Excessive red vertical deflection in CRT monitor?

[D.M. Procida]

Hi. I have an iMac G3, with a built in CRT display. Convergence at the
centre is perfect. Horizontal convergence is pretty good right across
the screen. Vertical convergence is terrible.

The problem is that at the top of the screen, the red beam lands higher
than the others, and at the bottom, lower. So simply moving the beams
around relative to each other doesn't solve that problem. Is there
likely to be an adjustment for this?

Purity isn't absolutely 100%, but it's certainly good enough - there are
no blotchy or mottled ares on the screen, just a bit of tinge towards
some edges.

Thanks,

Daniele

 

[Ol' Duffer]

If I understand it correctly, differences in the colors'
relative scan sizes is mostly due to the electron beams
not being centered in the yoke's magnetic field. If the
electron guns were perfectly centered in the CRT, and if
the yoke were molded and wound perfectly symmetrically,
there would be no need to adjust. But in the real world,
it is usually done by "warping" the yoke, i.e. the factory
jams rubber wedges or cardboard strips between the yoke
and CRT to shift it sideways. If the amount of warp was
significant to start with, the yoke's plastic structure
can flow with age and drift out of convergence. Or if
the unit has been dropped or jarred enough to dislodge
the yoke or bend the electron gun assembly inside the CRT.
Or if the glue on the wedges or strips has dried up and
they have fallen out. My experience with this problem
suggests that if it is bad enough to be objectionable,
the chances of correcting it are poor.

 

[[email protected]]

Since that probably uses a Sony Trinitron crt, the Hstat control if it
has it moves the red and blue one way, and there will be a coil or
couple on the yoke with a ferrite that needs moved to adjust the
landing.

It is also possible the gun internally has shifted and is way out of
adjustment.

 

[D.M. Procida]

Since that probably uses a Sony Trinitron crt

Not on CRT iMacs, unfortunately.

Daniele

 

[Andy Cuffe]

On Thu, 3 Feb 2005 09:59:12 +0000,

Hi. I have an iMac G3, with a built in CRT display. Convergence at the
centre is perfect. Horizontal convergence is pretty good right across
the screen. Vertical convergence is terrible.

The problem is that at the top of the screen, the red beam lands higher
than the others, and at the bottom, lower. So simply moving the beams
around relative to each other doesn't solve that problem. Is there
likely to be an adjustment for this?

Purity isn't absolutely 100%, but it's certainly good enough - there are
no blotchy or mottled ares on the screen, just a bit of tinge towards
some edges.

Thanks,

Daniele

The yoke has probably shifted on the neck of the CRT. This will
affect convergence mostly and purity a little.
Andy Cuffe

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