On Sat, 19 Sep 2009 07:29:21 +0900, "Brenda Ann"
wrote:


"ian field" wrote in message
...

As I understand it, the electron beam is aimed slightly off centre and the
beam trajectory is corrected by an external magnetic field (the ion trap)
the ions being heavier than electrons are not deflected and collide with
the glass wall of the tube neck.

It would be interesting to know how you cantered the picture without the
ion trap.


Actually, the ion trap (there was none on the CRT when I got the set)
doesn't seem to do much with the centering, mostly it just makes the picture
darker (about 70% or so darker). The other (parts) set I have had the ion
trap, so I used the one from that set. On closer observation, I don't think
there ever has been one on this CRT, which is why I'm wondering if it is a
replacement that doesn't require one. The tube is in exceptionally good
condition for a set this old.



The ION trap is a simple device. To prevent IONs from hitting the
phosphor screen, and creating a big brown spot in the center, the
electron gun in the neck is built with a 'bend' in it. The ions, being
what they are, are not affected by magnet fields. Electrons, are
however. By bending the gun slightly the ions hit outside a small
aperture on the accelerator grid. The ION trap magnet bends the
electron beam allowing it to go through that aperture without
problems.

Later day tubes were created with an 'aluminized layer' on top of the
phosphor that was able to handle the IONs without allowing the
phosphor to burn. And that caused the ION trap magnet to 'go away'.

So when you say the thing makes the picture 'darker' in fact it is
just simply letting the electrons strike outside the hole in aperture,
reducing the beam strength.