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Bakhruddin
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re: The incident of the shadow mask
Bakhruddin   1/12/2011 1:47:52 PM
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Your story is a familiar one. I worked for RCA in the Picture Tube R&D center for quite a number of years. Still miss the a/c hum of the hand degaussing coils and wands that were used in our PT test sets. A tremendous about of energe and $$ went into inventing designs to improve convergence and mis-register that resulted from less than perfect shadow mask / faceplate alignmnents and the results of electrostatic and magnetic coupling/degrouping electron beams - not to mention nonlinearities in the deflection system. Ah, the "good old days". Flat screen plasma and lcd came along and made all those display problems a thing of the past (and my job, as well!).

GPBobby
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re: The incident of the shadow mask
GPBobby   11/26/2010 12:11:19 PM
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During the late '70s I operated my own retail electronics business with an RCA franchise. The degaussing circuit was of limited value, designed to neutralize only slight magnetic perturbations such as reorienting the set with relation to the earth's magnetic field (aka rearranging the furniture) and the occasional invasion of the vacuum cleaner. The real determinate of picture quality, however, was the overall convergence, a process of aiming the electron streams to compensate for tube geometry. This compensation process was very complex (when changing a CRT) involving initially the positioning of the magnetic deflection coils (yolk) and a number of adjustable permanent magnets near the guns in the neck of the CRT. This was followed by electrical adjustments which basically altered the deflection current in the yolk to compensate for the physical distance differences in the length of beam travel for each color. As was mentioned, the convergence board, located near the top of the cabinet for "easy" access consisted of adjusting both R and L values for each color. There were both static and dynamic adjustments. These adjustment were made by looking at a mirror reflection of crosshatch pattern placed on the screen by a signal generator. Additional adjustments included color tracking to make sure the screen color stayed uniform over various brightness levels. My small business succeeded in competing with large furniture companies precisely because I converged every set before displaying it and again after delivery. The factory convergence was usually approximate and some adjustments were no doubt changed during shipping. It was also necessary to "burn the set in" with a dozen or so hours of operation to allow for thermal stabilization. The "big box" stores did not want or could not afford (after discount pricing) to perform this procedure. My pictures won out every time.

David Ashton
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re: The incident of the shadow mask
David Ashton   11/20/2010 12:21:15 AM
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I've never taken one apart, so I stand corrected. As the CRT takes a good 15 seconds to warm up, you're probably right on the second one too, though I have seen sets that still had some "waves" of colour on them when they first came on.

Flurmy
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re: The incident of the shadow mask
Flurmy   11/19/2010 4:28:10 PM
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"CRT monitors have a “degaussing” circuit consisting of a couple of turns of wire around the front of the screen." A couple of turns? Well, I never dismantled one, but I think the turn are much more. The wire resistance is about 25 ohms. "This is supplied from an AC source via a device called a posistor, which initially allows a large current to flow in the coil but over a minute or so reduces it to almost zero." A minute? Perhaps you mean a few seconds.

Flurmy
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re: The incident of the shadow mask
Flurmy   11/19/2010 4:23:55 PM
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I got many calls from my colleagues in the loudspeaker production line complainihg that the control computer screen was almost unreadable. My standard answer, before going down there to degauss, was: "how many times should I tell you to keep the loudspeakers far from the monitor?" Eventually the LCD computer screens became available but, being "cool", the first CRTs being replaced were, you guessed it, the ones of the boss; then the ones of the secretaries then, at last and much later, the CRT of the production line that were surrounded of strong magnets (not to speak of the magnetizer). That's life!

prabhakar_deosthali
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re: The incident of the shadow mask
prabhakar_deosthali   11/19/2010 11:24:35 AM
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In India, when I was in a TV manufacturing company, when technicians needed to do this trick at the customers place we had coined a term as "AARTI UTARO" . In India when we perform Pooja( Worship) of our god we light oil lamps and holding them in our hand we slowly move the lamp around the face of the god while singing some prayers. This is called Aarti. So whenever a customer complaint about such funny colors on the TV used to come to the service center, the manager used to instruct the technician to go to the customer's house and perform AARTI of the TV set ( of course not with the lamps but with a degaussing coil) By the way many of the magic acts performed by the famous magicians are based upon some simple principles of physics!

David Ashton
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re: The incident of the shadow mask
David Ashton   11/16/2010 4:19:23 AM
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One of my favourite quotes. To the customers it is, anyway.....

KD4GT
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re: The incident of the shadow mask
KD4GT   11/16/2010 2:14:50 AM
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Any sufficiently advanced technology is indistinguishable from magic.

David Ashton
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re: The incident of the shadow mask
David Ashton   11/14/2010 12:02:58 AM
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Didn't your Analyzer have a degausser built in? I have seen a TV that had the degaussing circuit go faulty and over time the colours got fairly zany. Degaussing IS a great way of impressing customers. It's a simple procedure, both in theory and in practice, but as Zeeglen said above, "I guess to the customers it did look like a magical witch doctor performing a exorcism ritual..."

timbo_test
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re: The incident of the shadow mask
timbo_test   11/13/2010 7:24:46 PM
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We used to sell test instruments that used Color display tubes. A few years ago I visited one of our customers, a university research lab. There in the corner was a fairly old Signal Analyzer hooked up to an external monitor. It was working UK, but the built-in display was just a mess of pink, blue and grey. "We used to like that box" but now the display is broken and we hardly use it at all", one of the research associates told me. "Have you tried degaussing it?", I asked him. "Oh?" he said, "what does that do?". The next time I visited I took an old degaussing coil from our repair centre and showed him! Less that a year later they traded in the old box, still working well, and bought a modern, up-to-date, replacement.

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