Hello Lynne and others,
You say,
>But yes I do remember a TV we had where you had to wait about 30 to
> 40 seconds after powering on the machine for the sound to pop up. Then
> the picture about 20 seconds later.
Ah, yes. That definitely was all-valve design. Some
trivia you might enjoy now is in order since that set is
probably long gone by these days.
Let's start with the screen. Many years ago, there was
the front of the CRT, the actual screen, and then a glass pane
mounted in front of it between you and the actual screen. You
probably had no idea, but it probably saved yours and many other
people's health and even their lives and I am not exaggerating by
too much.
The first CRT's were made of relatively thin glass and
all CRT's must be evacuated of almost every molecule of air in
order to work at all. You might say that the weight of the whole
atmosphere squeezes in on them from all sides at around 3 or 4
KG per centimeter. That doesn't sound like much but it is based
on surface area so a large screen has a noticeable amount of
force there all the time.
It's glass and all it takes is a blow to the screen or
somebody picking up the tube by the neck or, in the case of
something that might happen during service by a technician,
someone bumps the neck accidently, the glass shatters and the
tube will implode. One who doesn't know or care about the
details would say it exploded because the glass flies in all
directions. Occasionally, CRT's spontaneously implode due to
strains in the glass which were accidently manufactured in to
the tube when the glass cooled.
Then there was the voltage used to energize the CRT and
make the picture appear. On monochrome sets, it was around
20-thousand volts. Not much current, but enough to possibly be
lethal or at least knock you in to the next county if you
contacted it. That's why TV screens get such a static charge on
them. There is a huge charge behind the screen just looking for
a way out.
Color TV CRT's use more like 30 or 35-thousand volts for
their screens and so we get in to another hazard.
Both monochrome and color sets emitted what are called
soft X-rays. They normally did not really emit dangerous levels
but could under certain malfunction conditions which would not
be obvious to you, the average viewer. It was one of those
cases, though, of better safe than sorry so sets by the sixties
and seventies had safety shut-down circuits that turned off the
picture if voltages became too high due to component failure.
Others of the valves besides the CRT could also produce
X-rays, but those usually had metal shields around them and you
were relatively safe from them as they were hidden inside the
cabinet.
The CRT was also a giant capacitor, not by design, but
as a result of its construction. Old televisions had CRT's that
could hold a charge in dry weather for hours after having been
on and knock the daylights out of you if you touched what is
called the anode connection which protruded from the side of the
bottle. I speak from experience, here. This next part never
happened to me, but if you got shocked that way, you might
flench and hit the neck of the tube.
By the sixties, the screen was much safer as CRT
manufacturers began bonding a very, very thick pane of glass
right to the screen. That glass made it much stronger and also
was full of lead to stop X-rays.
The product safety agencies of every developed country
did all that was reasonable to keep the most dangerous designs
out of our televisions, but TV's really stretched the envelope
of technology back in the day and there, unfortunately, was the
occasional em plosion, fire or fatality resulting from accident
or mishandling.
Martin
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