4 Apr
2012
4 Apr
'12
11:28 a.m.
On 04/03/2012 04:07 PM, Tony Duell wrote:
Your belief is not a factor, you didn't work for DEC.
What the heck has that got to do with anything...
I stopped accepting the 'This is right because I say so' sort of arguemtn
when I was abotu 3 years old, and I have no intention of acceting it now.
It's very likely you and Ron are right, but, to quote the text over the
doorway of Kirkaldy's works in Southwark : 'Facts, not Opinions'.
All well
and good. One tiny point. I worked for DEC Terminals Product Lin=
e
when those items where in current production.=20
I could counter that by saying that while you may well hvee(SP?) designed
things that used said CRT, and may have handled the CRTs for many years,
this does not mean you know exactly how the CRTs themselves were
designed, and whether they remain sade(SP?) if you remvoe(SP?) the front glass layer
I don't beelvie DEC designed the CRT, they bought it in. I am nto(SP?) even
sure all came from one manufacturer, in which case it may well be that
some CRRTs(SP?) are safe without the outer glass, some are not. _I_ don't
know, and I'd rather not be showered in glass fagments to find out.
Rod and I both worked for DEC, I also worked with the
terminals people
during the Vt220, VT24x and VT320 series. DEC actually did interact
with CRT designers to get the exact product they wanted to DEC
specifications.
I am scpetical. If only because the CRTs in every DEC terminal I've been
inside has been a standard part with a normal type number.
Also, no manufacturer uses custom parts if they can avoid it. An
of-the-shelf part is cheaper and supply is more certain. And there is
absolutelly nothign odd about the dispaly circuitry in any VT series
terminal, or the performance requireds (resolution, etc) to require a
custom-made CRT
It's very likely DEC talked ot the CRT manufacturers, of course. But
mainly yo determine whcih CRT(s) would be suitable.
Anotehr thing, even if the CRT was custom-designed, I doubt very much the
requirment would have included 'Must be implosion-saft if soem hacker
removes the outer glass layer'. It would me much more likely to state
something like 'The CRT must have integral implosion protection to
<standard>) and the manufactuer decides how to comply with that. Given
that, any modication to the CRT could invalidate it.
I'm old enough to rember and worked on old tVs
that didn't have such
a safety layer. It was never an issue. For sport my brothers would
As am I...
FWIW, though, no TV set sold to the public in the UK ever had a CRT
pointing at the viewer without some kind of implosion protection. The
earlists sets (pre-wr) mounted the CRT vertically wit hthe screen on top,
the picture was viewed in a mirror. If the CRT imploded, the gass spreyed
upwards.
Later sets with the CRT conventionally mounted had a laminated glas or
(later) perspex sheild in front of the CRT. THen came tension bands and
laminated faceplate CRTs. Now, safety was takne a lot less seriously back
then, but they thougt this was enough of a danger for it to be necessary
to do something about it. And CRT implosion was, and is, not common.
take the
tubes (CRT) from junkers and haul them out back where they would
stand back 20-30 feet and pelt them with rocks. I've observed they
can be quite robust fromt he (SPELL!) front but quite fragile from the
neck side.
Sure, the scree nad flare are thick glass, they have to be to withstand
atmospheric pressure. It's darn hard to break them.
But that is not the point. We are not only worried about somebody hitting
the screen. The other daner is stresses in the glasss causing it to
fracture spontaneously. And that does happen (I have met a TV engineer
who had a CRT crack across the screen one day for no apparent reason).
That's the big problem it can happen without warning.
The tubes
were just fine without the face plate and met all implosion
requirements. Some (Green I think) did not have them at all.=20
Interesting. I am
suprised there are any differentces(SP?) between the CRt(case)
other than the phosphor. Why was this? Did all colours of CRT come from
the same manufacturer?
It probably is safe to run the CRT without the outer glass sheet, and I am
sure you know what you are doing.
[1] A common
way to do this was to use one of those neon tester
screwdrivers which would light up if touched on a live chassis. If so,
revese the mains conenctions. A _NOT FUNNY_ 'joke' was to convice the
newbie that if he took his screwdriver apart and turned the neon round,
it would then light on a dead chassis. (Of course it does no such thing,
it still lights on a live one).
Still dangerous as the mains line can still be intercepted. You do
learn to behave well
to live long. I alwas felt that anything mains powered without
isolation was designed to
kill anyone trying to repair them. Sort of the first generation
"No User Serviceable Components Inside".
Hmmm.. Although at least over here j-random-public would pull off the back
and swap valves...
To be honest, I don't mucu care for non-isolated TVs and radios. I much
prefer a nice mains transformer and parallel heaters. But I can see the
economics of it..
And if you repair old comptuers you are goign to come up against an SMPU
sometime. And in many (but not all) of those there will eb considerable
non-isolated circuitry. So you have to know how to work on it safely.
-tony