June 1969 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.
Color television was a big hit with homeowners and
was adopted fairly rapidly in the 1960s even considering the relatively high cost and
low number of network color broadcasts in the beginning years. The enthusiasm underwent
a severe reduction when word got out that large doses of
x−rays were streaming out of the front of the cathode ray tube (CRT) for sets
that did not take precautions to prevent it (which was the majority of sets initially).
The major cause was extremely high voltages applied between the electron gun and phosphorescent
raster grid - in the neighborhood of 35 kV or more - when the high voltage regulator
circuit malfunctioned. Note that even when everything was working properly, a small amount
of x−ray radiation was emitted. The x−ray problem received a lot of attention in the
electronics trade magazines which targeted service technicians because of their higher
than average close proximity exposure to energized CRTs. If a 25 kV zap didn't maim
or kill them instantly, long-time exposure to x−rays might do the job at a slow
rate. LCD and LED screens don't have that problem. See the "TV X-Rays" column in the
subsequent April 1970 issue of Radio-Electronics.
How the Cathode-Ray Tube Works,
TV X-Rays Are Back.
TV X-Rays Are Back
Suffolk County (Long Island), New York - A 14-month
study of 5000 color sets conducted by the Suffolk County Health Department indicates
that 20% of the sets were delivering excessive x-rays.
The study covered sets from 37 manufacturers, and at least one color receiver of each
brand was found to be emitting radiation in excess of the danger level (0.5 milliroentgens
an hour at a distance of two inches from the surface of the set).
The door-to-door survey was conducted by Seymour Becker, a physicist with the County
Public Health Service.
Using these figures in an extrapolation, three million of the 15-million color sets
now in use in the United States are emitting excessive x-rays.
Mr. Becker reports he found 15 separate causes for the excessive radiation, which
was being emitted in all directions. Mr. Becker said that all x-ray emissions, even non-harmful
ones "technically can be reduced to zero."
The amounts of radiation measured varied from 0.5 mR at 5 cm to as much as 150 mR.
The average offender emitted 2 to 5 mR.
Power supply voltages in the malfunctioning sets ran as high as 40,000 volts with
an average of 32,000 to 38,000. Normal high voltage is about 25,000 volts.
Editors Note: An excessive high voltage is almost always accompanied by x-ray emission
we recommend all technicians to check the high voltage of every color set they service
and make any needed repairs. To the set owner we urge that you look at your picture carefully.
If it is out of focus or narrow (black edges at the right and left) have your set's high
voltage checked immediately. The troubles just described are often produced by excessive
high voltage. And excessive high voltage is often accompanied by excessive x-rays.
Posted April 3, 2019