Dynamic convergence

[MRX37]

I've mentioned this before, but I have a 27" Zenith TV that has rather poor convergence in the corners. No amount of playing with the yoke or CRT neck rings will fix it. The CRT isn't shot yet. It's still nice and bright and has good color.

I'm wondering... the CRT has a fairly small yoke for a 27" TV. I'm wondering if a bigger yoke would improve things? I've already tried repositioning the existing yoke a dozen times.

[zenith2134]

If you change the yoke out, and put in mostly any other kind except the original, it would likely look awful and/or fry the vertical and horizontal sections of the chassis since there may be an impedance mismatch.

Some of the more modern yokes aren't even removable, since they are integrated into the crt and are designed to never be replaced.

[andy]

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[MRX37]

I'm going to try swapping the neck rings. I remember at one point the convergence was fine. I wonder if the neck rings lost some of their magnetic power.

[andy]

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[MRX37]

Originally Posted by andy

The neck rings only affect the center convergence. Have you tried moving the yoke forward, or backward on the neck? Sometimes they slip backward a little over time. A defective yoke could also cause bad convergence.

Originally Posted by MRX37

I've already tried repositioning the existing yoke a dozen times.

I thought that explained it well enough, but to clarify: Yes I have tried repositioning the yoke. I have moved it as far forward as it'll go, and as far back as it'll go, with tiny increments in between. Moving it back worsens the convergence, and forward helps only marginally, and messes up the color purity.

Tilting the yoke improves convergence in one corner while the other 3 corners get worse. As I stated, it's something which has slowly gotten worse over time. Focus and color strength are fine.

[Kiwick]

you can try to degauss the CRT thoroughly from the backside, the inner sheetmetal shield and the electron gun/yoke assembly can get magnetized by powerful magnets (kids?) or even nearby lightning strikes (even if the set survived)

just be sure to remove the convergence rings before or you will likely destroy them magnetically,

[MRX37]

Originally Posted by Kiwick

you can try to degauss the CRT thoroughly from the backside, the inner sheetmetal shield and the electron gun/yoke assembly can get magnetized by powerful magnets (kids?) or even nearby lightning strikes (even if the set survived)

That makes no sense, seeing as this TV has never been exposed to such conditions. I was there when this TV was bought brand new from the store.

[wbain]

Degauss the TV and go from there. You will never get good convergence at the corners, but you can minimize the bad.

[andy]

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[MRX37]

Originally Posted by andy

Some CRT and yoke combinations will converge better than others. What model and age is the set? If I were you, I would look for a replacement yoke from a junk Zenith of the same age.

It's a 1998 Zenith A27A23W GA-3 chassis.

Interestingly, I have a Hitachi only a year older, (built by Zenith) and it has AWFUL convergence in the corners. I'm not going to try to fix that one as the CRT in it is just about shot.

Correction magnets sound like my best bet. If the magnets are big enough, can I place them on the yoke instead of under it? How powerful do the magnets need to be? How big do they need to be? I have lots of magnets and can easily break them down to the needed size.

[kx250rider]

Get some small round magnets, maybe about 1/2 the size of a dime, and try placing them here & there around the bell of the tube. Rotate them, and flip them upside down, etc, until you get it the best you can. Probably the best place to start with the first one is about 3" away from the yoke, and on the back of the tube nearest the bad corner convergence. If you get it too close to the rim of the tube, it will cause purity problems. That's how the factory does it. That's also a good trick for correcting impurities due to a damaged shadow mask. I became "the expert" on that after the Northridge Earthquake in '94

Charles

[MRX37]

Thanks kx250rider.

However odd this sounds, I replaced the CRT neck rings with the rings from a 25" Magnavox, and convergence has actually gotten better. Only issue is that I ended up breaking the convergence assembly to get at the rings in both the Zenith and Magnavox... so now I can't get the rings to stay in place.

I'm going garbage picking tonight. I'll probably find another 25" or 27" TV that I can steal the ring assembly off of. Who knows, it might be a Zenith with a bad CRT, LOL!

[Findm-Keepm]

The procedure suggested is also in the Zenith Service Manual for the set. It's attached, if you need it.

Cheers!

[MRX37]

Thanks, but I have a manual.

Replacing the CRT neck rings fixed the convergence. I know these rings are only supposed to affect the center, but maybe the originals started losing their magnetism. Like I said before, the loss of corner convergence happened slowly and gradually.

I know Zenith made junk CRT's, perhaps they designed the neck rings to lose their strength right around the time the CRT started to fail... only my CRT hasn't gone downhill yet (knock on wood).

I've managed to get the new neck rings to stay in place with the aid of an extra CRT neck clamp and a little bit of glue. Purity checks out fine, and the convergence in the corners is much better. It's not perfect, but I have some small magnets to try...