Curious behavior of a CRT


Morris Odell
 

My friend who is restoring a 547 was experiencing a lot of trouble with trace distortion. On switch on the trace was not straight, was tilted and appeared non linear. The geometry and trace rotation controls did not change things. Substitution of a known good CRT didn't seem to make any difference and the fault persisted when the Y deflection plates were shorted thus eliminating the VA as the source of the problem. It was starting to look like a faulty CRT even though it was extremely unlikely that two CRTs had the same problem. Eventually he referred to a Tek publication on scope servicing that mentioned the possibility of static build up on the glass supports inside the CRT. It advised curing the problem by deflecting the beam around but off the screen, with the intensity turned to max. The intention was to dissipate the static charge through the beam. It was successful and the scope now works just as it should!

I've been playing with electronics and scopes for more than 50 years but this is a new one for me. I was amazed that static charges could have persisted on the internals of the CRT for days or even weeks and wonder if there is some other explanation. Has anyone else had the same experience? I'd be interested to hear from anyone who knows something about this.

Thanks,

Morris


bill koski
 

Glass is a great dielectric material.
I've seen the 2nd anode on picture tubes, Especially old set with tube rectifiers, still have a good charge days and even weeks after the last time it was turned on.


Ed Breya
 

Interesting. Can you find that document and link to it? Glass is a good insulator inside of vacuum. Outside, not so much, due to surface effects.

Ed


Albert Otten
 

Interesting. Can you find that document and link to it? Glass is a good
insulator inside of vacuum. Outside, not so much, due to surface effects.
Ed
Perhaps TekScope 1969 June, page 14 under "CRT considerations"? Static charge on CRT-gun support rods etc. Same remedy.
PDF available at Tekwiki under "TekScope".
Albert


Morris Odell
 

Yes, apparently that's the source of the info and it was reprinted in 1989 with the same text.


Ed Breya
 

It's too bad they didn't say more on this. Without knowing more detail, I'd guess that it's an anecdotal, possibly serendipitous, and temporary fix for either an actual CRT failure, or an unanticipated CRT design characteristic. I'll explain more next time. Study up on your electron guns and optics for this one - should be a lot of fun.

Ed


TV7
 

Found the reference in this publication: https://vintagetek.org/troubleshooting-your-oscilloscope/

Page 37


Ed Breya
 

And don't discount the possibility of failure in external circuitry - two CRTs with exactly the same problem is quite unlikely.

Ed


emissionlabs
 

this fault condition of electrostatic charge on the glass rods, is decribed in the manual which TV7 posted.

A very small amount of gas, means the beam ionises the gas, and the ions go to where they see a negative field. When it's more of them, they distract each other, go another way, and hit the inner structure randomly. If that is a glass rod, I suppose they get stuck there. Only free electrons can discharge them. If that happens, they become gas again and they move away. I suppose that is the reason why they recommend in that manual , page 37. (page 43 of the pdf document) "spraying" around the electron beam outside the visible screen. That will also leave lots of free electrons everywhere, but these can migrate better. Electrons will stay on a glass surface much less good. They can jump from one atom to the other, and distribute their charge. So they get pushed out to where the glass has some contact with metal, and there always is. But ions can't do that. Their charge attaches them to the surface, wherever they land. The only thing which can get them off is free electrons. I could be wrong, but this may explain it.


Eric Schumacher
 

To no one in particular: Forty years ago I was involved with the development of a one inch CRT (IEE Nimo tube https://www.youtube.com/watch?v=xmWg7CtN0Ac ) We occasionally observed odd deflection effects that seemed related to the weather and could be effected by breathing or blowing on the CRT. We initially assumed this was due surface contamination of the glass envelope. However no amount of exotic cleaning would resolve our issues and even seemed to make them worse! At some point a cohort found a reference in a text book to the build up of static on CRT envelops. The suggested cure was to thoroughly clean the tube And dip it into a 50/50 mix of acetone and silicone oil then bake the tube. Viola! No more environmental issues with Nimo tubes.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of bill koski
Sent: Wednesday, September 15, 2021 7:02 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Curious behavior of a CRT

Glass is a great dielectric material.
I've seen the 2nd anode on picture tubes, Especially old set with tube rectifiers, still have a good charge days and even weeks after the last time it was turned on.


Roy Thistle
 

On Thu, Sep 16, 2021 at 10:51 AM, emissionlabs wrote:


If that is a glass rod, I suppose they get stuck there.
Perhaps the ions just get a electrons from the glass, and go on their way... leaving the glass charged.
You can charge a glass rod with a piece of silk.
On a dry day, the static charge will hold for several moments.
If the air is very dry, and the glass is very clean, the static charge takes longer to dissipate.
If there is very little air, and it is very dry, and the glass is very clean... like inside a CRT... the static charge might last longer.
--
Roy Thistle


emissionlabs
 

@Eric Schumacher. Was that baked when the whole tube was ready, or did you bake the empty envelope first? Do you remember the temperature? I suppose the silicone changes into some kind of mildly conductive layer after it gets decomposed by the heat. Any information about that would be very interesting. Do you think there is a way to make the glass conductive from the inside, without compromising the vacuum?


TV7
 

Tried replying before not sure if it worked...

The publication was https://vintagetek.org/troubleshooting-your-oscilloscope/

Page 37