Damage to the tube is is certainly the simplest solution and is consistent with any apparent lack of drift in the manifestations of the fault, which would probably occur if the fault was electronic. But must investigate this more systematically, leaving it with a vertical trace for an extended period.

Magnetism is also simple solution. Will have to get on with demag. which will take some fiddling about and am painting the house at the moment.

With regard to abuse, the only visible sign is a small nick out of the plastic screen surround.

What voltages would be on the plates? My meters are 1000V DC max.

The one I had had a couple of KV on the deflection plates. I note the handbook is one of those almost useless ones with scant info.

It may be a similar issue to the GOS I had. Tracking in the pots.

My BWD & Heathkit are not like that from a circuit point of view.

Marc

Have investigated the displacement from the top to the bottom of the screen and this is 1.5 of the large divisions of the 10 division square screen.

This does not change as the cro warms up; even though individual traces drift on warm-up, identical horizontal traces set at the top and bottom of the screen drift together maintaining the relative horizontal displacement of 1.5 division over the 10 division height of the screen.

To my way of thinking this is consistent with distortion due to a physical cause such as deflection plate displacement or magnetization, though it doesn't eliminate an electronic cause (parts of the circuit may be very stable.

Do you remember which pots had tracking?

http://www.eevblog.com/forum/beginners/tek-475-trapezoidal-display-distortion/.

After finding this forum mentioning trapezoidal distortion in a TEK cro decided to take the cover off and try their suggestion of gently tapping the tube to see if the trace moves indicating a tube problem. No movement. Also tried gently bumping the side of the chassis and bumping it gently on it feet. Again no movement. So if plates out of place, they are not reacting to gentle GBH.

Then as the lid went back on noticed significant movement in the traces. As the case lid is aluminium, looked at the handle attached. Plastic with stainless steel fittings - not magnetic - but the two 8mm long machine screws are steel. This was enough!

As the internal chassis is steel demag is now a priority.

Given the environment it was previously in, a magnetic issue was the most likely explanation and you appear to have confirmation with the changes caused by simply moving the top cover.
Electron beams are very sensitive to magnetic fields. The slightest hint of one is enough to produce a noticeable shift in the beam. CRO tubes usually have a mu-metal shield covering them to help shield out stray fields but in this case there is more magnetism than it can cope with.

You probably don't need to do so any more but if you're curious, the peak voltages on the deflection plates are usually around + & - 100 volts with respect to chassis ground. No guarantees that this is the case with your CRO but I doubt that it is much over 200 to 300 volts. Just don't try measuring the cathode voltage this is usually negative 1-2kV and will kill most multimeters.

You can get trapped with SS there are two specific types the Ferritic 400 series are what automotive trim is made of.

With Chromium and Iron in the mix some of this can be distinguished with a magnet. A directional compass may produce some interesting deflections.

Not a wonderful choice of metal, near something that will be affected by it. The fact that the tube etc can be magnetised by the planet, is why CRT TV's have degaussing coils.

Previous question: Not sure on pots

Marc

Re SS tried a piece of plastic fridg magnet on it and it didn't stick - also a stronger metal fridg magnet.

Will try a compass - good idea.

Re choice of screw metal, this is probably insignificant compared to the steel bracket supporting the tube and the rest of the internal circuitry. Also a steel saddle around the neck securing the tube. The mu metal shield is around the neck of the tube only.

Concerned that passing the cro through a degaussing coil may induce a voltage somewhere and cause damage - eg in the focusing coil around the tube at the neck - flare junction. Any thoughts?

So that one has magnetic deflection. That coil should not have anything magnetic in it. the tape head degausser is much more selective.

If you are not putting it into the field of a "Spot Welder", which will degauss a credit card & stop a watch. I don't see much of an issue. You can always keep a distance from it.

There are no coils arranged either side of the neck and parallel to it as would be expected for magnetic deflection.

The coil I see is around the neck. It is shown on the block diagram schematic of the workings, but it is not labelled. Focusing is not mentioned in this schematic so I have assumed the coil is for focusing, though old text books I have mention plates for focusing inside the tube.

I have good size low voltage transformers available so have decided to wind a coil out of a 14ohm coil of hook up wire.