Cathode Ray Question - No Focus without deflection coils

[Ozmo]

Hi - this has me puzzled. I have searched - even tried the new AI searches could not find anything on this.

Question is why, on small B/W CRT monitors, is the Vertical deflection coil needed to get focus?
All explanations out there are that the X&Y deflection coils deflect the electrons - but this is not what Im seeing in the tiny TV's Im working on now.
I'm trying to make into a audio visualisation console -similar effect to the display tech moan has sometimes in the background - (search for "SAE Mk6 FM Tuner - Vintage HiFi holy grail").

When I remove the connection from the X and Y deflection coils - I dont get a nice small dot - I get a fuzzy spot sometimes taking up half screen.
I am using 1/2" crts from the viewfinder of old video camcorders and larger 1" crts from professional camcorders - which use a safe 5volts. These have two coils - but dont give a nice spot when the coils are disconnected or controlled by my audio amps.

It works as expected with larger 5" karokie machines - but these screens are too big for my purpose (and are also higher voltage).

Somehow - the Y Deflection coil is needed to have focus - this ruins the effect Im trying achieve. Anyone know why?

Thanks all.

 

[Bluejets]

https://vsa.edu.in/how-deflection-system-in-crt-work-explain-with-diagram/

 

[Ozmo]

https://vsa.edu.in/how-deflection-system-in-crt-work-explain-with-diagram/

Thanks - Yes - this is just as how we were taught in physics class - and same answer from the AI searches - but its not what Im seeing in real life.
On my small CRTs....

If I disconnect the 2 Horizontal coil wires - I get a thin straight line "|" .... thats ok.

If I put them back and take away the 2 Vertical coil wires - I get a blury out of focus mess '**'.

So the Vertical is doing something More than just deflecting - its just a coil but its being used to focus as well - I've not found that documented anywhere and wondering what this could be - and how I can emulate this so I can get a fine point I can control the position of?

I have a number of small CRT tubes here I have been testing - and none I can use - when the coils are disconnected - you dont get the expected fine dot - when I apply my own voltages - I can move the beam - but its out of focus. There is no focus adjustment I can see on these.

ta

 

[Ozmo]

If anyone is familiar with this and can assist: the best ones I have are "HDVF-20A - 2 Inch Sony Professional CRT Viewfinder"

Page 4-3 & 5-7
Again - when I try to control the deflection coils by my hardware - its out of focus - and only replacing the original Y coil driver circuitry brings it back into focus.

Here is a link to a CRT from a Karaoke screen I got close to what I want - its super sharp - just the screen is too big for the cabinet I have in mind (and the smaller one is safer).

Looking for hints on how to do similar with the one in the PDF...

 

[kellys_eye]

Older CRTs generally used electrostatic focussing via specific structural elements in the neck of the CRT. These required relatively high potentials too. Given the smaller size (space limitations( it seems appropriate to utilise external focussing which is only really a simple superimposition of the required voltage/waveform on the scan coils.

Can you obtain the schematic of the original tubes and their original application?