November 1968 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
When you were a kid, did your mother
warn you about sitting too close to the television because doing so would cause
you to be near-sighted or otherwise "ruin" your eyes? Mine did, and I'm now
very near-sighted, but it is doubtful that sitting too close to the TV is the
reason. In fact, according to Linus Van Pelt's comments to his sister, Lucy,
in this 1962 Peanuts comic strip, ophthalmologists tried to counter the misconception
about too-close boob tube
viewing. There is another strip where Linus challenges Lucy's assertion that
reading in dim lighting can hurt your eyes. The real concern as it turns out,
according to professional alarmists, was the massive doses of x-radiation pouring
out of the front of the early color TV sets. To hear the distractors tell it,
you could almost see the skeleton of anyone sitting in close proximity to a
color TV screen. The high voltage (25-35 kV) on the cathode ray tube (CRT) produced x-rays
which emanated from the front of the set. The issue raised a huge international
panic. I have posted a couple other stories about it, including "TV X-Rays Are
Back," from a 1969 edition of Radio-Electronics.
The Color TV X-Ray Problem
X-Rays One Year Later
It's been a full year since the TV X-ray
scare started and that scare is just now fading away. Set makers were prompt
in their reaction, and the new sets don't produce enough X-rays to worry about.
Every manufacturer has taken drastic and thorough steps to avoid any possible
recurrence. As a result, the danger to color TV viewers is almost nonexistent.
However, until color sets are built so that they produce no X-rays at all,
the guy who gets into the guts of a TV receiver - the service technician, the
hobbyist, the home-brew tube jockey - may find himself circumventing the protective
schemes the set maker built into his product at the factory. Therefore, anyone
thinking of working on color TV should observe the following rules.
• Never operate color picture tubes that do not have a bonded faceplate
without a sheet of safety glass in front of the screen.
• Don't operate a black-and-white picture tube in a color set unless
you use a sheet of safety glass in front of the faceplate.
• Don't operate a color picture tube with its purity shield removed.
• Never raise the high voltage above the manufacturer's recommendations.
• Don't operate receivers with the high-voltage shield removed.
• Always replace shielding and protective devices.
Follow these simple safety rules and you'll be safe from X-ray exposure.
X-Rays and Your Color Set
X-rays are dangerous. Color TV produces X-rays. Therefore - color TV is dangerous.
Logical bit of logic, isn't it? Fortunately, it's not all that logical. For
all-X-ray exposure is not hazardous. The real hazard depends upon the level
of intensity. The National Council on Radiation Protection and Measurement has
set up a minimum safe level of radiation from any TV set. It states that the
maximum exposure rate at any readily accessible point 5 cm (about 2 inches)
from the surface of any home TV receiver shall not exceed 0.5 mR (milli-roentgens)
This radiation level is about half the output of most luminous dial watches.
Obviously these are very conservative figures. Dr. Victor P. Bond, associate
director of the Brookhaven National Laboratory, has said that these ratings
"are deliberately conservative, and are set well below the levels of exposure
that carry a significant probability of medical effects ..." In other words,
there's not enough radiation to worry about.
But, and this is the big but, no one has ever demonstrated that X-rays can
do us any good - other than those a doctor may use professionally for specific
medical purposes. And it is my belief that the fewer X-rays I am exposed to,
the better. None of us would care to discover, 20 years from now, that those
low-level X-rays were dangerous after all.
Of course, the service technician who works on color sets a good part of
the day and the hobbyist who gets a lot closer to his set than an ordinary viewer
risk being exposed to much higher levels of radiation for longer periods of
time than the average person.
What can be done? Quite a bit, and most of it has already been done. The
major X-ray producer in TV is the high-voltage section. When a high-voltage
vacuum tube is subjected to 20,000 volts or more, it becomes an X-ray source.
And should the voltage in the set jump from say, 25,000 volts, to 30,000
volts, X-ray output may jump 10 to 20 times.
To date, the biggest culprit in color sets has been the shunt regulator tube.
But new circuits and tube designs have just about eliminated this problem. The
high-voltage rectifier is another potential trouble spot. One solution is to
replace the vacuum-tube rectifier with a solid-state rectifier that cannot produce
radiation at any voltage level. Another alternative is proper shielding.
Another possible solution is color picture tubes, like the Sony Trinitron,
that do not require the high-voltage levels that produce radiation.
In the meantime, TV service technicians must insure that the high voltage
on each color set they repair is properly set. Manufacturers must continue taking
steps to design sets that cannot produce X-rays.
Color TV's can produce X-rays. But the X-ray output of these sets is strictly
limited and manufacturers are checking their sets carefully before they leave
the factory. If and when the X-rays can be completely eliminated, and I believe
that they can be, we won't have to think about them at all.
-Larry Steckler, Managing Editor
Color Television Articles
Posted December 5, 2018