Post #48,078
8/4/02 10:16:26 PM
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Do desktop monitors do this too?
Reason I ask is that my brother and myself have a monitor each that is really really dark but they are old and used. So dark, that in some video games it is really hard to see in them. Any fix for this, or just buy a new monitor?
A bummer that laptops cannot get a new LCD screen, it is built in.
I am free now, to choose my own destiny.
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Post #48,080
8/4/02 10:27:28 PM
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Yes.
I've seen monitors that are so dark with age, you can only just see that there's a picture.
Of course, in such a case, I doubt the phosphor would have worn out that much that quickly and would suspect another component that has worn out. But yes, monitors (and TVs) slowly dim over time.
Wade.
"Ah. One of the difficult questions."
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Post #48,086
8/4/02 11:19:03 PM
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Some possible degradation mechanisms in TV/monitor tubes.
I've got a 14" Sony that's about 9 years old that has a broad vertical stripe that is noticably dimmer than the rest of the tube. It doesn't have much life left.
Vacuum tubes like those in monitors and TVs work by having 3 essential parts: 1) a vacuum vessel (the glass tube), 2) a "cathode" - a thin tungsten or similar metal filament that his heated white hot to minimize the energy necessary to remove an electron from the surface. (It's called "thermionic emission".) The cathode is the source of the electron beam that's swept across the phosphor coating at the front of the tube. And 3) the phosphor. (Lots of other parts are necessary too - power supplies for accelerating the electron beam, power supplies for deflecting the beam, grids and plates for electrostatic fields to interact with the beam, etc. - but these are the most basic features.
Cathodes degrade over time. The tungsten can evaporate from the surface, coat insulators, and the energy necessary to remove an electron from the surface can increase due to contamination.
The vacuum seals can degrade over time. It's not really surprising - "Nature abhors a vacuum" after all. ;-) As the quality of the vacuum seals degrade, more atmospheric gases leak into the tube. The can include oxygen that will react with the hot tungsten filament, changing it's electrical properties and reducing its efficiency. More of the electron beam will be degraded by electron collisions with atoms in the gases. Etc.
And the phosphor can be degraded by electron bombardment. Lots of us have seen oscilloscope screens that are damaged by too much intensity at 0,0.
I'd suspect that cathode degradation and degradation of the vacuum are pretty common in old TVs and monitors that have been used for many years. But degradation in HV power supplies is also very common, as is failing electrolytic capacitors in almost any power supply that's part of the system. As you say, in modern monitors phosphor damage is pretty rare. It's not like the old days of green on black text screens that would easily get burned.
I must say, though, I haven't seen any article that discusses what's most common reason for darkening in Cathode Ray Tube screens.
In LCD screens, I believe it's common to have a flourescent lamp behind the screen for illumination. Fluorescent lamps also have HV cathodes and so they have similar degradation mechanisms to CRTs and office fluorescent lamps.
Cheers,
Scott.
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Post #48,088
8/4/02 11:35:07 PM
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Interesting data point there.
The monitor I mentioned was a Sony. Hmm.
Electrolytics are the biggest cause of spec drift in electronics. When working on an old device with lotsa 'lectros, if they fault isn't obvious replacing all the 'lectros is the accepted first step. Often works, too.
Wade.
"Ah. One of the difficult questions."
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Post #48,090
8/4/02 11:52:55 PM
8/4/02 11:57:35 PM
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Tungsten cathodes - not -
Tungsten is used for the filament, but raw filament cathodes went out around the time Edison died. Recent cathodes have a tungsten filament folded up inside a metal sleve (filament is coated with an insulating material so the folds don't short out).
The metal sleve is coated with a rare earth material that is a much better emitter of electrons than tungsten. Unfortunately, this coating will eventually become poisoned by stuff that evaporates off the internal components of the CRT.
Back in the '50s, when TV tubes were really expensive, there were "do or die" devices that would overvoltage the filament, hopefully not enough to blow it, but enough to boil the contaminents off the sleve.
Color tubes don't seem to burn images like monos did, thought the phosphor may degrade with time, so screen savers have been primarily entertainment in recent years.
Disclaimer: I am not an expert in cathode ray tubes, but having been involved with vacuum tubes since well before the CK722, and a [link|http://www.geekculture.com/geekculturestore/webstore/tubesrock.html|vacuum tube enthousiast] to the present day, and having had a father who worked as a process engineer (now known as a "manufacturing engineer") for Tung-Sol when it was a major manufacturer of vacuum tubes, I am not entirely unaware of issues involving cathodes (or anodes, for that matter).
-------
PS I own one of those big 4-pin carbon anode transmitting triodes just like on the Ts (see link). It's about 8" high.
[link|http://www.aaxnet.com|AAx]
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Post #48,091
8/5/02 12:04:49 AM
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You're right, of course. :-) Thanks.
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Post #48,107
8/5/02 7:30:28 AM
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Can ya ID the T-shirt tube?
Ancient - 4 pin. Rectifier is about all ya can do with 2 left after fil. It's before my time, tho I've seen most.. back to the De Forest Audion.
The demise of Heathkit and the other kit suppliers seemed to parallel the rise of dumbth - as well as cheap disposable electronics. Fewer and fewer it seems, are much interested in really understanding how things work. As teaching tools (while scoring a decent stereo in the process) - I think the logic of tubes was much easier to grok than 'holes' and current-controlled devices - as all semis are. Also bulletproof. (If occasionally lethal to the terminally sloppy with sweaty palms).
Anyway.. I think even the music was psych. enhanced in a dim living room - with two pair of KT-88s (and bright rectifier in background) epitomizing POWER you can See! Still have a box of (small signal) new tubes in garage.. Russia seems to be the new tube superpower (!). Early Tek portables had/have some tiny glass HV rectifiers and.. nuvistors! for the pre-FET input amps.. Now Those were cute! itty bitty ceramic tubes the size of a large xsistor - triodes thru pentodes.
{sigh} I resisted picking up a Keithly tube (+chopper) picoammeter on eBay.. You can do femtoamps with a National Semi op-amp now - in fact Bob Pease scratched out a circuit whereby you can notice a change of ~10 more/less electrons! Eerie almost.
Tubes \ufffdber Alles
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Post #48,143
8/5/02 11:26:25 AM
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It's an Amperex 211D
It turns out I have two of them. It's a a high power transmitting triode. One of the two heater pins is also the cathode pin (heater floats at cathode voltage). That leaves one pin for plate and one pin for control grid.
Anode is sintered carbon because metal would buckle at the temperture these things ran at. Hight 7-1/2", dameter 2-1/4"
[link|http://www.aaxnet.com|AAx]
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Post #48,262
8/5/02 8:53:44 PM
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The last glowing anode I saw was in the early 1960's,...
a ham friend of mine built a transmitter from scratch.
In the garage I have a Sylvania 23Z9, (I just checked) so I have a private mini-museum too! It's a Compactron, from the dying days of tubes.
Alex
"Television: chewing gum for the eyes." -- Frank Lloyd Wright
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Post #48,109
8/5/02 7:54:49 AM
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Original Sonys..
Some of those (KV-1212) I have seen operating over 20 years later - dunno their daily avge. usage - but of these a few had reached the poor-contrast stage. That seemed to be as much about gas as emission, but then all interact - the gas increases drain which asks more of the cathode and of HV current. Death spiral eventually.
The mission for an oscilloscope CRT is simpler (if more precise re beam dynamics). I have a number of scopes whose CRTs are now from 20 to near 40 years of age. (For the ones I have - the rest is (mostly) solid state) Their 'getters' worked/work well and the beam is as crisp as new on most. Of course these were made in Tektronix' own tube fab, state-of-the-art when that meant something -- it will be interesting to see if (like the occasional light bulb) -- some of these are still operating in 50 years. I'd bet a few bucks: they will. (Electrolytic caps will have been replaced, natch)
We'll never again see analog scopes of this quality, though there is one very expensive ('image-tube' + LCD final display) modern scope in production - which can sorta compete on writing rate = 550 MHz BW IIRC. Still doesn't equal the GHz Teks which shall eventually lose their emission, when used daily. (Most folks reserve these for tough jobs and do Not leave them on for routine stuff - but some Corp users don't care.)
Odd to see a scientific tool of undoubted utility: just disappear through [??] = with NO replacement in sight.
Ashton
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Post #48,123
8/5/02 9:47:04 AM
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The luminance isn't from the LCD
A bummer that laptops cannot get a new LCD screen, it is built in.
The LCD controls what light gets through, but the light itself is created by a [link|http://www.howstuffworks.com/question580.htm|Cold Cathode Fluorescent Lamp]. The light can be changed if needed.
Darrell Spice, Jr.
[link|http://home.houston.rr.com/spiceware/|SpiceWare] - We don't do Windows, it's too much of a chore
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Post #48,152
8/5/02 11:55:22 AM
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Had that happen to me a year or two ago
I normally work in a rather darkened office, so it's hard to tell when exactly it started kicking in, but when you start straining to see what's on a monitor in an office dimmer than something lit by a 50 watt incandescent bulb, it's easy to tell *something* is going wrong. Don't know how old the monitor was, either, but I don't *think* it was older than 5 years.
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