Re: Mechanism of CRT Double Peaking

Fabio Trevisan

My first Tek, I used to own until a few months ago, had a weak CRT since day one (when I got it from a local, Brazilian auction site).
One of the first thing that I noticed, besides the low intensity, was that it had this double-peaking thing (which I was unfamiliar with).
It was when I joined this group and the messages should still be there.
Back then David Hess (where's him? Haven't seen anything from him for a while) pointed me to Ed Breya (both are renowned members), as Ed had some experience in rejuvenating CRTs.
At the same time, I learnt about the extensive information available on the tekwiki site (google for TekWiki... you'll find it).
At TekWiki, there in the "Manuals, Catalogs and Other Publications", there's a section called "Concepts Series".
Between the "Concepts Series" publications, the very first one is the "Oscilloscope Cathode-Ray Tubes, 2nd ed.
LInk here for your convenience:
Between pages 10 and 14 there's a detailed explanation of how the electric field lines from the grid interact with that of the cathode, to create an electrical equivalent to a photo camera's "aperture" ring... which is actually what broadens or pinches how much of the Cathode surface is "opened" for emissions.
So,, follows what I conclude from the understanding:
When set to a lower intensity, the cathode effectively emits only from the center, and as we crank up the intensity, it opens up more and more area of the cathode to emit the beam. Therefore, the center of the cathode is the most used portion and is the one that wears out first.
As the CRT ages, the central spot of the cathode wears out and you can only obtain some intensity as you open the "aperture"more and more, exposing the next section (an emitting ring now, not anymore a circle).
That concentric wearing "pattern" ends up imprinted on the cathode and, in my opinion, is what causes the double peaking...
On a worn out cathode, when you start cranking up the brightness, initially the aperture is drawing electrons from the "worn out" area which, despite it's worn out, it responds to the intensity control...(more C.W. more brightness, only that at a lesser degree"...
As you keep opening up the "aperture", you expose more and more of the outer rings (which are also more worn out than the center, so the intensity decreases)...
Up until a point that you open the aperture so much that you expose the outermost ring of the cathode which is still not worn out... (thus, enters the second "peak").
At this point, although you managed to get more electrons and more beam intensity, the size of the spot is already too big and the Focusing anode can no longer focus the beam correctly... (or the Focus Tracking potentiometer can no longer track correctly the increase of the intensity control) and ultimately, the Focus and Astigmatism is ruined.

Part of my conclusion can be flawed, but overall, I think this is the mechanism.



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