FROM THE BENCH
E-mail: fromthebench@gcmagazine.co.uk
Spot drop What are the tolerances for faulty TV screen pixels? Can they be cured? Alan Bennett reports he incidence of TV screen
semiconductor at that point. This is done in
pixel defects is reducing as
production wherever possible. I’ve still got
manufacturing techniques improve,
a CRT jacker machine which zapped away
manufacturers have their own standards
but still they crop up, and viewers, once
leakage caused by conductive particles
for pixel defects. Some publish them clearly
having spotted them, can be difficult
between electrodes. I also remember the
on their trade websites, some even on their
to deal with. TV setmakers have very
terrifying process, used in my workshop
public pages. Toshiba and Philips are very
specific rejection criteria for under-
under guarantee by the regunning man,
up front on this, as can be seen on their
guarantee claims.
of beating the glass faceplate of a picture
tesc.toshiba and atyourservice sites. Other
tube really hard with the sole of his shoe to
manufacturers seem to keep very quiet
dislodge particles from the shadow mask,
about bad pixels. What is sure is that none
A large TV screen contains over two million
thus curing black dots on the picture. Any
of them will exchange a screen unless it
pixels, each of which is made up of three
LCD and plasma screen dot picture faults
falls below the clearly defined standards
coloured dots (red, green, blue). Thus
are generally present from the start and
they set.
there is a total of well over six million dots
seldom come or go thereafter.
on screen, and if a few of them are faulty
The best way to see stuck pixels or cells
Possible cures
it’s still a tiny percentage of the whole. A
is to feed the screen with a black frame
There are various claims for pixel repair
complete ‘dead’ pixel makes a small black
from a pattern generator or pause a video
methods. Where the defect is a physical
dot on the picture, while a ‘stuck’ one
disc at a moment when the picture is all
one, like a foreign body, a rubbing or a
produces a much more intrusive (especially
dark, then look for white or coloured dots.
sharp tapping of the screen – getting
on dark scenes) small bright white dot. A
For best visibility of dead spots, get up a
progressively harder – on the bad spot
single dead element/dot/cell gives a small
full-screen white field and look for black or
with a small (say 5mm radius) rounded-end
coloured dot on screen (yellow, turquoise or
coloured dots. You need to be close to the
wand has been suggested and claimed to
purple) only when that part of the screen is
screen to see them well.
work. I would be afraid of damaging the
lit up: this is the least offensive defect. One
screen further in doing either of these
stuck-high element/dot/cell shows as a tiny
Tolerances
bright coloured dot – red, green or blue
Some setmakers in the top bracket only use
YouTube shows how to fix faulty pixels on a
– on all the time and thus again showing
screens with zero defects, selected from
up most on a dark part of the screen. My
the production run. Mass-market screens
picture above, shot in the workshop here,
can have a few pixel errors and still be
shows a green dot on a relatively small LCD
acceptable in the eyes of the manufacturer:
screen.
three bad cells on a 42-inch screen is one
plasma screen by hitting it with a baseball bat. Wow! Other methods seek to make a repair by electrical means: that is by feeding to the screen a special picture signal. It can come from a DVD or a software program – e.g. UDPixel and JS Screenfix. These cannot repair dead pixels, but are claimed to get rid of some kinds of bright or stuck pixels in LCD screens. They rapidly switch userselected colours on and off, thus agitating the appropriate dots and somehow effecting a cure. I have to confess that I have not tried them myself, but I would be very interested to hear from any reader who has, or indeed has cured pixel defects by rubbing or striking blows.
These defects may be present on
in 2,000,000 or 0.0000005%. In fact the
any type of screen, and on LCD and
criteria for replacement under warranty
plasma types they generally arise from a
used by screen and setmakers are based on
microscopic impurity or pollution of the
visibility at normal viewing distance; on the
screen substrate at some point during
type and size of the defects; and whether
manufacture, resulting in a short- or open-
they are clustered together. A general
circuit switching transistor – e.g. a TFT
standard, ISO13406 –2, sets out to precisely
driver, or a physical defect which might be
specify four levels of acceptance; of these
repairable (see later). With some types of
most manufacturers work to type 2, having
stuck-pixel defect it may be possible to zap
a maximum of two dead pixels per million
them out in the factory by addressing the
and two clusters of dead colour dots. You
faulty dot and electrically blowing off the
can find more details in Wikipedia.
to any great extent. One site I found on
JAN/FEB 2011
Defects
Individual TV and computer monitor
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