a nice sinewave.

 

[StopGo]

Owww eewww!

I would agree, it looks magnetic, but what is that solid bar at the bottom. It looks like a paddle from a tennis video game ?!

 

[Overclocked]

Owww eewww!

I would agree, it looks magnetic, but what is that solid bar at the bottom. It looks like a paddle from a tennis video game ?!

Speaking of which, you can get Pong on an O-Scope.

That line Also looks magnetic. Is your Oscope still covered by warranty?

 

[Dean Huster]

Your first check should always be the power supplies, for both voltage and ripple, because everything depends upon them. In this particular case, a supply may not be the culprit. Magnets only have quasi-permanent effects on cathode-ray tubes when they involve color masks (TV picture tubes where the color can be temporarily goofed up or permanently wrecked when a really strong magnet distorts the mask) or if the scope has a ferrous scan expansion mesh in the CRT at the end of the electron gun (most Tektronix scopes of the 1970s through the 1980s had these), but it would take a really strong magnet next to the neck of the CRT to distort that. Was the scope ever dropped?

After the supplies, check the sweep waveform at the horizontal deflection plates. If the ramp is not linear, you have a problem there.

I notice that your scope has on-screen readout. Has that been affected at all? If not, blame the sweep/amplifier circuits. With no signal, does the vertical positioning operate as normal? If so, keep the vertical out of the lineup for now. Have you tried X-Y operation as a method of pinpointing the problem. If the scope displays a nice straight line with the same sinewave signal to both H and V, then the horizontal amplifier is probably OK and the blame goes to the sweep circuit. Your scope has enough knobs and functions that you should be able to at least confine the problem down to a general area.

Dean

 

[spuffock]

I didn't hit a bus, I haven't been near the MRI at the hospital, I didn't drop the scope..
The blodge at the bottom is the on-screen display, suitably out of focus on my toy camera.
Let me rummage through this box of old spectacles....thats better.
This is a faster sinewave, gives a better idea of the shape of the anomaly.
I've tried tilting the scope about and laying into it with a small rubber mallet, but it doesnt seem to shift anything.
I hope for a rational explanation.
I'm a bit paranoid about thinngs that can reach inside a vacuum tube and go SCRUNCH!

 

[_nox_]

rational explanation - hmmm... How does the CRT of a scope work ? Is it based on a magnetical~ or electrostatic electron deflection ?
Maybe the tube itself is damaged ... either the coil is deplaced or one of the plates - you could try to shake it a bit ( to try whether something changes )

 

[Russlk]

Look inside to see if there is a magnet loose. Perhaps there is oscillation in the horizontal amp? Definatly wierd.

 

[Nigel Goodwin]

How does the CRT of a scope work ? Is it based on a magnetical~ or electrostatic electron deflection ?

It's electrostatic.

 

[spuffock]

Another clue, if you put the scope well out of focus, there seems to bbe the shadow of a mesh on the screen.
I'm convinced that something has happenned to the tubbe.. I have another scope , the twin of this one, but I'll be damned if I'll swap tubes to bbe sure of it.
I'm also damnned if I'll put up with this accursed keyboard generating doubble letters all the time.

 

[Nigel Goodwin]

Another clue, if you put the scope well out of focus, there seems to bbe the shadow of a mesh on the screen.
I'm convinced that something has happenned to the tubbe.

To be honest, from decades of experience repairing TV sets, it does look like a tube fault! - probably the deflection plates have moved?.

 

[_nox_]

probably the deflection plates have moved?

this was my point. << ... either the coil is deplaced or one of the plates - you could try to shake it a bit ( to try whether something changes ) ... >>

 

[audioguru]

It worked fine until you took it out of your car's boot.
Maybe you or your car built up static electricity then zapped the 'scope's screen? The static charge might be between the glass on the front that you can touch and the front of the CRT. Try removing the front glass then discharge the front of the CRT.

 

[_nox_]

It worked fine until you took it out of your car's boot.
Maybe you or your car built up static electricity then zapped the 'scope's screen? The static charge might be between the glass on the front that you can touch and the front of the CRT. Try removing the front glass then discharge the front of the CRT.

is this possible ?

 

[Nigel Goodwin]

is this possible ?

I wouldn't have thought so?.

 

[StopGo]

That 2nd picture really brings out the detail in the problem.
The distortion has a pattern visible in both X and Y directions. This would mean any electrical fault would need to involve a coupling between the X sweep and Y deflection circuits.

Because these are distinctly different parts of the electronics, I would agree this appears to be tube related. If it is electrical then it must be somthing fundamental to both circuits... e.g ground ref. or voltage / PSU.

Can you open the scope up and check the tube for obvious signs of fault ?

 

[Overclocked]

It worked fine until you took it out of your car's boot.
Maybe you or your car built up static electricity then zapped the 'scope's screen? The static charge might be between the glass on the front that you can touch and the front of the CRT. Try removing the front glass then discharge the front of the CRT.

Isnt that the same as degaussing?

 

[audioguru]

No. Degaussing is magnetic cancellation, I think this problem is electrostatic.

When the 'scope was in the car's boot, was the car hit by lightning? If it was then the car and the 'scope would be magnetized. Then degaussing the 'scope would fix it.

 

[Dean Huster]

A CRT is electrostatic in operation, so a bit of ESD around the CRT isn't going to affect anything. It would take a severe shock (as in G forces) to the scope to cause any of the internal gun structure to pop apart, e.g., scan expansion mesh, deflection plates, etc. Did you smack it against something?

If the scope just plain stopped functioning normally by gently changing its position, I'd definitely pop the case off and check some waveforems using another scope.

In all my years with Tektronix, I have yet to see a traditional scope that needed "degaussed". There were some transition models before LCD screens came into vogue that used magnetically deflected color CRTs -- THOSE might need degaussing if subjected to a strong magnetic field, but those scopes are pretty rare these days.

Dean

 

[zevon8]

I have a Tektronix 2235 that was shipped from central USA to Toronto, Canada by parcel post, packing material was only ( to my horror ) scrunched up newspaper, and not much of it. I bet that box got chucked around like rugby ball, both sides of the border.

I checked it over pretty close before powering it up, and when I did, it was fine. My hats off to Tektronix, pretty impressive. Neither postal system had any reason to be gentle, no fragile stickers, or any thing of the kind written on the box, just a plain USPS box.

I wonder if your scope has one or more cold ( dry ) solder joints that didn't like the movement/vibration of a road trip. As mentioned in another post, this has all the hallmarks of DC ripple, easily caused by poor connection to a capacitor.