[sci.electronics] TV degauss problem

yjwu@unix.cie.rpi.edu (Yeun-Jung Wu) (08/26/89)

	I got a Toshiba 20" stereo TV eight monthes ago. It works fine until
recently that a huge purple spot appeared on the screen. It happened when I 
took a magnet near the screen to see the effect of magnetic field on electron
trajectory. I know that the TV set had a problem of slow recovering from 
the change of earth magnetic field when I moved it from place to place. 
Last time I wanted to use the sheet metal cabinet on which I put my TV set. 
So I moved my TV set somewhere. When I put it back to its place and switched
it on, one quarter of the screen on the right turned purple. I knew that 
probably I have to wait a long time to let the "degauss" circuit to work but
I was not patient enough. Then I took a magnet (a ring magnet from raw speaker)
to see whether I can wake it up by subjecting it to weird magnetic field.
I saw the pretty pattern on the screen and tried rotating the magnet to see
its effect. From my previous experience I know that I can't play it too long.
So the screen ended up with the purple part and some dipole pattern. That 
purple part on the right of the screen faded away gradually but a huge purple
spot remained on it forever. I know that I can "cancel" the spot by 
positioning the magnet near screen with some angle but it seemed stupid to 
have a magnet in front of the TV set to make it work properly. 
	How does the "degauss" circuit work? Dose it take into consideration
that the set will experience different component of earth magnetic field when
we orient it differently? 
	This problem got me pretty upset so I decided to open the case to see
what I can do. I cannot locate the "degauss" circuit because there are only 
wires go to the vertical and horizontal scanning coils and I did not see any
part of the PCB marked as "degauss" section. I adjusted some of the small 
magnets near the electron guns. The situation get better and there is a smaller
"green" hue spot near the upper center now. It is not that bad and only shows
up when there is a uniform background. Right now I am using my Apple ][+ to 
generate a white background to optimize the balance. But how comes that I have
to adjust those magnets? 
	I appreciate any opinion concerning this mystery. Please forward your
mails to my mbox. E-mail address follows as
        
	yjwu@unix.cie.rpi.edu

	Thank you for your patient with my poor English!

 

brian@ucsd.EDU (Brian Kantor) (08/26/89)

Most consumer TV degaussing circuits consist of a coil of magnet wire
around the front face of the tube, and a simple analog timer circuit.
The idea is that every time the set is switched on, a moderately heavy 
alternating current is run through the coil and is gradually reduced to
nearly nothing over the space of a few seconds.  This has the desired
effect of demagnetizing bits of metal that may have picked up a slight
magnetic "charge" from the Earth's and other ambient magnetic fields.

Your experiment with the magnet could easily have place more
magnetism into the TV set than this weak degaussing system can cope
with.  If you're lucky, a degaussing coil (such as the ones Sun
ships with their color workstations, or which can be bought at
large electronics suppliers or maybe even Radio Shlock) is a much
more powerful degausser and may, properly used, be able to clear
the problem.

If the magnet you were fiddling with was strong enough and you got
it close enough, you may have bent or misaligned the shadow mask
in the CRT, which would cause the electron beams to land off-register
with the colour phosphor dots on the tube face.  There is no cure
for this problem except to replace the tube.  However, if the
misalignment is not large, you may be able to tune it out with the
convergence controls in the set.  This is a complex procedure that
takes more patience than I normally possess, so I don't recommend it.
Don't even TRY it without a copy of the manufacturer's alignment
procedure in front of you - and you'll need a bar/dot generator.

I wish you good luck.  You're going to need it.

irwin@m.cs.uiuc.edu (08/28/89)

In years past, I used to do television servicing and still have a
degaussing coil. In using them one placed the coil near the set, turned
it on, worked it all around the set, and then slowly backed away from
the set, prior to turning off the coil.

My guess as to the best solution for you is to look up the number of
a tv shop, call them and ask them if they would degauss your set if
you brought it by the shop, and at what cost. It would only take them
about a minute or two to do it. (I would not be inclined to tell them
that I moved this, tweeked that, etc) Just say you were playing with
a magnet near the set and did not know it would bother it.

barry@hprmokg.HP.COM (Barry Fowler) (08/28/89)

brian@ucsd.EDU (Brian Kantor) Writes:

etc, etc....


>However, if the
>misalignment is not large, you may be able to tune it out with the
>convergence controls in the set.  This is a complex procedure that
 ^^^^^^^^^^^

WRONG! WRONG! WRONG!

>takes more patience than I normally possess, so I don't recommend it.
>Don't even TRY it without a copy of the manufacturer's alignment
>procedure in front of you - and you'll need a bar/dot generator.

>I wish you good luck.  You're going to need it.

In all due respect to Mr. Kantor, the convergence controls DO NOT
have an effect on this described problem.  The convergence controls 
control red, blue, and green SHADOWS in varous portions of the screen
and DO NOT have an effect on COLOR PURITY, which is the problem created
by the basenote writer holding a permanent magnet to the CRT.

NEVER hold a permanent magnet hear a color CRT.  You will often permanently
magnetize the shadow mask of the CRT, creating the problem described in the
basenote.  My suggestion is to purchase a degaussing coil from a TV
parts supplier.  With the coil turned on and the set turned on (actually
the set doesn't really have to be on but it provides feedback), 
rotate the degaussing coil, first close (around 6") in overlapping circles
in front of the CRT.  Continue while slowly distancing the coil from the 
CRT until you are about 6 feet away from it.  Then turn the coil 
perpendicular to the CRT and shut off power to the degaussing coil.
This should remedy the problem IF the CRT isn't permanently ruined.

If you are a "technical type":

There are also two "purity rings" on the neck of the CRT.  These also have
an effect on color purity.  To adjust them, turn the color level to minimum
color intensity.  Adjust the red screen control (G2) to the screen is 
a full red raster.  Rotate the purity rings independently until the non-red
shadows on the screen go away and the only color on the screen is red.
Then, reset the G2 controls for a good black and white picture.

If the above procedure seems too complicated and you aren't sure of what
you are doing, DO NOT attempt to do it.  You will create a whole bunch more
work for the serviceman and will be charged, rightfully so, an additional
amount for goofing things up because it takes LOTS of time to reconverge
the set properly.  I often had this task and it ain't fun....

Good luck,

Barry


Disclaimer:  HP is not to be held for any of the above mentioned information.
	     

yjwu@unix.cie.rpi.edu (Yeun-Jung Wu) (08/29/89)

	I got several good recommendations since last Saturday. I am 
pretty much sure now that the problem is due to the permanent
magnetization of the shadow mask. The manufacturers do not use aluminum 
for the mask material probably because it is not mechanically strong 
enough to support the fine pattern and they do not need nonmagnetic stainless
steel neither because it is in a vacuum seal and its high cost. 
	So what I need is a degaussing coil which is a coil with A.C. current
passed through, I guess. I remembered that long times ago my dad had one 
such coil to demagnetize screwdrivers and many other tools. Right now I have 
a induction motor from a cheap turntable. I can take the rotor away but the
effective area is a little bit smaller and to cover the whole screen will take
me a long time. 
	Last night I gave my TV set a heavy dose of static magnetic field 
(that magnet again) but I tried my effort to smear it more uniformly 
out through the whole screen. I really screwed up at some time and the screen
went crazy; it looked like the spectacular sky in the north pole during the
active sun spot period. But finally it is much better than before; the non-
uniformity is quite invisible if it is not under the white background. 
I adjusted the purity rings on the neck to make sure that the letters in text
mode of my Apple ][+ is convergent on most of the screen. (I only adjusted 
one pair of them and I marked where their original positions are.)
	The accident give me a good lesson and I do learn a lot from all of 
you. Thank you for your suggestions and effort.