CRT monitor geometry problem

[Guest]

Someone just gave me a 19" CRT monitor, and I noticed a geometry
problem with it. It seems that the top edge and bottom edge of the
image are not parallel with the top and bottom of the monitor. The
right side of the top edge and bottom edge of the image touch the top
and bottom of the screen, but the left side of the top edge and bottom
edge are a few millimeters from touching the top and bottom of the
screen.

This is something that a trapezoid adjustment would fix, and the
monitor does have a trapezoid adjustment. But it only adjusts the
sides of the image, not the top and bottom. There is nothing I can do
about this problem (without opening up the monitor), right?

 

[GT]

Someone just gave me a 19" CRT monitor, and I noticed a geometry
problem with it. It seems that the top edge and bottom edge of the
image are not parallel with the top and bottom of the monitor. The
right side of the top edge and bottom edge of the image touch the top
and bottom of the screen, but the left side of the top edge and bottom
edge are a few millimeters from touching the top and bottom of the
screen.

This is something that a trapezoid adjustment would fix, and the
monitor does have a trapezoid adjustment. But it only adjusts the
sides of the image, not the top and bottom. There is nothing I can do
about this problem (without opening up the monitor), right?

How long did you give it to warm up? Make sure its warmed properly (might be
an hour or so) before adjusting things.

 

[Bob Myers]

How long did you give it to warm up? Make sure its warmed properly (might
be an hour or so) before adjusting things.

But even after that - no, it's very likely that there is nothing you can
do to completely adjust out this error. You might get lucky and find
an additional adjustment inside, but that's more and more unlikely
these days. If you don't see it on the front panel or in the OSD, it's
probably not there at all.

Bob M.

 

[VanguardLH]

in message

Yes, there generally are quite a few adjustments inside the monitor
but unless one is familiar with working on CRT's I'd not do it.
There is HIGH VOLTAGE in there!

And the adjustments on the yoke assembly are independent. Once you
find you went the wrong way with a few settings, you might not be able
to back out to what you had before unless you record exactly how many
turns or partial turns you made, in what order, and if you moved the
yoke itself. If you go inside, figure on if you fix it then fine, if
you don't then you already planned on discarding it.

You might also try changing the refresh frequency in Windows. You
might observe different effects in alignment that then you can adjust
out.

 

[Calab]

Someone just gave me a 19" CRT monitor, and I noticed a geometry
problem with it. It seems that the top edge and bottom edge of the
image are not parallel with the top and bottom of the monitor. The
right side of the top edge and bottom edge of the image touch the top
and bottom of the screen, but the left side of the top edge and bottom
edge are a few millimeters from touching the top and bottom of the
screen.

CRT monitors are designed to have about a 1/4 inch of blank display all the
way around the picture. Set it this way and you probably won't notice the
misalignment.

 

[larry moe 'n curly]

Someone just gave me a 19" CRT monitor, and I noticed a geometry
problem with it. It seems that the top edge and bottom edge of the
image are not parallel with the top and bottom of the monitor. The
right side of the top edge and bottom edge of the image touch the top
and bottom of the screen, but the left side of the top edge and bottom
edge are a few millimeters from touching the top and bottom of the
screen.

This is something that a trapezoid adjustment would fix, and the
monitor does have a trapezoid adjustment. But it only adjusts the
sides of the image, not the top and bottom.

You may want to ask in sci.electronics.repair because there are a lot
of TV/monitor technicians there. I think I've read there that a
problem like this is in the horizontal deflection circuitry, possibly
the damper diode, which may be a separate device or built into the
horizontal output transistor.

however I have since removed the yoke

Good luck reinstalling it exactly as it was and getting the color
convergence restored.

 

[Bob Myers]

Since when? Ideally, the image shoud cover 100% of the usable area.
(Of course, the tube itself is larger than that.)

That all depends on what you define as the "usable area," and
whether or not the opening in the monitor bezel encroaches on
that area.

In most CRT specs (the tube itself, not the monitor), there is a
"recommended image area" which is smaller than the full extent
of the phosphor screen. In part, this area is determined by what's
going on re the curvature of the inside of the glass faceplate as you
get to the edges of the tube (obviously, at some point that inner
surface is no longer anything even remotely resembling "flat," and
the geometry, etc., starts to go bad quicky in those regions. In
part, the area is also determined by the performance of the yoke
(at least a "typical" yoke as expected for that tube type, the focus
electronics, etc., convergence, etc., also going away as you get
close to the extreme edges. For monitor tubes, this results in
the optimum image area being somewhat smaller than what you
might expect from just looking at how big an area the phosphor
covers.

TVs are a bit different in that they are commonly "overscanned" -
i.e.., the scanned image actually extents beyond the edges of the
phosphor screen by about 5-10%. This basically hides edge
geometry and other problem by placing them where the viewer
can't possibly see them.

Bob M.

 

[Bob Myers]

Hmm... Can't say that I've ever heard of that. Doesn't make a lot of
sense, really - if the performance "sucks" beyond a certain
scan-angle, why have phosphor there? I suppose choice is nice...

Because of the way the phosphor is deposited. It's
basically poured into the faceplate glass as a slurry,
over a photoresist mask which is produced by using the
shadow mask or aperture grille as an exposure mask.
This results in phosphor being deposited well up the sides
of the glass, which is a good thing because then you also
need to aluminize the back of the phosphor (done later on
in a vacuum-dep process, after the funnel has been mated
to the faceplate) to provide the actual "2nd anode" of
the tube.

The distortions are still present near the edge of the phosphor screen
where they can be seen, of course (although certainly far less obvious
than they would be if you could see the "edge" of the scan.

Yes, but often hidden by the bezel of the TV.

Personally, I loath the concept of overscanning in televisions. I
know why it's done, but it throws-away information in a situation
(NTSC) that has none to spare. My last CRT TV had some service-menu
options to reduce/eliminate overscan, which I did, despite noticable
distortion of the image in the last half-inch on one side.

The practice should, of course, become less common with the move
to digital television and flat-panel display technologies, but unfortunately
it is still with us even with many HDTV transmissions.

Bob M.

 

[Bob Myers]

Well, when I wrote "why have phosphor there", I meant "why have
phosphor and shadow mask" there, or, phrased another way, why have
usable display area there (if one really ought not use it).

Again, because of the way the phosphor screen is produced.
and other needs (re the aluminization step I mentioned) to make
the CRT producible. "You can't easily NOT have phosphor
there" is the short form answer, and you don't WANT that
area to be completely open, for other reasons. But again,
just because it's there doesn't mean it's a good idea, from
the standpoint of focus, purity, convergence, and geometry
concerns, to actually try to use it. TV is another story, and
clearly much less concerned with this issues vs. the typical
monitor application.

Bob M.

 

[Bob Myers]

But you CAN easily not have shadow-mask there.

You don't exactly "have shadow mask" there in the first
place; the mask is, of course, separated from the phosphor
screen itself by a small distance. But you DO want to have
the phosphor in that area of a color CRT patterned with
RGB dots or stripes, hence the mask IS going to be able
to cover that region. If you were to design the tube otherwise,
any time the beam strayed outside of the intended image
area you'd get unwanted colors. Or else you'd fill the area
with some non-emissive material (at extra cost), and the image
would simply cut off (not a desirable result, esp. given that
the raster size and position cannot be held exactly). And the only
way to easily pattern the phosphor is to make sure that the mask
IS large enough so as to cover the region all the way to the
faceplate edges.

There are other alternatives between shadow-mask and completely open,
right?

Sure - but they'd cost more to build, and for no good reason.
As things are (or at least, as things WERE), the manufacturer
of a monitor tube would simply publish the image area within
which the performance specifications were guaranteed, and
the monitor designer tried not to have the product set up to
significantly exceed that. (If it did, though, it still worked, as
opposed to having the image simply cut off - it just was no
longer guaranteed to meet all of the performance specs.)
And again, TV tubes were a different story altogether - the
specs were much looser, and for that matter the geometry,
focus, etc. not as critical.

CRT and CRT monitor design was and is always full of
compromises.

Bob M.

 

[Bob Myers]

I suppose this thread has run it's course, but I still thinking that
it's "normal" to use that area. I know I max-out my monitors - I'm
not paying for area that I'm not using. 8)

The thread probably has run its course, but I did think of
one more item to note - if you don't want to "pay for area
you're not using," you should take a look at the televisions
of the very earliest days of broadcast TV, the late 1940s and
early 1950s. Raster-scan CRTs were still in their infancy
as consumer products, and the only way you could make
a set with acceptable image quality over a reasonable area
was to use this tube with a *round* faceplate (and
a very long funnel - a low deflection angle) and then just
show the viewer a part of that area through the opening
in the TV cabinet. Rectangular-faceplate tubes in which
the entire screen could be used acceptably and shown to the
viewer had yet to be developed.

Take a look at this:

http://www.tvhistory.tv/1946-RCA-621TS-7in.JPG

....a 1946 RCA TV - black and white, of course - with
a SEVEN INCH (typical size in those days!) round CRT
behind that roughly-rectangular bezel opening.

The development of color television using "tricolor"
tubes (in the 1950s) didn't help matters any, as it was incredibly
difficult at that time to maintain convergence, purity, etc.
over the area needed for an acceptable picture. Take a
look at the insides of this 1954 Westinghouse 15" model,
one of the very first color TVs offered for sale:

http://www.tvhistory.tv/We54chs33.jpg

(This set cost $1295.00, by the way, at a time when a
new Cadillac Eldorado convertible was under $5k!)

www.tvhistory.tv is a fantastic site, by the way - lots of
fun just browsing through the classic TV, esp. from this
era and earlier. Tom Genova, the webmaster there, has
done an incredible job assembling photos of old TVs,
service manuals, news items, etc., spanning the 75+ year
history of television.

The CRT has had a great run as the electronic display
of choice for most of the past 100 years, and I think
pretty much everyone in the industry feels a bit of a twinge
at its inevitable passing. But it has come to be time to move
on...

Bob M.