Crazy scope

[spuffock]

The spot will go through the same convolutions if you move it around the screen with the shift controls. This rules out any cross coupling in the amplifiers, hum related things or dry joints. X and Y waveforms at the deflector plates look good on another scope. Anything electrostatic somehow stored in the tube would not have survived the wash of the beam for this long, even tubes meant for storage would not hold anything for more than a few minutes with the read beam on. I suppose it could be something stored magnetically on a ferrous mesh, but I've got some fair magnets about the place and I can't shift it much, and then only temporarily. My bet is a mesh that has become detached, but then again, laying into the side of the tube with a rubber mallet would be expected to shift it some, but no. I shall eventually open the tube and find out for sure, once any hope of getting a replacement tube from Mitsubishi for its novelty value has gone.

 

[tunedwolf]

I would say looking from the pictures you posted...that you have farked the shadow mask in the tube...A classic giveaway is a jellyfish like display...

I once seen a tube rebuilder show me how to temporarily fix that, he took a large wooden sweeping brush and swung it right at the middle of the faulty picture tube( it was a 20 inch tv tube)...the shadow mask popped right back into place, was like magic...Sadly though, these sorts of antics would be frowned upon nowadays If the shadow mask is the cause of your trouble, the tube is for the bin...


rgds

 

[Nigel Goodwin]

I would say looking from the pictures you posted...that you have farked the shadow mask in the tube...A classic giveaway is a jellyfish like display...

You only have shadow masks in colour tubes, isn't this a monochrome scope tube?.

 

[tunedwolf]

yes, sorry, I should have been a little clearer...if this is a colour version, it would have a shadow mask, but looking at the pics again at the beginning of the thread, I now realise it's not....just ignore me, dunno why I thought colour...maybe I read it was somewhere in the thread..

Could the scan yoke have moved on the neck of the tube ?

 

[eblc1388]

Could the scan yoke have moved on the neck of the tube ?

Nearly all small size CRO tubes use electrostatic deflection. The yoke on the CRO tube is probably used for trace rotation only.

 

[spuffock]

The manual says there is a post deflection mesh. Driving over road humps has set up a mechanical resonance in the gun, and torn it loose.That's my story and I'll stick to it until something better comes along.

 

[Nigel Goodwin]

Could the scan yoke have moved on the neck of the tube ?

Again, you're thinking TV and not scope!.

Scopes are electrostatically deflected, NOT magnetically - which wouldn't work, it would have a VERY, VERY limited bandwidth, and not be very linear!.

 

[spuffock]

The final answer to this one, I opened up the tube very carefully. It has a post deflection mesh, and it looks like it has been impacted by something about the size of a grain of sand. Such a huge effect for such a tiny little dint.

 

[Dean Huster]

Yep. A dimple in the mesh will cause that kind of distortion. Whenever those get dented like that, it's either a bit of foreign material that got loose and hit it while moving the scope around (the electron guns are notorious for little glass shards appearing out of nowhere) and popped into it. If a bit of dust or a nearly microscopic fragment lodges in the mesh, it causes a little shadow dot or flare on the screen -- I don't recommend smashing the scope on its front panel on top of a table to knock the intruder loose as a fix.

That mesh is so fine that it will hold water without dripping. A friend who worked as a repairman for electron microscopes took a piece of mesh and ran it under the microscope for me. It's pretty interesting and very easy to see that it's an etched product and not woven wire as a screen door would be. And delicate isn't the word for it. Definitely that part of the gun structure most liable for damage.

The good news: It doesn't affect the operation of the scope at unaffected areas of the screen. Bad news: Tek CRTs don't come with a small price tag. Back in the days when I was on the Tek tech bench repairing the little 200 series handhelds, the CRT for those was $200 and that was back in 1976 when a 465 CRT was still around $400-500.

Dean