Oscilloscope horizontal line is curved. How fix this?

[Andycap]

I have seen this before when the deflection coil has slid back along the tube neck. (From TV repair days but worth a check)

[Gyro]

Unfortunately oscilloscopes CRTs are electrostatic deflection, not magnetic.

[Wallace Gasiewicz]

Yes they are mostly electrostatic but some CRTs in scopes have magnetic yokes also.

[bdunham7]

Yes they are mostly electrostatic but some CRTs in scopes have magnetic yokes also.

That's a bit confusing...

A CRO--a standard scope that displays the signal on a high-speed CRT in the conventional manner--typically has a trace rotation adjustment coil and everything else is internal and electrostatic.

Some DSOs with a CRT display may use a small television-like CRT tube with magnetic deflection. 

Both of the scopes in this thread are of the former variety.

[floobydust]

I don't understand the notion of damaged CRT's as if people are throwing oscilloscopes on the floor or playing football with them. Wouldn't that also break their glass? They do have G-force ratings, electron guns I've never seen "bent" inside without other consequences like broken PC boards or enclosure.

I think warped deflection can also be caused by other things:
Magnetized CRT or mounting frame, or trace rotation needs cal or its circuit is not working. This is usually a tilted trace.
Problems with the deflection power supply, I've seen bad electrolytics cause it to have ripple or poor regulation. This is non-linear distortion in both H and V deflection.

OP's square wave had brightness dots on the rising & falling edges, which means trouble in the form of power supply ripple. You can hetrodyne this at certain sweep speeds to measure it lol, but almost always means bad electrolytics capacitors in the power supply.

The Iwatsu SS-6421 though looks non-linear like pincushion distortion, like a ball but it's electrostatic deflection. Tons of calibration menus too so "soft" cal must be stored somewhere. On the pc boards there's trace rotation (horizontal) , R195 ORTHO (vertical), R63 QUICIC Centering, R52 MAG Centering etc. This is not a scope for the faint-hearted. First try the S/W calibration, see if it got erased or a battery is dead. You'd have to run the service manual through a translate program:
https://www.iti.iwatsu.co.jp/manual/pdf_j/analog_osc/ss-6421_6521-cal-j-t-change.pdf
https://www.iti.iwatsu.co.jp/manual/pdf_j/analog_osc/ss-6421_6521-oj-t.pdf

[bdunham7]

I don't understand the notion of damaged CRT's as if people are throwing oscilloscopes on the floor or playing football with them. Wouldn't that also break their glass? They do have G-force ratings, electron guns I've never seen "bent" inside without other consequences like broken PC boards or enclosure.

It certainly does happen, and often the glass does break internally but not the envelope. Just look at the video in the thread with the 2235.   I agree that it probably hasn't happened here.

Problems with the deflection power supply, I've seen bad electrolytics cause it to have ripple or poor regulation. This is non-linear distortion in both H and V deflection.

My thoughts as well.  He needs to get another scope on the deflection plates and the amplifier supply, which I'm going to guess they share.

You'd have to run the service manual through a translate program:
https://www.iti.iwatsu.co.jp/manual/pdf_j/analog_osc/ss-6421_6521-cal-j-t-change.pdf
https://www.iti.iwatsu.co.jp/manual/pdf_j/analog_osc/ss-6421_6521-oj-t.pdf

OP is in Japan, so that probably will be of great help.

[james_s]

I don't understand the notion of damaged CRT's as if people are throwing oscilloscopes on the floor or playing football with them. Wouldn't that also break their glass? They do have G-force ratings, electron guns I've never seen "bent" inside without other consequences like broken PC boards or enclosure.

It can happen fairly easily due to shipping damage, or if the scope is stored sitting on end and gets knocked over. The glass is quite strong and the internal structure of the electron gun is fragile. the deflection plates are often supported only on one end and then stick out into space, a good whack can bend one or break one of the glass rods that holds the whole structure together. I have seen scope tubes with internal damage that were still intact more than once.

Color raster CRTs are easily damaged too, a good whack can move or distort the shadow mask causing a permanent purity error (colored blotch), I've seen that multiple times too.

[Wallace Gasiewicz]

Concerning Yokes on electrostatic CRTs.

There are some tubes that are electrostatic that also  have coils on the tube to adjust the trace.
I would refer you to this HP  publication

http://hparchive.com/Manuals/HP-143A-Manual-SNP_925.pdf

Go to Page 19 / 51: Page 4-4 of HP manual
Quote:
The TRACE Align Coil, L1and the ORTHAGONALITY coil,  L2 are located under the CRT shield and control the alignment of the trace within the graticule

These are adjustable with pots.

I realize this is an old HP scope and not the scopes we are discussing and I really do not know if the scopes at question here have these coils.
This is an O Scope CRT electrostatic tube type scope and not the TV tube display type scope.
But these adjustment coils are fairly common.

I think the Orthagonality adjustment is sometimes called Geometry adjust.
Orthagonality controls the Pincushioning.

[Gyro]

The spot welds within the gun assembly can be quite fragile too, particularly if you hit some kind of resonance during shipping. I once had a Tek scope where the first grid connection weld had failed where it joined the wire that comes through the base seal. Extremely frustrating to look through the glass and see a tiny fraction of a millimetre gap with absolutely no way of getting to it!

[bdunham7]

I realize this is an old HP scope and not the scopes we are discussing and I really do not know if the scopes at question here have these coils.
This is an O Scope CRT electrostatic tube type scope and not the TV tube display type scope.
But these adjustment coils are fairly common.

That's an interesting find!  It is a rather unusual hybrid CRT and fairly low bandwidth (unless you get a sampling plug-in).  I'm guessing that there were other old low-BW large screen scopes that were similar, but I haven't worked on them.  The difference is that it is wide-screen and needs a lot of expansion to operate--and that is where the geometry issues crop up and the various coils are used.  There is a picture of the CRT in the manual so you can see the TV-like shape of it. I suppose you could make a television tube with electrostatic deflection too...and they did!

I think the Orthagonality adjustment is sometimes called Geometry adjust.
Orthagonality controls the Pincushioning.

Orthagonality on things that have it is just trace alignment (rotation) in the vertical direction.  Instead of one trace rotation adjustment that is more typical, you have trace rotation that you set the horizontal trace with and then you make a separate tweak to orthagonality (literally "right-angledness") to a vertical trace.  Pincushion/Barreling is a separate issue and the HP143A you show has that as well.   The (Iwatsu) OP's scope does resemble a barreling issue, so perhaps his is an unusual example of a small-screen high-BW CRT that nonetheless has those adjustments.  If so, given his description of the problem, that circuit may have failed entirely.

[Wallace Gasiewicz]

My HP 1980 scope has an adjust for the pincushion effect
Has a pretty large screen also

[ginji]

Hello everyone.

The following instructions are for handling and calibration procedures, which I have already obtained, but the contents of these instructions are not sufficient to solve this problem. Unfortunately, the service manual does not seem to be available.
https://www.iti.iwatsu.co.jp/manual/pdf_j/analog_osc/ss-6421_6521-cal-j-t-change.pdf
https://www.iti.iwatsu.co.jp/manual/pdf_j/analog_osc/ss-6421_6521-oj-t.pdf

The CRT of the IWATSU SS-6421 is of course electrostatically deflected and has TRACE ROTATION COIL and ORTHOGONALITY COIL.
This problem could not be solved by these TRACE ROTATION and ORTHOGONALITY adjustments.
We do not have a service manual for the SS-6421, but we are examining the schematics with reference to service manuals for other models.

[m k]

My first impression from first picture was that deflection is so off that it's turning back.
Obviously not the case here.

[G7PSK]

When I was repairing watches they were degaussed using a capacitor discharging through a coil, not sure what size the capacitor was as it was inside a sealed unit but big enough to make a loud noise and the watch to jump when the button was pressed.

[CaptDon]

A capacitor discharging through a coil causing the watch to jump would magnetize the hell out of it, not demagnetize it. De-magnetizing uses an alternating current field that slowly decreases to zero (or the alternating field is slowly moved away from the device).

[CaptDon]

I had a PPI display which used a 5CP7A crt. One day when I powered the unit on the trace was of center (low and to the left) but slowly came back to nearly correct positioning. Being low and left indicated some negative Vertical and Horizontal ailment. I went crazy checking all kinds of things to no avail. I finally shorted the V and H deflection plates together, all 4 to one common short. Guess what, from a cold start after being powered off for an hour or two I was still seeing the exact same problem!!! How could that be with no differential on the deflection plates??? Turns out Broken gun support inside the C.R.T back at the cathode/grid 1 area!!! 5CP7 was a poor substitute for the 'A' version (crappy focus, crappy astig) but a 5ADP7A worked great after much adjusting. Twisted the base off the bad C.R.T. and crunched the glass nipple. Didn't want anyone to get hurt with a possible implosion. I hear these old 5" scope C.R.T.'s are pretty violent when the go boom.

[thinkfat]

A capacitor discharging through a coil causing the watch to jump would magnetize the hell out of it, not demagnetize it. De-magnetizing uses an alternating current field that slowly decreases to zero (or the alternating field is slowly moved away from the device).

But that is exactly what will happen in this case. Coil and capacitor form a tank circuit. The discharge through the coil will cause a dampened oscillation until the energy has turned into heat.

[CaptDon]

Hmmm, sounds kind of brutal? I would have though the initial magnetic field would either damage the internals or produce a polar field so strong that even the decaying alternating field couldn't degauss it. I like the part "Made the watch jump", not on my watch!! I have had to de-mag my older watches using a bulk tape eraser. Sadly I used work around magnetrons, klystrons and crossed field amplifiers as well as more recently the systems with waveguide circulators all of which will put a hurting on a mechanical watch. Some of the old multi-megawatt magnetron magnets when separated from each other can lift hundreds of pounds of salvage out of the water. Those dead maggies were highly sought after!!!

[ginji]

Hello everyone.

I have solved the problem.
I measured the voltage at the CRT and pin number 7 is 0 volts.
Pin number 7 is labeled B1, but it is probably IS or ISD.
(IS:Insulation Shield).
I removed the circuit board and traced the circuit one by one, and found that the 820KΩ resistor in the high voltage power supply circuit was broken.
After replacing the resistor, the voltage on pin number 7 went up to 950 volts.

For reference, below is the voltage of the CRT under normal conditions.
Pin number 1: (H),  2: (K) = -2500V,  3: (G1) = -2500 V,  4: (NC),  5: (Q1-) = -480V,  6: (P2) = 40 V,  7: (B1) = 950 V,
 8: (Q1,2+) = 586V,  9: (Q2-) = -380V,  10: (Q3-) = -160V,  11: (G2) = 40V,  12: (?) = 5V,  13: (B2) = -710V,  14: (H)
Thank you very much.

[Pyral]

Unfortunately I have experience with internal damage on CRTs, and that isn't it. The CRT damage looks like what happens when you try drawing a straight line over a rough surface. The line will be straight, then when it reaches a wrinkle in the shadow mask it will follow the crease until the actual beam pattern gets far enough away to go back to the expected path.

Here is a video of what internal damage looks like.


If you suspect internal damage, set your scope so that the beam is as wide and unfocused as possible so it takes up most of the screen. I forget what settings I used but I managed to get the whole screen illuminated. If its damaged it will be obvious because the crease will be visible instead of a uniform color.

[bdunham7]

Hello everyone.

I have solved the problem.
I measured the voltage at the CRT and pin number 7 is 0 volts.
Pin number 7 is labeled B1, but it is probably IS.
(IS:Insulation Shield).
I removed the circuit board and traced the circuit one by one, and found that the 820KΩ resistor in the high voltage power supply circuit was broken.
After replacing the resistor, the voltage on pin number 7 went up to 950 volts.

Excellent work.  Thanks for posting the solution.

[ginji]

In the internal structure of the CRT, the IS（or ISD） is located between the X and Y deflections, and it is also responsible for correcting distortions.
I did a lot of research on the structure of CRTs and found very few references to IS, and only one reference to it, which indicated that the cause of the problem was most likely related to IS.
I spent a lot of time researching and thinking about the problem, as there were very few cases of oscilloscope bright line curves.

Now I can use this classic analog oscilloscope (IWATSU SS-6421) again for a while.

Thanks, 73!

[m k]

Your problem is not very regular so google style(dominance) search is pretty useless.
And you must find exact wording those others have used.

I have used approach where I've concentrated to least significant google hits with long list of excluded words.
Picture search is also many times better but then you have to guess what picture leads forward.