wrote in message
...
On Jun 3, 4:30 pm, "William Sommerwerck"
wrote:
"Smarty" wrote in ...
As a newcomer to this group, I hesitate to be a contrarian
or appear to be in any way argumentative or disagreeable.

You didn't hesitate to do it to me. Don't pretend to be "diffident,
modest,
and shy", because you aren't.

I don't have as much experience servicing TVs as others in this group.
But I
have noticed that impurity is less visible with a monochrome signal than
a
color one. Clearly, the visibility will vary with all the factors stated,
and
possibly some we've missed.

I can understand your confusion about this point, and it is a point of
confusion I share. But I must remind you of what Sherlock Holmes said,
that to
speculate without data weakens the mind.

To argue about what is or is not theoretically visible is a complete
waste of
time, when a simple experiment would resolve the issue. (And if anyone
reading
this thinks I will slide the yoke on my 13" Trinitron to see what
happens --
they are mistaken.)

There are two things present that have to be understood. There is
convergence, which is making sure the 3 electron beams hit the same
spot/area at the same time. There is purity, which is making sure the
3 beams hit their respective phosphors.

Let us assume that for some reason, all three electron guns come in at
an incorrect angle thru the shadow mask or grid or screen. The blue
electron gun hits 50% on the blue phosphor and 25% on the red and 25%
on the green phosphor. The red gun hits 50% on the red phosphor and
25% on the blue and 25% on the green phosphor. The green gun hits 50%
on the green phosphor and 25% on the red and blue phosphors. All
three phosphors are illuminated at 100%, so only differing electron
beam strengths due to compensating for differing phosphor efficiencies
will be noticeable in any color shading of white and gray areas of the
picture.

Assuing that the convergence of the three images is correct, and only
purity is bad, it will take a trained eye to tell that there is a
purity problem on a B+W picture. But, as soon as a color picture is
presented, colors will be noticeably "off".

That is 100% correct, and an adept description of how such errors are barely
noticeable on a black and white picture. Understand also that we are talking
pure CRT physics here. Don't get confused by colour signal weightings that
are part of the encoding and transmission process. Whilst there are some
differences in the efficiencies of the phosphors, and the eye is non linear
in its response to the visible spectrum, those differences are not huge, and
for white through shades of grey, the three beam currents will not be wildly
different for a CRT that's in good emmissive order. Hence the reason that
mass beam landing errors are not anything like as important to the
reproduction of an accurate grey, as you might imagine. The fact that such
errors are much easier to see on a colour picture may well be a perceptual
one, as the human eye / brain combination, is extremely good at handling
colour perception. Single beam landing errors - convergence errors - are of
course, much easier to see on a black and white picture.

As to questioning the credentials of the people that have replied to this,
being new, you should probably be aware that the William has a lifetime's
experience at the sharp end of service, sales and technical writing, Bob has
lectured the stuff, I spent many thousands of hours of my life working on
this stuff at nuts and bolts level, and Phil, for all his occasional rants
and outbursts, is a highly qualified service engineer whose technical
understanding and ability is without question.

I don't think that any of us have actually said that purity errors are *not*
visible on a black and white picture - they are - but we have all agreed,
based on a collective several lifetimes actually working on this stuff
professionally, that they are nothing like as obvious as they are on a
colour picture, and that someone who is not as well versed in the subject,
may have difficulty seeing such errors under those circumstances, or
identifying exactly what the problem was if they could see them.

Whilst I'm sure that very severe purity errors concentrated in a small
area - such as you used to get when little Johnny helpfully waved a magnet
at the tube face - might show rather more than a 'general' large area purity
error, the fact remains that typical large area errors caused by failure of
the auto degaussing circuit, or turning the set whilst it is on, manifest on
a black and white picture as typically a pastel coloured 'stain' or an area
that looks 'dirty'. I can't really think of a better description than those.

Arfa



I taught color TV servicing at an out-of hours course at Bell
Laboratories 50 years ago and have kept my hand in repairs ever
since. Getting good convergence and purity was/is always a
challenge. Taking tv's on a swivel base and maintaining pruity as the
set is rotated through 120 degrees was always a challenge. The new
flat-screen non-crt tvs are a blessing!!!