Saga and general setup for large CRT TVs

"I'm having problem(s) with a brand new 40" Mitsubishi tube (direct view) TV. I'm writing this with hopes of getting some basic information so that the dealer doesn't bamboozle me.

From first viewing (5 minutes after the delivery man departed). I noticed a discoloration patch in the top right hand corner (purple when the background is blue/greenish when background is white)."

(From: Tony (ard12@eng.cam.ac.uk).)

As you probably know, a colour TV produces a red picture, a green picture and a blue picture on the screen at the same time. You eyes interpret that as a coloured picture. If you look at (a normal, non-projection) TV screen through a magnifying lens, you should be able to see red, green and blue dots, and no other colours.

Now, there are 3 basic adjustments to getting a good colour picture :

1. Purity. This means that the red picture is only red, the green picture only green, etc. This is the one that needs setting up on your set - you have a purity problem

2. Convergence. This means that the 3 pictures line up over the entire screen (or as much of it as possible). If this one is wrong, you'll see coloured fringes around objects in the picture.

3. Grey scale. This sets the overall colour of the picture - it means that white is really white, etc. It varies the relative intensities of the red, green and blue pictures.

(From: VK.)

"I called the store in a panic and they calmly told me to press the "degauss" button to eliminate the problem (which I quickly learned was spurious magnetization, caused perhaps by storage near a speaker in the warehouse?). Result? Better but not cured."

(From: Tony.)

Yes, spurious magnetization (or more correctly a different magnetic field around the tube from the one present when it was set up) will cause purity problems.

(From: V. K.)

"The next day I visited the store, and the manager said (again) that this was an easily fixable problem, requiring a few waves of a degaussing coil. To appease me, he sends the salesman home with me with small (1 foot diameter) coil in tow. Salesman (boy, actually) waves the coil in front of and around set but can't seem to remove the discoloration."

(From: Tony.)

Argh... Here's what should have been done IMHO.

1. The set should have been degaussed (a fancy word for demagnetized).

2. They should have connected a 'pattern generator' to the set. This is a piece of equipment that generates various test signals. They should have selected 'red raster' (which will appear to you as a pure red screen), and set up the purity adjustments on that. You should ask to see the pure red raster (and pure green and pure blue if the generator will allow it), and make sure there are no strange-coloured patches. If you like, you can examine the screen through a magnifying lens to check that there are no dots of other colours appearing - I do that when I'm setting up a new TV or monitor.

3. They should then have displayed a 'cross hatch' on the screen. This is a grid of white vertical and horizontal lines. Convergence errors are shown by the lines splitting into 2 or more colours (normally one of the 'primary colours' - red, green, or blue, and its complementary colour (cyan, magenta, and yellow).) Note, however, that it's _very_ difficult or even impossible to get perfect convergence over the entire screen on a modern tube, and that you'll not notice small errors near the corners on a TV screen. Note that some engineers prefer to set up the convergence on some other type of display (dots, for example), but you should at least be able to see a cross hatch pattern (all pattern generators provide that one)

4. They should have then displayed a 'grey scale' test display. This is a pattern of vertical grey bars of different brightnesses, from black to white. They should all have been a neutral grey, without colouration.

Note that convergence and purity interact to some extent, and thus if either is adjusted, both must be checked (and rechecked). Grey scale adjustments interact with nothing else.

I would want to see the set on a pattern generator (at least the patterns I've mentioned above) and identify the problems.

(From: V. K.)

"To demagnetize the TV, he says that a large coil is required, that encompasses the whole unit; service rep will 'be in touch'."

(From: Tony.)

I've never heard of that - the correct procedure is to wipe the coil around the front, top, sides and bottom _NOT the back_ and then move it 2-3m from the set before turning it off. It doesn't matter whether the set is on or off for this, btw. I've not heard of putting a large coil round the entire set. (See the section: Degaussing (demagnetizing) a CRT.

(From: V. K.)

"After the sales boy leaves, I could SWEAR that the picture quality in general is decreased, with people (especially their extremities like lips and ears) appearing pinker than before, and also more general interference (fringes/noise) noticeable."

The convergence and purity are set by ring magnets on the neck of the tube. It's possible that the degaussing procedure has slightly demagnetized these, and if so, the whole set will need to be set up. Similarly, if any part of the set was magnetized at the factory, then the adjustments may have been set up to compensate, and then after demagnetization, they'll need to be reset.

(From: V. K.)

"So my questions are these. Can the original problem truly be FIXED with proper sized coil and application?

(From: Tony.)

I don't think the size of the coil will make any difference. I would want to see that set on a pattern generator, so I could be _sure_ as to what the problems are. If the dealers don't have a pattern generator, then they're not fit to be fixing TVs IMHO.

(From: V. K.)

"Could I be imagining that the waving of the small coil degraded the picture quality?"

(From: Tony.)

It's possible, but fairly unlikely. See above

(From: V. K.)

"Should I demand replacement to a new set? Can I legally ask for this, or is it like a new car...you own it, now you deal with the service guys forever.

(From: Tony.)

I don't know US law, but in the UK, if a product is defective, you can demand a refund of the money paid (not a replacement or a repair, a refund). IMHO, a TV with incorrect colours is defective...