[SOLVED] Tektronix 2205 Distorted Traces - Everything Else Seems Perfectly Fine - Please Help.

I have this Tek2205 scope, I was made in the 80's and I really love it's design inside - all off the shelf common components. It works perfectly fine when turned on. All knobs and controls are ok except the wave forms are distorted (see pics and video). It was suggested that's caused by magnetisation of the CRT shield, but it didn't work after I tried a homemade degaussing coil (which may not have worked at all :< ).


Anyone have any suggestions? It's hard to get hold of a degaussing coil these days I may have to make a more powerful one....

Thanks

Looks as though a deflection coil got misaligned on the CRT.

BradtheRad said:

Looks as though a deflection coil got misaligned on the CRT.

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thanks for your comment. do you mean there's something physically wrong with the coil or it's an electrical problem?

Looks as though a deflection coil got misaligned on the CRT.

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There are no deflection coils used with oscilloscopes, just deflection plates inside the CRT.

FvM said:

There are no deflection coils used with oscilloscopes, just deflection plates inside the CRT.

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Right, my 50-year-old scope has plates, not coils. But I couldn't be sure about every other scope. I've seen coils in tv's.

yongee said:

thanks for your comment. do you mean there's something physically wrong with the coil or it's an electrical problem?

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A tilted coil would be one way to explain a physical cause for the problem if we were talking about a tv rather than an oscilloscope.

Instead we will assume your scope has deflection plates. It is not likely for a plate to get tilted, misaligned, etc.

The problem is that your horizontal scan has changing amplitude. The further the beam appears down the screen, the wider the horizontal scan gets.

If it were a computer CRT monitor we would say the image has a trapezoidal shape. This shape can be adjusted by calling up the various settings (in monitors made in the past 15 years or so).

Does your scope have such a choice of settings?

Or maybe something has happened where the horizontal scan circuit is now affected by the vertical deflection circuit. To troubleshoot this would require you open the scope and try some meter readings. You would need to find the test points for the circuitry driving the horizontal scan. Readings would be a couple hundred volts.

The horizontal scan width has an adjustment. Probably a potentiometer. If this developed a bad contact (perhaps the ground) then the horizontal scan might be influenced by other circuitry.

I thing you have a problem in the horizontal output amplifier (X amplifier).

Also you have the trace to short.

You need to see all the capacitors in that circuit.

Also see if the voltages are clean - no ripple,and if their values are inside tolerances.

If the capacitors are good and the voltages are ok,its possible you have some
semiconductor with problems.

Good luck

This kind of fault is sometimes very simple to fix. The base of the CRT on these scopes runs quite hot and the springs pushing against the CRT pins tend to weaken. Open it up and try GENTLY moving the base PCB, if the fault stops and starts, power it down and carefully push the springs together to retension them. Be careful, there are high voltages on the tube base.

If that doesn't work, the next likely culprits are the capacitors in the power supply. Their ESR increases with age and the timebase signals start to couple together through the supply lines. Most capacitors are easy to replace and their values are not too critical but if the value is under 100uF, try to find exact value replacements as some are used for generating timing pulses which must be the correct length.

Brian.

If the circuits are ok,maybe the scope got dropped.
There is signals of something broken ?
See this link for the same problem :

https://www.electronicspoint.com/2205-crt-problems-t108945.html


Good luck

SDumas

Other thoughts:

1. does the "trace rotation" control work? It's possible the current through the rotation coil is completely wrong. It should provide SLIGHT tilt adjustment, it does, forget this as a cause of the problem.

2. I can see the trace is also too narrow, it should scan to both edges of the screen. Look along the neck of the tube and see if there are pins on the SIDE of it, if there are, check the connections are good but be careful not to stress them as the glass is quite fragile at these points. The symptoms suggest you might have a 'floating' deflection plate which is picking up charge from the adjacent ones which are 90 degrees to the electrostatic field. The side connections (if there are any) are the deflection place pins, not all models have them.

It's a very repairable scope and should give superb performance. I have a 2465 and 2232 here in daily use after about 25 years!

Brian.

Thank you everyone for trying to help! I really appreciate that.

But sadly SDumas was right.

sdumas said:

If the circuits are ok,maybe the scope got dropped.
There is signals of something broken ?
SDumas

Click to expand...

I have probed around with meters inside the scope and every voltages seem spot on and all caps have low ESR etc. Then I decided to pull the CRT out and saw this:



Well I guess that's the end of such a lovely scope - the inside looks almost new (I know it's been 30 years since it was made) ... What a shame! Maybe I will try to get another dead scope with good CRT to bring it back to life in the future.

Good learning point though

Yes, that's a shame.

By the way, I see a coil around the glass, and it's tilted. That just happens to look like what I first thought the problem was.

Don't give up yet!

Although new tubes are no longer available, there are plenty of old but good ones around. The same tubes were also used in several models so there is a good chance of finding a replacement.
The place to enquire is on the Tek users group on Yahoo Groups. You have to register but it's free and they don't ask for personal details, go to "groups.yahoo.com" then put "Tekscopes" in the search box. It's an amazing resource for Tektronix users and I would say you had a good chance of finding a replacement if you ask nicely on there.

Brian.

Taking another look at your post #10 video...

It shows a part shaking back and forth inside the CRT. It's plausible to believe this has to do with the deflection plates.

However the shaking part would not necessarily create the problem seen in your post #1 video. I still wonder if that tilted coil doesn't have more to do with the trace getting wider toward the bottom.

It is dangerous to suggest this, but if you were to un-tilt the coil, it might solve your original problem.

On the other hand it might do something bad to those metal strips that appear to be attached to the coil. It looks as though the metal strips go through the glass. To move anything around could ***** the glass.

So it becomes a question whether the tilted coil did in fact have anything to do with the broken part, and whether you can safely reduce one or both problems, to make the scope usable.

BradTheRad, sadly, the fault is a displaced deflection plate. The broken glass support holds one corner of one of the plates. What has happened is the X and Y are no longer at 90 degrees to each other, hence the odd shape distortion. The coil is the trace rotation coil, passing a few mA one polarity or the other makes the whole picture rotate along the tube axis but even if not exactly concentric it would have minimal effect. In fact, on a good tube, mounted precisely, the coil current would be zero anyway. There is a drastic measure that might partially fix it, that is to mount the tube upside down. All the wiring to the base and anodes would have to be extended though.

Brian.