Convergence/Vertical Deflection Problem for Green

[Andy]

Using Google, I was able to get some great advice on convergence from
previous posts on this group. But I feel I have a problem that the
convergence magnet rings cannot solve.

I am fixing a Samsung CT-5012V2 television. The deflection yoke was bad
and I replaced it with an equivalent yoke the electronics store gave
me. It's not the exact same part number, but it's either that or order
an expensive yoke off the Internet. Somehow new deflection yokes are
much cheaper in my country than any yoke on the Internet.

I used a cross hatch pattern bitmap and displayed it full screen on my
computer and fed the video out from the computer to the TV. Its the
cheapest pattern generator I have.

I got the convergence on the vertical lines almost perfect. The problem
is with the horizontal lines on the cross hatch pattern. The
convergence is good at the centre, but near the top and the bottom of
the picture, the green separates from the red and blue and moves closer
to the top and bottom of the screen. The red and blue cnvergence
remains good for the whole picture.

It appears that there is a greater vertical deflection for the green
than the red or blue. The green moves closer to the top and bottom of
the picture while the red and blue remains together. The rings at the
back of the tube cannot solve this. I determined through
experimentation that the centre pair adjusts the red and blue relative
to each other and the back pair adjusts the position of both the red
and blue. They do not affect the green.

Does anyone have a clue on how to solve this?

 

[kip]

Get the correct Yoke....
kip

 

[Andy]

I was thinking that the yoke is the only item left that can cause this,
but I cannot think of a reason why? Why would a yoke cause a greater
vertical deflection on the green and not the same vertical deflection

 

[[email protected]]

I was thinking that the yoke is the only item left that can cause this,
but I cannot think of a reason why? Why would a yoke cause a greater
vertical deflection on the green and not the same vertical deflection
on other two electron beams? How is this possible?

It could also be the placement of the yoke, dynamic convergence is
adjusted by tilting the yoke slightly. Static convergence (same error
everywhere on the screen) is adjusted by turning the premagnetised rings
on the neck behind the yoke.

 

[Andy]

Yes, I learnt about dynamic convergence from searches of previous posts
on this group, and tried it already.

The vertical lines on the cross hatch pattern converge almost perfectly
from my adjustments. This should mean that I have the yoke positioned
correctly.

The horizontal lines on the cross hatch pattern have almost perfect
convergence only for the red and blue. The green horizontal line
separates as it goes closer to the top and bottom of the screen.
Tilting the yoke does not help correct this.

On only logical explanation I can think of is that the vertical
deflection is greater for the green beam than for the others. I just
don't understand how this is possible.

I noticed some additional metal components on the replacement yoke that
is not present on the original. Since I have nothing to lose, I will
try removing or adjusting them.

 

[Andy]

I got the yoke! I made a great effort and checked out a lot other
places and finally found it at a small repair shop I didn't know
existed until now. It's the same as the original, with the exact same
part number - and it even cost cheaper than the first yoke I bought. I
will try it out tonight and post if it works.

 

[Andy]

Scrap that. I just found the correct yoke with the exact part number.
Will be trying out the new yoke.

 

[Andy]

FIXED! Within minutes after setting the purity, a few quick turns on
the rings and I could get the cross hatch lined up pretty good. Of
course I still need a few more hours to get it perfect.

So the problem was the wrong yoke. I guess I need to be a Physics
genius to know why that first yoke gave that behaviour. Maybe it was
designed for a tube where the electron guns are arranged differently.

I'll have to absorb the cost of the new yoke since it was my mistake,
but I learnt so much from the experience, it feel it's worth it. Plus,
the cost of the two yokes was still less than half of what it could
have cost if I bought it on the Internet.