On Fri, 29 Apr 2005 02:43:31 -0700, Roy Lewallen
wrote:

I haven't kept up well with LED technology,
but I haven't heard of any LEDs that have a built-in regulator,
switching or otherwise.

Hi Roy,

Constant current LEDs have been around since the late 70s. They were
marginally common then, but I rarely see them now.

As for the spectrum of LEDs, the all require different battery
voltages to bring them to full output. Light power is always
expressed as a function of current, not voltage however. The voltage
is somewhat likened to the forward conduction knee.

IR LEDs have the lowest knee voltage, roughly at the same potential as
common diodes: 0.6V. Blue LEDs have the highest knee voltage, roughly
3V to 3.5V. The colors Red, Orange, Yellow, Green, Aquamarine, Blue,
UV exhibit the order of ascending voltage (and in the conventional
order of wavelength for a rainbow). The White LED is simply a Blue or
UV LED coated with phosphor (a solid state fluorescent light).

73's
Richard Clark, KB7QHC

Phosphor? Wow, that fact had missed me totally...
The lifetime rating on those white LEDs are incredible!!!
From seeing crt tubes degrade over time, I'd expect there to be some sort of
problem with the phosphor over such a long lifetime? A crt fails/dims due
to air entering the tube, possibly?

regards,
john

On Fri, 29 Apr 2005 09:05:27 -0700, "John Smith"
wrote:

A crt fails/dims due
to air entering the tube, possibly?

Hi Brett,

I've seen air enter the tube - perhaps, but there are more
conventional explanations. Those episodes of "seeing" it enter were
slightly less than catastrophic (accidentally broke the final seal,
which, as it happens, is the right way to de-activate them).
Principally it is the oxidation of the cathode (one reason why tubes
have scavengers) from what air had remained from the production cycle.

Back when I did TV repair as a teen, and then later in the Navy as
part of a Christmas charity project, we used what was called a
"rejuvenator."

This piece of gear would apply several hundred to a thousand volts
between the cathode and the first grid (usually a cylindrical shield
capped with a plate with a pinhole in it) and hold the voltage there
for a couple of seconds (manufacturer's recommended time). This would
produce a current that would rip the surface of the cathode open
revealing a new emitting area and the tube would come to life for
another 6 months.

Well, during the extended project in the Navy, we found any number of
tubes that refused this Lazarus treatment. We simply held the button
down for 30-60 seconds as they sizzled. Who cared? It often
convinced the tube that there was more to life than consignment to the
dumpster (the guys loved to give them the heave-ho to thundering
explosion). The only downside to this aggressive treatment was that
in ripping open the surface, part of the current was a debris stream
towards the grid that would occlude the pin hole. Sort of self
defeating when you think about it (and good reason for the recommended
couple of seconds instead of our heavy thumb).

All rather remote possibilities for the White LED. LEDs suffer other
problems in commercial service. Usually with aging where the
half-life is quicker than expected. Another problem is with what is
called sun bleaching (usually the problem of LED traffic lights). I
would suspect either, or both, to be issues with vendors and poor QA
or selection. There are a world of "ultra-bright" specifications that
wander the map.

73's
Richard Clark, KB7QHC

Richard is quite correct, tho to older hams, believe the
term he uses for "Scavengers", were called "GETTERS", and
were to claim any free gas's in a tube (this is anchient
history!). And, another method that was used , was an
"Extender", which was in reality, nothing more than a step-
up transformer for the filament, raiseing the voltage (and
hence the cathode temperature) by 1 to 2 tenths of a volt.
these were also (a variation of this principle) available
on telephone microwave carriers , where the klystrons cost
several hundreds of $$$ . These, normally set the voltage
at 6.3 volts, but you could (at your own risk) turn it as
high as 7 volts. But, normally didn't exceed 6.5 volts.
and to give an idea just how prudent, the receive local
oscillators is some of that equipment, (according to the
date codes), lasted in excess of 20 YEARS continious service!
It wasn't the phospors that deteriorated, but the free
electrons that the cathode could radiate!
And, as a side note, Southern Pacific, found an outfit that
would replace the filiment/cathode and rebuild those same
klystrons , at about 1/4 the price of the new ones- the
original equipment was made by Lenkurt , about 1960 vintage
and only , about 8 years ago was finally retired, replaced
with solid state, mostly digital, but some analog equipment!
If this hasn't boored you by now, dont know what it will take!

As info, Jim NN7K (really dates me) !

Richard Clark wrote:
On Fri, 29 Apr 2005 09:05:27 -0700, "John Smith"
wrote:


A crt fails/dims due
to air entering the tube, possibly?


Hi Brett,


Principally it is the oxidation of the cathode (one reason why tubes
have scavengers) from what air had remained from the production cycle.
Snip
Back when I did TV repair as a teen, and then later in the Navy as
part of a Christmas charity project, we used what was called a
"rejuvenator."
snip
This piece of gear would apply several hundred to a thousand volts
between the cathode and the first grid (usually a cylindrical shield
capped with a plate with a pinhole in it) and hold the voltage there
for a couple of seconds (manufacturer's recommended time). This would
produce a current that would rip the surface of the cathode open
revealing a new emitting area and the tube would come to life for
another 6 months.
snip again!
73's
Richard Clark, KB7QHC