akers@oliven.UUCP (Rob Akers) (01/28/86)
During the Superbowl we had two color tv's on, one to watch the game and the other to act as a monitor for the VCR which was recording the game. Watching two sets was distracting, so I placed a box (which happened to contain two small loudspeakers) directly against one screen to block the picture. I removed the box after about 30 seconds and found that the color on the set had changed drastically. There were now 3 horizontal "zones" of color on the screen. The top zone had an overwhelming red tint, the middle zone had an overwhelming blue tint, and the bottom zone had an overwhelming green tint. Thinking I had possibly knocked the antenna loose I checked the connections but they were fine. Adjusting the antenna and the tint control did not correct the problem. I continued to watch the set for about 15 minutes; the zones of color remained during this time. I turned the set off for a few minutes, turned it back on and the problem still existed. By this time I was concerned that the set had been permanently damaged, and I still didn't understand what had happened. I moved the box containing the speakers in front of the set, and the zones of color became much more intense. I turned the set off for about 30 minutes. When I turned it back on, the picture was fine.(thankfully!) Apparently the magnetic field from the speakers had caused the problem. There are two things I don't understand: 1) Why did the problem remain for so long (15 minutes) after the speakers were moved (far away) from the set? Is the picture tube capable of holding a magnetic charge? Is there metal sheilding around the tube which could hold a magnetic charge? Could this permanently damage the picture tube? 2) Why did the magnetic field cause the distinct red, blue, and green zones to appear? Is there a relationship between color (frequency ?) and the amount of deflection from the magnetic field? Is it related to the distance of the gun(s) or lens(es) from the magnetic field? Excuse the dumb questions, but physics was a long time ago and I am fairly ignorant of the inner workings of a color television set. Recommendations of good books on the subject would be appreciated. The set is a 19 inch Toshiba, about two years old. Any ideas or commments out there? Thanks in advance, Rob Akers
bl@hplabsb.UUCP (Bruce T. Lowerre) (01/31/86)
... > There are two things I don't understand: > > 1) Why did the problem remain for so long (15 minutes) after the speakers > were moved (far away) from the set? Is the picture tube capable of holding > a magnetic charge? Is there metal sheilding around the tube which could > hold a magnetic charge? Could this permanently damage the picture tube? yes, yes, yes. > 2) Why did the magnetic field cause the distinct red, blue, and green > zones to appear? Because it's a color TV (this sounds facetious, but true). > Is there a relationship between color (frequency ?) and the amount of > deflection from the magnetic field? No, the same electron guns are used for all three colors. > Is it related to the distance of the gun(s) or lens(es) from the magnetic > field? No. > Excuse the dumb questions, but physics was a long time ago and I am > fairly ignorant of the inner workings of a color television set. > Recommendations of good books on the subject would be appreciated. > ... > Any ideas or commments out there? You didn't ask the really good question. "Why did the problem clear up when you shut off your TV and then turned it back on?" Answer: The set probably has a built-in de-gaussing coil which is active only during warm-up. You lucked out in that the set was able to fix itself.
eric@apollo.uucp (Eric Peters) (01/31/86)
Automatic degaussing has been around so long that folks have forgotten (or never knew) that the shadow mask of a standard color TV becomes permanently magnetized from stray magnetic fields, and it needs to be "degaussed" occasionally. The shadow mask is a thin sheet of steel with a quarter million tiny holes in it right behind the TV screen. Its purpose is to assure that only one electron beam can shine on each colored spot on the screen. How it does this is another story. Even the magnetic field of the earth will eventually magnetize the shadow mask, especially if the set is moved around a lot. It used to require a service call to demagnetize it. But in all modern sets there is a "degaussing" coil around the face of the tube, and it is controlled by a thermistor. It's arranged so that when you turn the set on and the thermistor is cold, it operates the degausser. When the thermistor warms up after a few seconds, it turns off. When you put speakers, or any magnetic objects, near the TV screen, their permanent magnets magnetize a portion of the shadow mask. When it becomes magnetized, the electron beams are bent as they pass through it, and thus the beams don't get to their correct colored phosphors. All sorts of "wild" effects can happen to the color -- You saw one example. Note: A strong magnetic field can physically deform the shadow mask perm- anently. It is very thin and supported at the edges. If it should become bent, it cannot be repaired. So don't fool around with strong magnets near the screen of any color TV you care about. Anyway, once you let the set cool off (to cool the thermistor), it will automatically degauss the next time you turn it on. Sometimes it can take two or three cycles to completely clear the color. You may be able to find a serviceman's degaussing coil, which is much more powerful than the one in the set, and do a very thorough job. But the operation is a bit tricky. You can actually end up magnetizing the shadow mask worse than it was! Eric Peters (...decvax!wanginst!apollo!eric)
wmartin@brl-smoke.UUCP (02/03/86)
I've got a Zenith System 3 that has a color splotch in the lower right quadrant that comes and goes, sometimes being more noticeable than other times, and which is probably due to a degaussing problem. I've tried degaussing the set with a reel-to-reel tape bulk eraser, which produces a similar sort of rapidly fluctuating magnetic field to a regular degaussing coil, but, if it cleans it up, the effect is temporary. What I can't figure out is why such a magnetized area would would fade away and then return over a timespan of some months; I could understand it appearing and never going away (due to either a permanent damage to the shadow mask or to the failure of the set's degaussing circuits), or appearing once in various random locations and then going away when the set's internal degausser wiped it out, but why would it persist in the same region of the screen, fading slowly and strengthening slowly, over months and years? (One clue: it is in the area of the screen nearest to the set's own internal speaker -- could there just be a design defect allowing the magnet in that speaker to affect the picture tube internally? If so, it's another nail in the coffin on Zenith System 3 sets. Don't ever buy one of those dogs!) Will