The mains varying field is minimized by use of a toroidal mains transformer, but the more recent mains powered speakers seem to be coming with *plug top* PSUs, which take the problem further away.

Magnetic fields don't really 'pull' on charged particles, they result in a force at right angles to the field lines with a direction dependent on the charge (negative for electrons, positive for protons) and field (North or South). The magnitude of the effect also depends on the energy/speed of the particles and their mass. For the case of a CRT:

If the field is horizontal with respect to the screen, the picture will mostly shift up or down.

If the field is vertical with respect to the screen, the picture will mostly shift left or right.

If the field is in the direction of the tube axis, electrons going toward the right will experience a shift in the opposite direction as those going toward the left (as the beam is deflected). Presto: The picture will rotate.

The rotation knob or setting ion some TVs and monitors varies the current in a coil wrapped around the CRT bell just beyond the neck which has its axis parallel to the CRT's axis and adds a magnetic field to counteract the component of the ambient field along that direction.

(From: Bob Myers (myers@fc.hp.com).)

No, it's not nonsense. The fields generated by the deflection coils, etc., ARE much greater in magnitude than the Earth's field, but they're AC fields. The DC offset of these fields is relatively small, and the Earth's field (also DC) IS sufficient to cause a visible shift in the position of the raster and affect the beam landing, etc.. This is why, for instance, there ARE often problems when trying to use a "Northern hemisphere" monitor in the Southern hemisphere.

Having said that, however, this isn't really something the average user needs to worry about. In the detailed specs for any monitor, there generally ARE a set of specific ambient conditions under which certain performance specs are intended to be checked. These usually include the ambient magnetic fields (which also tells you what magnetic environment was used at the factory for adjustment), and the orientation of the monitor within those fields. For the vast majority of monitors, the specified ambient conditions simulate average magnetic fields in the U.S. or Europe (which are very similar), and the monitor is specified as facing east or west within those fields. Strictly speaking, one has to establish those conditions (and so match the factory adjustment environment) in order to evaluate the monitor for compliance with these specifications.

Monitors are aligned in whatever field the manufacturer (or large OEM customer) SPECIFIES. This USUALLY involves an east or west alignment, as this results in no field component in the CRT's Z-axis (the axis "down the throat" of the CRT, perpendicular to the center of the screen).

However, this doesn't necessarily mean that optimum performance at YOUR location will be obtained with the unit facing east or west, as local fields can vary considerably from the specified nominal field. The field identified in the specs is just that - it is part of the conditions under which those specifications are to be checked.

But the *specific* conditions for a given installation can vary considerably from the nominal, and so the only advice I can give the individual user is that if you're happy with the performance, don't worry about it. If you DO think that a local DC field (the Earth's field or any other) is causing a problem, THEN try to move or rotate the unit to see if you can find a better orientation or location. Of course, *AC* fields, such as those from a nearby power line or electrical equipment, are something else entirely.

They use magnetic field compensation for the professional types. This is too expensive for us mortals, so we get a CRT that has been optimized for one field condition only: North, South or Neutral. Not all displays are CRTs. LCDs for instance are not sensitive to the earth magnetic field. And not all CRTs use a shadow mask for colour selection. For instance, in Tektronix colour oscilloscope they use a white CRT with a colour LCD shutter in front of it. That too would not be affected too much by the earth magnetic field.

You see, plenty of ways out for aircraft, ships, and the Space Shuttle.

The vertical component of the earth magnetic field varies as a function of latitude, particularly between hemispheres a vertical magnetic field will influence the color purity of a CRT.

The magnetic shielding of a CRT will, after degaussing, not provide complete compensation for the vertical field, especially for the space between shadow mask and screen.

That's why manufacturers produce different displays for different hemispheres: northern, southern and neutral. They do this by adjusting for optimum purity in a simulated magnetic field.

Re-adjust the purity, this involves moving the deflection coil, adjustment magnets, adding more magnets, etcetera. This is a big job and success would not be guaranteed.

Simulate a southern hemisphere location by creating a vertical magnetic field around the TV, put two big multi-turn wire loops (Helmholtz coils) above and below the TV and run a DC current through them. Might be expensive and certainly would provide a 'different' look!

Replace the picture tube with a northern hemishpere type, this is very expensive.

Mount the picture tube upside-down inside the TV cabinet. Then reverse the wires for the line (H) and field (V) deflection to put the picture correct side up again. For this case, you might have some problems with: The mounting nuts for the tube are hard to reach and may have left thread (look carefully before turning! The wires to the inverted picture tube panel being too short, they can probably be easily extended. The distance between high-voltage anode connection and the chassis (circuits) being too short (safety!) Condensation dripping into the anode contact. Bang & Olufsen once made a compact style television where they wanted the anode contact to be away from the top of the cabinet, so the back cover could fit tighter. So they mounted the tube upside-down. Consequently they had to order southern hemisphere tubes for a northern hemisphere TV set. That works, of course.



"Any truth to the rumor that how you position a projection TV in a room (N,E,S,W) can affect the image quality? Does the Earth's magnetic field truly have that much of an effect."

Yes, it is true.

It makes a difference whether you talk about a front or rear projector. Front projectors are expensive and critical enough that they will be converged after installation, so that takes care of any convergence errors. Purity errors are of course no issue with 3 separate CRTs...

Rear projectors are converged in the factory, the customer does only the static convergence (4 pots) after he has decided which direction the set will face. This takes care of problems due to the horizontal component of the earth agnetic field.

In a rear projector the CRTs are mounted almost vertically. The vertical component of the earth magnetic field causes a rotation error. Normally this is not an issue because that component does not depend on the orientation of the set and it is more or less constant over the entire continent.

It makes a biiiig difference though if you manufacture PTVs in Belgium and then export them to Australia... That means opening the cabinet and re-adjusting for rotation.

A front projector has its tubes mounted horizontally. The rotation error will depend on the direction the set is facing. This is easily adjusted through the convergence.

Back to CRT FAQ Table of Contents. Picture Quality and Appearance Issues Why Does the Intensity Appear So Non-Uniform in Bright Areas? Actually, the intensity variation is likely to be even worse than you might think - possibly as much as 2:1 from the center to the corners. In most cases you do not notice it. With large deflection angle tubes, fewer electrons make it to phosphor dots near the edge of the screen. It is simple geometry. (From: Bob Myers (myers@fc.hp.com).) It is extremely difficult for any CRT display to maintain perfect brightness and color uniformity across the entire image. Just the geometry of the thing - the change distance from the gun to the screen as the beam is scanned, the changing spot size and shape, etc. - makes this nearly impossible, and there can also be variations in the phosphor screen, the thickness of the faceplate, etc.. Typical brightness-uniformity specs are that the brightness won't drop to less than 70% or so of the center value (usually the brightest spot on the screen). On color tubes, the lack of perfect brightness uniformity is aggravated by the lack of perfect *color* uniformity and purity. What appear to be "dark spots" on a solid gray image may actually be beam mislanding (color purity) problems, which may to some degree be remedied by degaussing the monitor. Again, *some* variation is normal; if you think you're seeing too much, you can try degaussing the thing and seeing if that helps. If it doesn't, then the question is whether or not the product meets its published specs, and that is something you'll have to discuss with the manufacturer or distributor. Comments On Color Purity, Set Orientation, and Doming "The problem with my TV is that bright parts of the picture change color. For example, white areas may shift towards yellow or blue depending on the orientation of the set. What are the possible causes of doming? I have noticed that the magnitude of the doming effect varies with TV orientation even after degaussing several times at the new orientation. Does this help identify the cause of the doming in my case?" (Portions from: Jeroen Stessen (Jeroen.Stessen@philips.com).) The problem with regular shadow masks is 'doming'. Due to the inherent principle of shadow masks, 2/3 or more of all beam energy is dissipated in the mask. Where static bright objects are displayed, it heats up several hundred degrees. This causes thermal expansion, with local warping of the mask. The holes in the mask move to a different place and the projections of the electron beams will land on the wrong colours: purity errors. The use of invar allows about 3 times more beam current for the same purity errors. Both local doming and magnetic fields compete for the remaining landing reserve. Due to improper degaussing, the doming problem may be more visible. And applying a tube designed for the wrong hemisphere may very well increase the doming complaints. It is possible to deliberately offset the nominal landing in order to get more doming reserve (the shift due to doming is always to the outside of the tube). You would do this using spoiler magnets put in the right places. Permanently setting the contrast lower is not a real cure because the customer might not like such a dark picture. A better picture tube (Invar shadow mask) *is* a good cure (in most cases) but there is the cost price increase. (This is mainly due to the fact that Invar metal is harder to etch.) Also see the section: What is Doming?. Difference in Color Rendition Between CRTs (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).) There can be several reasons why primary colours (especially red) may look different between picture tube brands: Different phosphor composition. In the beginning everybody was looking for the phosphors with the highest luminous efficiency. Nowadays with the trend to avoid heavy metals, particularly cadmium, in consumer products the composition had to be changed. This shifts the colour point. Back scattering. Not all electrons that hit the shadow mask are absorbed. In fact, a quite high percentage is scattered back into the empty space between gun and mask. If they bounce back again from internal metal parts, then they may find their way to the screen and activate an arbitrary phosphor element. This increases the black level and reduces saturation of the primary colours. Red turns a bit towards orange. Even with good phosphors, large area colours will be less than perfect. Triple-CRT projection TVs do not have this problem, fantastic red! Colour filters. Toshiba has developed a process where they put individual colour filters between glass and phosphor. This makes the black much better and also improves the colour points when unwanted spectral lines are suppressed. There can also be differences with respect to the NTSC system, like wrong matrix from YUV to RGB. The definition in the Japanese NTSC system differs from the USA NTSC system and the signal processing should take that into account. Contour Lines on High Resolution Monitors - Moire These fall into the category of wavey lines, contour lines, or light and dark bands even in areas of constant brightness. (Some people may refer to this phenomenon as "focus or Newton's rings".) These may be almost as fine as the dot pitch on the CRT or 1 or 2 cm or larger and changing across the screen. If they are more or less fixed on the screen and stable, then they are not likely to be outside interference or internal power supply problems. (However, if the patterns are locked to the image, then there could be a problem with the video board.) One cause of these lines is moire (interference patterns) between the raster and the dot structure of the CRT. Ironically, the better the focus on the tube, the worse this is likely to be. Trinitrons, which do not have a vertical dot structure should be immune to interference of this sort from the raster lines (but not from the horizontal pixel structure). You can test for moire by slowly adjusting the vertical size. If it is moire, you should see the pattern change in location and spatial frequency as slight changes are made to size. Changes to vertical position will move the patterns without altering their structure - but they will not remain locked to the moving image. If they are due to the raster line structure - your focus is too good - the patterns will remain essentially fixed in position on the face of the CRT for horizontal size and position adjustments - the patterns will remain fixed under the changing image. How to eliminate it? If moire is your problem, then there may be no easy answer. For a given resolution and size, it will either be a problem or not. You can try changing size and resolution - moire is a function of geometry. Ironically, I have a monitor which is nicer in this respect at 1024x768 interlaced than at 800x600 non-interlaced. Some monitors have a 'Moire Reduction Mode' switch, control, or mode. This may or may not be of help. One way to do this is - you guessed it - reduce the sharpness of the beam spot and make the picture fuzzier! You might find the cure worse than the disease. Another cause of similar problems is bad video cable termination creating reflections and ghosting which under certain conditions can be so severe as to mimic Moire effects. This is unlikely to occur in all colors with a VGA display since the termination is internal to the monitor and individual resistors are used for each color (RGB). I think it is ironic that some people will end up returning otherwise superb monitors because of moire - when in many cases this is an indication of most excellent focus - something many people strive for! You can always get rid of it - the converse is not necessarily true! Moire and Shadow Mask Dot Pitch (From: myers@fc.hp.com (Bob Myers).) The density of the holes in the shadow mask set an upper limit on the resolution supported by that monitor. Lower resolutions work just fine; there is no need to have the logical pixels in the image line up with the physical holes in the mask (nor is there any mechanism to make this happen), and so you can think of this as the "larger pixels" of the lower-res image simply covering more than one hole or slot in the mask. As the effective size of the pixels in the image approach the spacing of the mask holes, individual pixels are no longer guaranteed to cover enough phosphor dots on the screen to ensure that they are constant color or constant luminance, but an image will still be displayed which ON AVERAGE (over a reasonably large area) looks OK. Actually, the specified "top end" format ("resolution") for most monitors usually is at or slightly beyond this point - the effective pixel size is somewhat UNDER the dot pitch. Isolated Spots on Display These could be a problem with the video source - bad pixels in the video card's frame buffer or bad spots on a camcorder's CCD, for example. Or, they could be dirt or dead phosphor areas in the CRT. Except for problems with the on-screen character generator, it is unlikely that the monitor's circuitry would be generating isolated spots. You can easily distinguish between video problems and CRT problems - missing pixels due to the video source will move on the screen as you change raster position. CRT defects will remain stationary relative to the screen and will generally be much more sharply delineated as well. There is a specification for the number and size of acceptable CRT blemishes so you may have to whine a bit to convince the vendor to provide a replacement monitor under warranty. Purple Blob - or Worse Have you tried demagnetizing it? Try powering it off for a half hour, then on. Repeat a couple of times. This should activate the internal degausser. See the section: Degaussing (Demagnetizing) a CRT. Is there any chance that someone waved a magnet hear the tube? Remove it and/or move any items like monster speakers away from the set. Was your kid experimenting with nuclear explosives - an EMP would magnetize the CRT. Nearby lightning strikes may have a similar effect. If demagnetizing does not help, then it is possible that something shifted on the CRT - there are a variety of little magnets that are stuck on at the time of manufacture to adjust purity. There are also service adjustments but it is unlikely (though not impossible) that these would have shifted suddenly. This may be a task for a service shop but you can try your hand at it if you get the Sams' Photofact or service manual - don't attempt purity adjustments without one. If the monitor or TV was dropped, then the internal shadow mask of the CRT may have become distorted or popped loose and you now have a hundred pound paper weight. If the discoloration is slight, some carefully placed 'refrigerator' magnets around the periphery of the tube might help. See the section: Magnet Fix for Purity Problems - If Duct Tape Works, Use It!. It is even possible that this is a 'feature' complements of the manufacturer. If certain components like transformers and loudspeakers are of inferior design and/or are located too close to the CRT, they could have an effect on purity. Even if you did not notice the problem when the set was new, it might always have been marginal and now a discoloration is visible due to slight changes or movement of components over time. Magnet Fix for Purity Problems - If Duct Tape Works, Use It! The approach below will work for slight discoloration that cannot be eliminated through degaussing. However, performing the standard purity adjustments would be the preferred solution. On the other hand, the magnets may be quick and easy. And, where CRT has suffered internal distortion or dislocation of the shadow mask, adjustments may not be enough. In any case, first, relocate those megablaster loudspeakers and that MRI scanner with the superconducting magnets. The addition of some moderate strength magnets carefully placed to reduce or eliminate purity problems due to a distorted or dislocated shadow mask may be enough to make the TV usable - if not perfect. The type of magnets you want are sold as 'refrigerator magnets' and the like for sticking up notes on steel surfaces. These will be made of ferrite material (without any steel) and will be disks or rectangles. Experiment with placement using masking tape to hold them in place temporarily. Degauss periodically to evaluate the status of your efforts. Then, make the 'repair' permanent using duct tape or silicone sealer or other household adhesive. Depending on the severity of the purity problem, you may need quite a few magnets! However, don't get carried away and use BIG speaker or magnetron magnets - you will make the problems worse. Also note that unless the magnets are placed near the front of the CRT, very significant geometric distortion of the picture will occur - which may be a cure worse than the disease. WARNING: Don't get carried away while positioning the magnets - you will be near some pretty nasty voltages! (From: Mr. Caldwell (jcaldwel@iquest.net).) I ended up with the old 'stuck on a desert island trick': I duck taped 2 Radio Shack magnets on the case, in such a way as to pull the beam back.!!!! A $2 solution to a $200 problem. My friend is happy as heck. RCA sells magnets to correct corner convergence, they are shaped like chevrons and you stick them in the 'right' spot on the rear of the CRT. How Much Tilt is Acceptable? This was in reply to a concern that a 1 degree tilt on a 27" TV was a problem. Yes, you may not like it, but unless there is a user tilt adjustment, the laws of physics prevail! (From: David Kuhajda (dkuhajda@mail.locl.net).) A 1 degree tilt given the effect of the earth's magnetic field is well within tolerance for a 27" TV set. The larger the picture tube, the more the tilt effect of the earth's magnetic field is noticeable. Even a shielded speaker may have just enough magnetic field to cause some slight tilt. 1 degree, however, is anything but a serious problem. Probably you would notice it you turned the TV 180 degrees on its axis that the tilt would then be going the other was. Factory standard is to have the picture straight when the back of the TV set is facing magnetic north. The actual measured tilt we have seen is as much as 3 degrees on a 36" tv set. This is why the higher-end larger TV sets have an adjustment for picture tilt. What is Doming? The shadow or slot mask inside the CRT is a thin sheet of steel or InVar positioned a half an inch or so behind the phosphor screen. The flatter the screen, the more susceptible it will be to thermal expansion effects: With individual phosphor dots spaced as as little as .13 mm apart (for a .22 mm dot pitch CRT), it doesn't take much inaccuracy in their position to result in a noticeable effect. (See the section: How to Compute Effective Dot Pitch.) As a result, high resolution CRTs tend to be more susceptible to doming problems. (Portions from: Jac Jamar (jamar@comp.snads.philips.com).) Doming is a deformation of the shadow mask or its support structure caused by heating and subsequent expansion in bright (high beam current) areas of the picture. This causes a shift in position of the finely spaced holes or slots in the mask. The result will be color purity problems - discoloration and brightness variations. For a .28 mm dot pitch CRT, a change of only .14 mm in the position of a hole or slot can totally shift the display from one of the primary colors to another. InVar shadow masks can sustain a significantly higher current density than steel shadow masks (by as much as 3:1) without noticeable problems. Trinitrons are more resistant to local doming effects as long as the wires are under enough tension. However, expansion of the suspension components can still result in doming with an overall bright picture. The onset and disappearance of color purity problems will generally lag the cause due to the thermal mass of the affected components. For local heating resulting from picture highlights, this will be only a few seconds since the thermal mass of a small area of the mask is not that great. However, for effects having to do with expeansion of the suspension or support structure, it may take up to 30 minutes to reach equilibrium. The orientation of the TV or monitor with respect to the earth's magnetic field and even whether the CRT was set up for the Northern or Southern hemispheres may affect the resulting color shift. Thus, the picture may tend toward yellow while the monitor is facing one way and blue when rotated 180 degrees on its base (even if degaussed at each position). Reducing the brightness/contrast or setting the brightness limiter will prevent doming but may result in an unacceptably dark picture. Shadow mask doming in itself is not something that becomes defective and has to be repaired. It is a characteristic of the CRT assembly. However, shifts in the position of purity adjustments can results in increased sensitivity to slight doming. Purity problems resulting in discolouration and/or brightness variations are due to mislanding of the microscopic electron beams (the electron beams after the mask) on the red/green/blue phosphor stripes or dots. The mislanding is in general caused by: Influences of ambient magnetic fields (such as the earth magnetic field).

Shadow mask doming.

Tolerances occurring in the production of CRTs.

Less than optimal setup of the purity adjustments (yoke position, rings on CRT neck, etc. Only when the sum of these influences exceeds the 'guardband' provided in the CRT design, discolouration (or brightness variations) becomes visible. If discolouration complaints arise, this will normally not be due to changes in doming behaviour, but to changes in shielding against magnetic fields. The ambient magnetic fields are shielded by means of iron components inside (or sometimes outside) the tube, which have to be 'degaussed' to give good shielding. For this in a set degaussing coils and circuits are provided. A discolouration complaint will thus often be due to insufficient degaussing. TV sets and monitors which are kept in 'stand-by' mode for a long time may never be degaussed adequately because the degaussing circuit may only operate for a short time after the unit is switched on from cold - whether this is so with your unit depends on the design). In this case, they can pick up magnetic fields from magnets moved nearby or other equipment. The solution in this case is to switch the TV or monitor completely off or pull the plug if in doubt, let it cool down for half an hour or longer and switch it on again. If necessary this procedure can be repeated a few times (I had to do this at home once when my children had been playing with magnets). For monitors with degauss buttons, you can usually hear a hum when the degauss is activated.

Similarly, if the orientation of a unit with respect to the earth's magnetic field is changed, it requires degaussing. So if you put your TV in another corner of the room or rotate your computer monitor on its tilt-swivel base, you have to activate its degauss circuitry (by letting it cool down or in the case of a high-end monitor, using its degauss button) or degauss it manually (see the section: Degaussing (Demagnetizing) a CRT).

The PTC resistor (thermistor or posistor) in the degaussing circuit can become defective. This prevents proper degaussing after switch-on. Since lower resolution CRTs are used for most TVs compared to similar size computer monitors, doming would not be nearly as much of a problem if they were both run at similar brightness (energy density) levels. However, TVs are very often used at higher brightness levels resulting in more of a thermal load on the mask which offsets the lower resolution. Afterglow - Phantom Patterns on CRT After Shutoff Why is there a splotch of colored light at the center of the CRT after I kill power to my TV? Why does this not happen if the plug is pulled instead? It seems to last for hours (well maybe minutes at least). (Portions of the following from a video engineer at Philips.) A broad diffused glow (not a distinct spot in the middle of the screen) that lasts for a few seconds to minutes is called 'afterglow' and may be considered 'normal' for your model. The warm CRT cathodes continue to emit electrons due to the high voltage that is still present even though the signal circuits may have ceased to operate. For more sharply defined spots there are two phenomena: Thermal emission from a cathode that has not yet cooled off (and this could take several minutes) gives a more or less circular spot near the centre. It is actually 3 spots from the 3 cathodes, we at Philips call them 'Christmas balls'. Field emission from sharp whiskers on any electron gun part gives a much sharper spot, sometimes with a moon-shaped halo around it. Even with the filament off, there may be some electron emission from these sharp points on the cold cathode(s) due to the strong high voltage (HV) electric fields in the electron gun. I do not know how likely this is or why this is so. The shape of the spot is an inverted image of the shape of the emitting area(s) on the electron guns cathodes. The visibility of both effects depends in the same way on the decay time of the high voltage (HV/EHT) on the anode. When turned off with the remote or front panel button, you are not actually killing AC power but are probably switching off the deflection and signal circuits. This leaves the HV to decay over a few minutes or longer as it is drained by the current needed to feed the phantom spot or blob. When you pull the plug, however, you are killing AC input and all the voltages decay together and in particular, the video signal may be present for long enough to keep the brightness (and beam current) up and drain the HV quickly. Whether this actually happens depends on many factors - often not dealt with by the designers of the set. A proper design (who knows, yours may simply have been broken from day 1 or simply be typical of your model) would ensure that the HV is drained quickly or that the other bias voltages on the CRT are clamped to values that would blank the CRT once the set is off. If the problem developed suddenly, then this circuitry may have failed. On the other hand, if it has been gradually getting more pronounced, then the characteristics of the CRT or other circuitry may have changed with age. In most sets it is left to chance whether the picture tube capacitance will be discharged by beam current at switch-off. It may simply be due to the behaviour of the video control IC when its supply voltage drops that causes the cathodes to be driven to white and this may not be formally specified by the manufacturer of the IC. Some of of the latest sets have an explicit circuit to discharge the EHT at shutdown. As noted in the section: "Safety guidelines" the HV charge on the CRT capacitance can be present for a long time. A service technician should be very aware of that before touching HV parts! Interestingly, most sets for the Asian Pacific market have a bleeder resistor built in that will discharge the EHT without the need for a white flash at switch-off. These will in fact drive the beam to black at switch-off via a negative voltage to the CRT G1 electrode. The AP market is very sensitive to proper set behaviour, they don't like a white flash. In short, it all depends on the demands of the particular market, the chance of the picture tube producing a spot/blob, and the mood of the designer. So, it may not be worth doing anything to 'fix' this unless the splotch is so bright (more so than normal video and for an extended time) that CRT phosphor damage could result. This is usually not a problem with direct view TVs but would definitely be a concern with high intensity projection tubes. On the other hand, your phantom blob may provide for some interesting conversation at your next party! Discussion on the causes of color flare On the right side of high intensity colors, some CRTs will exhibit a flare - the color will appear to be stuck at its highest level. This often occurs with older CRTs even at modest drive but can usually be forced to happen with any CRT if the drive level is turned up very high. (From: Andy Cuffe (baltimora@psu.edu).) I think it's due to the electron gun clipping when it's overdriven. Even a new CRT will bleed if it's driven hard enough, but most TVs are designed so you can't turn up the contrast that much. Once the CRT goes into clipping, it must take a short time to start working normally again after the drive level falls below clipping. The same thing happens when certain problems develop in the video amp. All CRTs do it when they get weak enough. Samsung seem to be worse than most. In general, all CRT manufacturers have been cutting costs. A larger percentage or 8 year old or newer TVs that I see have bad CRTs than ones that are more than about 16 years old. I just picked up a heavily used 1982 Zenith from the side of the road and it has a better looking CRT than most new TVs. (From: Michael Shell (mikes1987@yahoo.com).) I suspect that you may be right. I used to work a lot on older tube color televisions and I don't recall the bleeding problem. The first time I remember seeing it was on a 1978 Sampo. I have seen it EVERYWHERE since then. Has anybody seen the problem on a, say 1970, tube type set with good video drivers and correct CRT voltages (so you know it to be the CRT). (From: Andy.) My theory is that a weak CRT (or a good one with reduced G2) represents a higher impedance load to the video output transistor. The biasing of the video outputs would have to be designed for the load created by a good CRT. When the output load impedance goes high enough, the voltage can go high enough to saturate the transistor (the CRT isn't pulling enough current to keep the C-E voltage from going close to 0). tubes, being like FETs (in that they are majority carrier devices) which are used in high speed digital circuits because don't have any delay in getting out of saturation. (From: Michael.) I think we may have a winner. A scope on the cathode should be able to confirm it. (From: JURB6006 (jurb6006@aol.com).) Hmm, that's one of most technical questions I've heard in a while. First of all, realize that most sets do this with a lowered G2 voltage. While on the surface, lowering the G2 seems to mimic a weak CRT, this is not the case. Only in some ways it does, apparently. Some sets have an unusual resistance, and others seem to have an unusual propensity to "bleed" (we call it flaring at my shop). I was around before ICs, and have had an opportunity to study the design of the video output circuit(s) of TVs without ICs. I do understand circuit design, and have reached the following conclusion (this completely excludes sets with AKB): It depends on how the video output transistors are driven. It seems that if there is a path for current feedback, the set will flare. This is the worst in sets that drive the emitters of the outputs. Other sets, which could be designated as "voltage drive sets" either have such solid drive to the emitters that CRT load doesn't affect it or they just drive the base(s) of the outputs. Engineering-wise, I think it basically boils down to the output impedance of the video output stage, and on a single ended stage, as most are and were, this impedance is different for negative and positive going slopes in the waveform. There are other theories, this is only one. I speak of this because the flaring effect was almost always noticed on some chassis, back when all the CRTs were interchangeable. It followed the set, not the CRT. (From: Michael.) OK, I agree with what you are getting at here. Consider what happens when a transistor is overdriven. There are so many excess carriers in the device that it takes a long time for recombination to occur. This delay will result in the transistor taking a longer than normal time to switch off. As noted above, often bad video driver circuitry/designs can cause bleeding (flaring) too. These cases could be caused by overdriven transistors (see above), feedback loops, or some type of ringing effect. I am interested in this as well, but my real fascination is when the CRT is the trouble maker. Unlike a semiconductor, a CRT is a pure majority carrier device - no holes, just electrons flying around in there. What bothers me is this: Say we have a test pattern consisting of a solid red square in the middle of an otherwise black screen. We turn up the saturation/contrast (and have a weak CRT), we will see bleeding to the right of the square. Instinctively, we FEEL we know what is going on. But think about it. The instant the electron beam leaves the square, the voltages on the CRT grid/cathodes are such (or should be) that the red gun should be shut off. (It is only like a nanosecond from the gun to the screen.) If the CRT cathode is weak, or the G2 voltage is too low, then I would expect the beam to cutoff even faster! Yet, the phosphors in the bleed DO see electrons exciting them! So, what is happening here? Did charge somehow build up somewhere in the tube? OR has the tube changed properties in such a way to cause trouble for the video output stages in a manner which would cause problems like Jurb6006 suggested? In the later case, it should be possible to design or modify the output stages to be resistant or immune to this problem. (I am not suggesting it would be worth it though). The thing I really despise is that it seems to happen on CRTs that still have perfectly acceptable brightness. (From: JURB6006.) When they flare, yet the CRT isn't weak, it is usually due to a low collector Supply voltage to the vid outs. Unfortunately there is no way to tell on a normal scope whether the effect is being driven to the CRT or if the effect is IN the CRT. Actually on what I call "voltage driven" units, you CAN see some type of clipping when either the CRT is weak or the G2 is low, but let's say on an older Sony, it seems like the clipping is omnidirectional. Yet when this happens, the purity is not extremely affected. On the sets that flare profusely, is it possible that the designers stumbled on a rudimentary form of AKB? (From: Asimov (warpcastgate@dynip.com.) It's analogous to how speakers and amps distort when the clip. When the clipping occurs in the CRT it's bandwidth falls to zero and you see then a type of ringing or smear. It's like the beam gets cutoff then saturates repeatedly very fast at a video rate. Also the electrostatic voltages which set the convergence get all thrown out of whack when this happens. This last is why a CRT with weak emission will also show poor purity, bad convergence, and a loss of tracking. (From: Andy.) I wonder if it would be possible to modify the video output circuit to eliminate the bleeding in a TV with a weak CRT that's not too far gone to be usable? (From: Michael.) I plan to try this in the next month or so. If I have any luck, I will post the results. In order to do this, voltages are gonna have to climb to keep that transistor out of saturation. This is going to result in more heat. Could it be possible that energy conservation mandates from the government resulted in flaring? If so, the same thing that causes my CRTs to flare also causes my toilet and shower to lose power. From the point of view of the cathode, the CRT is a current controlled device like a BJT. From the point of view of the grid, it is a voltage controlled device, like a MOSFET. I can't remember why the video drive is applied to the cathodes rather than the grids, but I know there was/is a good reason. So, barring a radical change, such as CRT grid drive, I think we want to use BJTs, but we need total control over Ik, regardless of how Mr. CRT feels. So, we would drive the CRT cathode with the NPN BJT collector, and have a fairly large Vce - which implies a bigger transistor with more heat sinking. I had thought of an alternative - using a transistor that comes out of saturation faster, but as you mentioned before, we lose detail in saturation and thus would still not like the resulting picture. Another simple solution would be to "tweak a few V's". Instinctively, I feel we can do this. Then why haven't the OEMs? I thought up some humorous explanations for flaring during the course of this discussion. Maybe somebody will get a laugh: A crust spot on the cathode, which only emits at high drive levels, causes the electrons that pass through it to have a reduced velocity - by a factor of a 10000. These delayed electrons form the flare. The front of the cathode starts to wear out. Under high drive levels, electrons are emitted from the BACK of the cathode. The G2 voltage then pulls them around, but the result is a corkscrew spiral path to the screen whose total linear length is on the order of a 1000 feet. Hence the delay. Purity is not affected due to "circular symmetry". A crust forms on the cathode. Under high drive levels, electrons are trapped within the layers of this crust. Even when the video drive cuts off current to the cathode, the trapped electrons continue to leak to the surface resulting in a flare. This is known as the "cathode becomes a capacitor" theory. The phosphor of old CRTs become "flaky". When hit by electrons, ionic radiation is emitted radially outward. This radiation makes other adjacent phosphors super sensitive to future electron exposure. Hence, we are able to see a nearly zero electron beam which results in the flare. And finally: It's all in your head. Buying a new TV raises endorphens in the human brain, fixing the problem for a couple years. This also explains why more expensive, feature rich TVs have the problem less often. The CRT restorer should be applied to the USER not the TV. In skillful hands, it can also cure one of "color vision" allowing the use of much more inexpensive B/W sets.



Back to CRT FAQ Table of Contents. Magnetic Fields and Degaussing Degaussing (Demagnetizing) a CRT Degaussing may be required if there are color purity problems with the display. On rare occasions, there may be geometric distortion caused by magnetic fields as well without color problems. The CRT can get magnetized: if the TV or monitor is moved or even just rotated.

if there has been a lightning strike nearby. A friend of mine had a lightning strike near his house which produced all of the effects of the EMP from a nuclear bomb.

If a permanent magnet was brought near the screen (e.g., kid's magnet or megawatt stereo speakers).

If some piece of electrical or electronic equipment with unshielded magnetic fields is in the vicinity of the TV or monitor. Degaussing should be the first thing attempted whenever color purity problems are detected. As noted below, first try the internal degauss circuits of the TV or monitor by power cycling a few times (on for a minute, off for at least 20 minutes, on for a minute, etc.) If this does not help or does not completely cure the problem, then you can try manually degaussing. Note: Some monitors have a degauss button, and monitors and TVs that are microprocessor controlled may degauss automatically upon power-on (but may require pulling the plug to do a hard reset) regardless of the amount of off time. However, repeated use of these 'features' in rapid succession may result in overheating of the degauss coil or other components. The 20 minutes off/1 minute on precedure is guaranteed to be safe. (Some others may degauss upon power-on as long as the previous degauss was not done within some predetermined amount of time - they keep track with an internal timer.) Commercial CRT Degaussers are available from parts distributors like MCM Electronics and consist of a hundred or so turns of magnet wire in a 6-12 inch coil. They include a line cord and momentary switch. You flip on the switch, and bring the coil to within several inches of the screen face. Then you slowly draw the center of the coil toward one edge of the screen and trace the perimeter of the screen face. Then return to the original position of the coil being flat against the center of the screen. Next, slowly decrease the field to zero by backing straight up across the room as you hold the coil. When you are farther than 5 feet away you can release the line switch. The key word here is ** slow **. Go too fast and you will freeze the instantaneous intensity of the 50/60 Hz AC magnetic field variation into the ferrous components of the CRT and may make the problem worse. WARNING: Don't attempt to degauss inside or in the back of the set (near the CRT neck. This can demagnetize the relatively weak purity and convergence magnets which may turn a simple repair into a feature length extravaganza! It looks really cool to do this while the CRT is powered. The kids will love the color effects. Bulk tape erasers, tape head degaussers, open frame transformers, and the "butt-end" of a weller soldering gun can be used as CRT demagnetizers but it just takes a little longer. (Be careful not to scratch the screen face with anything sharp. For the Weller, the tip needs to be in place to get enough magnetic field.) It is imperative to have the CRT running when using these whimpier approaches, so that you can see where there are still impurities. Never release the power switch until you're 4 or 5 feet away from the screen or you'll have to start over. I've never known of anything being damaged by excess manual degaussing as long as you don't attempt to degauss *inside* or the back of the TV or monitor - it is possible to demagnetize geometry correction, purity, and static converence magnets in the process! However, I would recommend keeping really powerful bulk tape erasers-turned-degaussers a couple of inches from the CRT. Another alternative which has been known to work is to place another similar size monitor face-to-face with the suspect monitor (take care not to bump or scratch the screens!) and activate degauss function on the working monitor. While not ideal, this may be enough to also degauss the broken one. If an AC degaussing coil or substitute is unavailable, I have even done degaussed with a permanent magnet but this is not recommended since it is more likely to make the problem worse than better. However, if the display is unusable as is, then using a small magnet can do no harm. (Don't use a 20 pound speaker or magnetron magnet as you may rip the shadow mask right out of the CRT - well at least distort it beyond repair. What I have in mind is something about as powerful as a refrigerator magnet.) Also see the juggler's technique, below. :-) Keep degaussing fields away from magnetic media. It is a good idea to avoid degaussing in a room with floppies or back-up tapes. When removing media from a room remember to check desk drawers and manuals for stray floppies, too. It is unlikely that you could actually affect magnetic media but better safe than sorry. Of the devices mentioned above, only a bulk eraser or strong permanent magnet are likely to have any effect - and then only when at extremely close range (direct contact with media container). All color CRTs include a built-in degaussing coil wrapped around the perimeter of the CRT face. These are activated each time the CRT is powered up cold by a 3 terminal thermister device or other control circuitry. This is why it is often suggested that color purity problems may go away "in a few days". It isn't a matter of time; it's the number of cold power ups that causes it. It takes about 15 minutes of the power being off for each cool down cycle. These built-in coils with thermal control are never as effective as external coils. Note that while the monochrome CRTs used in B/W and projection TVs and mono monitors don't have anything inside to get magnetized, the chassis or other cabinet parts of the equipment may still need degaussing. While this isn't likely from normal use or even after being moved or reoriented, a powerful magnet (like that from a large speaker) could leave iron, steel, or other ferrous parts with enough residual magnetism to cause a noticeable problem. If you try the 'technique' below, make sure you don't smash the TV or your spouse! (From: Mike Champion (mchampfl@gdi.net).) I replaced the magnetron in my microwave and ripped apart the old one with my kids to 'see how it works'. Boy, there are some mother magnets in there! The kids and I had fun with them. You know - push me pull you; the paper clip boat; which Easter egg has the metal and which has the wood; etc. Dnough with this kid stuff - 'wanna see something really cool?', says I. Having been around monitors for a long time in the computer business, i showed them what what a REALLY powerful magnet will do to an electron beam in a cathode ray tube - my sharp 19" color TV. "Wow, dad!", "psychodelic!" "it looks like all the colors are flushing down the toilet!" Boy, was I DAD or what? The problem was that my experience with magnets and monitors were in the monochrome days! so the price I paid for such esteem in the eyes of my children were purple faces and green legs on my sharp 19" color TV! Uh-oh! well, maybe it will be allright by tomorrow. Well it wasn't. Now I'm getting worried! I used to do computer support at a television station so I called an old engineer friend there for help. He just hee-hawed! As he was drying his eyes, he suggested that I had probably just magnetized the mask and he'd loan me a degausser. I offered to buy him lunch for the favor. This was Friday and because of my friend's diagnosis T was able to relax about the problem enough to think about it. Hmmmm. degausser = alternating magnetic field... Strong magnetron magnet... Alternating... So I got this great idea! I took the ring magnet I used to mess it up, tied a string to it, suspended it on the string and spun it as fast as i could. I put it up to the CRT and brought it away slowly! Eureka! On Monday I called my smart-alek friend and cancelled the lunch! How Often to Degauss For TVs, there usually isn't much choice - degauss is automatic when the TV is turned on. However, some computer (and other) CRT monitors provide a degauss button or menu option. Some monitor manufacturers specifically warn about excessive use of degauss, most likely as a result of overstressing components in the degauss circuitry which are designed (cheaply) for only infrequent use. In particular, there is often a thermister that dissipates significant power for the second or two that the degauss is active. Also, the large coil around the CRT is not rated for continuous operation and may overheat. If one or two activations of the degauss button do not clear up the color problems, manual degaussing using an external coil may be needed or the monitor may need internal purity/color adjustments. Or, you may have just installed your megawatt stereo speakers next to the monitor! You should only need to degauss if you see color purity problems on your CRT. Otherwise it is unnecessary. The reasons it only works the first time is that the degauss timing is controlled by a termister which heats up and cuts off the current. If you push the button twice in a row, that thermister is still hot and so little happens. One word of clarification: In order for the degauss operation to be effective, the AC current in the coil must approach zero before the circuit cuts out. The circuit to accomplish this often involves a thermister to gradually decrease the current (over a matter of several seconds), and in better monitors, a relay to totally cut off the current after a certain delay. If the current was turned off suddenly, you would likely be left with a more magnetized CRT. There are time delay elements involved which prevent multiple degauss operations in succession. Whether this is by design or accident, it does prevent the degauss coil - which is usually grossly undersized for continuous operation - to cool. Ultra Cheap Degaussing Coil Pack Rat Trick #457384: Next time you scrap a computer monitor (or tv), save the degaussing coil (coil of wire, usually wrapped in black tap or plastic) mounted around the front of the tube. To adapt it for degaussing sets, wrap it into a smaller coil, maybe 4"-6". To limit the current to something reasonable, put it in series with a light bulb (60 to 100 W, maybe as high as 200 W but keep a finger on the temperature of the coil!). You need AC current to degauss, so just put the bulb in series with the coil and use the your local 120 VAC outlet. BE VERY CAREFUL that you actually wired it in series, and that everything is properly insulated before you plug it in (A fuse would be a real good idea too!!) A few circles over the affected area will usually do it. Note that it will also make your screen go crazy for a little bit, but this will fade out within a minute or so. Just a couple of points for emphasis: The coil as removed from the TV is not designed for continuous operation across the line as indicated above. In fact, it will go up in a mass of smoke without the light bulb to limit the current. The poor TV from which this organ was salvaged included additional circuitry to ramp the current to 0 in a few seconds after power is turned on. Reducing the coil size by a factor of 2 or 3 will increase the intensity of the magnetic field which is important since we are limiting the current with the light bulb to a value lower than the TV used. You don't need to unwind all the magnet wire, just bend the entire assembly into a smaller coil. Just make sure that the current is always flowing in the same direction (clockwise or counterclockwise) around the coil. Insulate everything very thoroughly with electrical tape. A pushbutton momentary switch rated for 2 amps at 115 volts AC would be useful so that you do not need to depend on the wall plug to turn it on and off. (From: Larry Sabo (sabo@storm.ca).) I've been using a couple of degaussing coils from "parts" monitors, connected n series. The combined resistance is about 27 ohms, for a current of around 4 to 5 amps. Sorry, I don't know the wire size, but it's very substantial, not like some of the thin, flimsy stuff I see. Works great! Bob Myers' Notes on Degaussing A couple of comments: first of all, it makes no difference whatsoever if the display is on while it's being degaussed. (Oh, some people DO like to watch the psychodelic light show, but it really doesn't help anything for it to be on.) Actually, there is a very minor case to be made for degaussing while OFF, at least for the Trinitron and similar tubes. (The field of an external degauss coil CAN cause the grille wires to move slightly, and they're a bit more flexible when hot - so it is conceivable, although certainly unlikely, that you're running a higher risk of causing the grille wires to touch or cross and become damaged.) Secondly, a good practice for degaussing is to slowly back away from the monitor after giving the screen a good going over. Once you're about 5-6' away, turn the coil so it's a right angles to the CRT faceplate (which minimizes the field the monitor is seeing), and THEN turn to coil off. This is to reduce the possibility of the field transient caused by switching the coil off from leaving you once again with a magnetized monitor. The last point is to make sure that you DON'T leave the coil on too long. These things are basically just big coils of wire with a line cord attached, and are not designed to be left on for extended periods of time - they can overheat. (I like the kind with the pushbutton "on" switch, which turns off as soon as I release the button. That way, I can never go off and leave the coil energized.) Oh, one more thing - make sure your wallet is in a safe place. You know all those credit cards and things with the nice magnetic stripe on them? :-) (Actually, I've got a good story about that last. I was teaching a group of field service engineers how to do this once, and being the "Big Deal Out of Town Expert", made VERY sure to place my wallet on a shelf far away from the action. Unfortunately, Mr. Big Deal Out of Town Expert was staying in a hotel which used those neat little magnetic-card gadgets instead of a "real" key. Ever try to explain to a desk clerk how it was that, not only would your keycard NOT let you into your room, it was no longer anything that their system would even recognize as a key? :-)) Degaussing after lightning strike Sometimes a nearby lightning strike may produce effects which mimic those of a nuclear explosion, at least in terms of EMP induced magnetization. This may be take the form rainbow patterns or purity blotches that the internal degaussing coil or even an typical external degauss won't cure. (From: JURB6006 (jurb6006@aol.com).) A lightning strike produces a VERY high magnetic field, something the degausser can't handle. Somehow connect a good, like 10 amp Variac straight to the degaussing coil(s) and turn it all the way up and down, fairly quickly. You might do better to get ready, flip the switch on and turn it down from the top, but DON'T blow the fuse, that might make things worse. Turn it down quick, but it is the top end that gets the job done. Thing is those coils can only stand it for a second or two, but that is way longer than it takes. You can also do this with it running, but you risk damaging the vertical circuit. However you can try different levels and see if less than max is enough to do it. At extremely high levels you risk damaging the shadow mask, that is if it is not already damaged. I almost scrapped a 36" Sony for this, same thing, near a lightning strike. The colors were seriously FUBARed. The coil to the variac trick did it. What happens is I think the purity shield itself gets magnetized, despite it's low coercivity. It takes a bit more than the standard degausser in the set to do the trick. Degaussing Humor - If it Works, Use It! Note: If you are forced to use this stunt, sorry, approach, make sure you don't end up smashing something important! :) (From: Mike Champion (mchampfl@gdi.net).) I replaced the magnetron in my microwave and ripped apart the old one with my kids to 'see how it works'. Boy, there are some mother magnets in there! me and the kids had fun with them. you know - push me pull you; the paper clip boat; which easter egg has the metal and which has the wood; etc. enough with this kid stuff - 'wanna see something really cool?' Says I. Having been around monitors for a long time in the computer business, I showed them what what a REALLY powerful magnet will do to an electron beam in a cathode ray tube - my sharp 19" color TV. "Wow, dad! psychodelic! It looks like all the colors are flushing down the toilet!" Boy, was I DAD or what? The problem was that my experience with magnets and monitors were in the monochrome days! So the price I paid for such esteem in the eyes of my children were purple faces and green legs on my sharp 19" color TV! Uh-oh! Well, maybe it will be all right by tomorrow. Well it wasn't. Now I'm getting worried! I used to do computer support at a television station so I called an old engineer friend there for help. He just hee- hawed! As he was drying his eyes, he suggested that I had probably just magnetized the mask and he'd loan me a degausser. I offered to buy him lunch for the favor. This was Friday and because of my friend's diagnosis I was able to relax about the problem enough to think about it. Hmmmm... Degausser = alternating magnetic field. Strong magnetron magnet. Hmmmm... Alternating... So I got this great idea! I took the ring magnet I used to mess it up, tied a string to it, suspended it on the string and spun it as fast as I could. I put it up to the CRT and brought it away slowly! Eureka! On Monday, I called my smart-aleck friend and cancelled the lunch! :) Can a Really Strong Magnet Permanently Damage the CRT? Even a magnet that can suspend your weight may still not have much range as they usually have metal pole pieces that concentrate the flux and work well only with a matching flat steel plate. The only thing in the guts of a TV or monitor (that is accessible from outside the cabinet) that can be damaged permanently is the shadow or slot mask of the CRT. If the magnet is strong enough to distort it, the CRT will be ruined. Even manual degaussing with a similarly powerful degaussing coil will then not totally clear up color purity problems. The shadow or slot mask is a very thin perforated steel or InVar sheet about 1/2 inch behind the glass of the CRT screen - which is itself about 1 inch thick or more. So, even up against the screen, your magnet is still at least 1-1/2 inches from the shadow mask. It would take a mighty powerful magnet to distort it. For Trinitron (or clone) CRTs with aperture grilles - tensioned fine wires in place of a shadow or slot mask, the force required would be even greater, or perhaps not. See below. (From: Jeff Mangas (jeff@edldisplays.com).) I work in a small monitor factory and some time ago we were using some very strong degaussing wands to remove magnetism from some of our chassis. We found that this caused a weakening of the shadow mask and it would take only a small shock/vibration to break the mask loose. We are not 100% sure that it was the degaussing that caused the problem but we only used these strong wands for a short time (lost several tubes while using them) and have not had any problems before or since. (From: Matthew Saunier.) It is possible to destroy a CRT with magnets - I once had a magnet out of an old 15,000 RPM hard drive (MASSIVE flux to snap those heads from one end of the drive to the other in 8 ms), and touched it to the glass of an old Trinitron monitor (21") I was going to toss. I could actually hear the aperture wires snapping (TINK! TINK TINK!). Unfortunately I was unable to break a stabilizer wire - I think that my initial assault took a lot of tension off of them. That tube was extremely ready for the garbage heap when I was through with it. Maybe 1/4 of the screen had black vertical stripes, and you could see a few shadows from broken wires that had curled up near the top of the screen. I didn't have a traditional CRT around that I wanted to break, but I assume that it wouldn't take as much force to pinch the shadow mask. WARNING about degaussing late model Sony Trinitron CRTs The following has been confirmed by others. (From: David Kuhajda (dkuhajda@locl.net).) You should NEVER use a big degauss coil on ANY SONY WEGA tube, or ANY SONY 27" or larger CRT made after 1997. Sony deliberately put a small amount of magnetic field into the strapping and aperture grill to compensate and improve the convergence. A BIG degauss will remove this and make the tube look very bad. A BIG manual degauss coil from about 3 feet away should have a low enough field to be safe. (Note: should) I NEVER use the large degauss coils on the Sony tubes after seeing the Sony video of how CRTs have been damaged. I USE a smaller degauss coil and work it on a Variac at a lowered AC voltage, and bring the voltage up each successive pass to degauss the CRT until it is cleared up. If the internal degauss is not taking care of the problem, you have other things to look at. Has the yoke or yoke purity rings been moved? Have the TV or monitor been dropped? Are all the connections good on the degauss thermister? If it is a three leg thermister it still may be bad as those leave a small current flowing on the older Sony coils. Have any of the extra purity magnets fallen off the yoke or CRT? Note that Sony tubes do NOT have shadow masks, but they have aperture grills which are an array of incredibly fine wires under tension. A BIG degauss coil can also rip the aperture grill away from the stabilization wires.



Back to CRT FAQ Table of Contents. CRT Related Adjustments Principles of Purity and Convergence Adjustment Purity involves bending all 3 of the beams so that they cross the space between shadow mask and screen at the proper angle and will land at a different place on the phosphors. Convergence involves adjusting the aim of 1 or 2 of the beams at a different angle so that they all land at the same place on the screen. Dynamic convergence circuitry is now virtually non-existent, except in high resolution monitor tubes and in Sony Trinitron tubes (they require a very basic horizontal convergence). All other tubes have the convergence correction built into the design of the tube and the coil. Sony has chosen a different trade-off between price and performance (which includes also sharpness). Most CRTs have a series - usually 3 pairs - of ring magnets mounted on the neck near the socket end. These are used for part of the purity adjustment and static convergence. (Coarse purity is set by the position of the yoke and dynamic convergence is set by the tilt of the yoke.) These rings consist of multi-pole magnets which due to their field configuration are able to affect the electron beams from the 3 guns in different ways. (Some CRTs employ internal structures that are premagnetized at the factory and cannot be adjusted in the field - though perhaps auxiliary magnet rings could be added if the original magnetization were lost for reasons we won't go into :-). This type of CRT will be obvious as there will be no adjustable rings to mess screw up!) The relative orientation of the rings in a pair affect the strength of the effect. In a nutshell, the electron guns in most modern CRTs are arranged in-line. For example: GRB. Some of the ring adjustments are designed to affect them all while others pretty much leave the center gun's beam alone and only affect the outer ones. Various options then exist depending on the magnetic field configuration. The three sets of ring magnets are shown below along with the position of the red (R), green (G), and blue (B) electron beams passing through them. Each is actually a pair of rings which may be moved relative to one-another to control the strength of the magnetic field. When the tabs are adjacent, the fields from the two rings nearly cancel and the rings then have no effect. Two typical orientations are shown (N and S are the poles of the ring magnets): Orientation 1: S S N N R G B S N R G B N N R G B S S S N 2-pole 4-pole 6-pole (purity) (red-blue) (red/blue-green) 0 Degrees 0 Degrees 0 Degrees Orientation 2: N N S S N N R G B R G B R G B S S S S N N 2-pole 4-pole 6-pole (purity) (red-blue) (red/blue-green) 90 Degrees 45 Degrees 30 Degrees (My apologies if I have the direction of deflection reversed - I can never remember the right hand rule for electrons moving in magnetic fields!) The 2-pole purity rings move the set of RGB beams more or less together to fine tune the position of the center of deflection. The field lines go generally across (at the orientation shown) between the N and S poles. Orientation 1 , the RGB beams will be raised. Orientation 2 , the RGB beams will be moved to the right.

The 4-pole red-blue rings move the R and B beams relative to the G beam but leave the G beam alone. The field lines go generally between adjacent N and S poles. On opposite sides of the rings, the polarity/direction of the lines oppose and thus tend to move the R and B beams in opposite directions. The G beam in the center does not experience any deflection from the 4-pole ring magnets since all the fields tend to cancel. Orientation 1 : The R beam will move up and the B beam will move down relative to G. Orientation 2 : The R beam will move up and to the right and the B beam will move down and to the left relative to G.

The 6-pole red/blue-green rings move the RB beams with relative to the G beam but leave the G beam alone. The field lines go generally between adjacent N and S poles. On opposite sides of the rings, the polarity/direction of the lines are the same and thus tend to affect the R and B beams in the same direction. The G beam in the center does not experience any deflection from the 6-pole ring magnets since all the fields tend to cancel. Orientation 1: The R and B beams will move up relative to G. Orientation 2: The R and B beams will move up and to the right relative to G. For purity to be perfect (or as good as possible), the electron beams must originate from the same effective center of deflection as used in originally laying down the phosphors. Moving the yoke forward and backward on the neck of the CRT can precisely set the deflection center along the axis of the neck. However, slight transverse errors may still exist due to imperfections in the yoke windings or positions of the electron guns. This is affected slightly by the earth's magnetic field as well. The purity magnet rings are those closest to the yoke and provide the means for moving the electron beams very slightly to compensate. The adjustment procedures generally use the red gun for the setting purity. Intuitively, one would think this should be the center (green) gun. However, since the red beam current is the highest (red phosphor is least sensitive), problems are likely to show up first with the red purity so it is used for the adjustment. In any case, it is a good idea to check all three guns for proper purity and tweak if needed before moving on to convergence. In an in-line gun, the green gun is always in the middle. The only reason for adjusting purity with the red beam is because it gives the greatest sensitivity: (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).) The red beam current usually has the largest amplitude.

A landing error of the red beam gives the best visible discoloration (much better than green, better than blue).

This makes the landing of the red beam the most critical. Detailed Purity and Static Convergence Adjustment Procedure Also see the adjustment information in the documents: Notes on the Troubleshooting and Repair of Computer and Video Monitors or Notes on the Troubleshooting and Repair of Television Sets. (From: Alan McKinnon (alan.mck@pixie.co.za).) The rearmost pair of magnets (seem from the service position behind the set in other words furthest from you nearest the front of the tube) affects purity. More on this later. The middle and front magnets are for convergence and work on pairs of colours. The effects can most easily be seen on a cross hatch test pattern (10 or so horizotal lines, 15 or so vertical lines). But first, purity: Without getting into the details of what happens inside the guns, I assume you need to know how to do the adjustments. You need some means of generating an evenly red screen. An (expensive) pattern generator is the preferred method. Fiddle the rear purity rings to distort the screen by bringing green and blue blobs into it. You will note that the magnets can be adjusted by moving both together, and moving them aart relative to each other. The best advice here is: adjust slowly and observe what happens. Once you have the screen evenly red, move on to convergence, which is the trick of getting the red green and blue beams to coincide on the screen to produce white, with the minimum of colour shadowing. With a cross hatch pattern on screen, you can see easily how convergence works, and how the magnets affect the picture. Each tube type is different in exactly how this is done, but the general idea is that one set of magnets affects two specific colours only, moving them apart and bringing them nearer, while leaving the third colour alone. The other set of magnets takes the colours affected by the other set, and moves them together relative to the third colour. Also, moving a pair of magnets together adjusts the colours in one direction (vert or horiz) while moving the magents apart adjusts the other direction. With all things in life, there is some overlap, so you need to look carefully and see what happens mostly - the adjustments are not cut and dried. Oh, and they are interactive to some degree. Keep checking purity after you do convergence. All of this is best shown with a picture, the colours are arbitrary, you may well find the details do not apply to your tv, but the basic principles will. These initial converence adjustments apply only to the centre of the screen by the way, the edges are done elsewhere: Rotating one set of magnets together might move red and blue together till they coincide vertically: | | | | | | | | | | | -----> | | | -----> | | | | | | | | | | R G B G R B G R&B And moving them apart relative to each other might move red and blue together horizontally: R ----- R -------- R&B--------- G ----- -----> G -------- -----> G --------- B -------- B ----- Moving the other set of magnets together might take the red and blue pair and move them to coincide with the green, vertically: | | | | | | | -----> | | ------> | | | | | | G R&B G R&B R&G&B (=white) And moving them apart relative to each other might move the red and blue pair and move them to coincide with green horizontally: R&B ------- R&B ------- -----> ----> ------- R&G&B G ------- (=white) G ------- Once the convergence is perfect in the centre of the screen (called static convergence) it's time to handle the edges and corners (called dynamic convergence for historical reasons). This is done by physically moving the entire yoke that is clamped around the tube neck with the deflection coild on it. It is anchored in place by a collar on the tube neck, loosen this slightly, butnot enough so that the yoke can move backwards. It is also held in place by rubber wedges glued or taped down. Take the wedges out. By gripping the yoke and levering it up and down, left and right, you will change he convergence in the corners. The effects don't work as you might at first suppose - moving the yoke left affects the lower right corner, this type of thing. Get the dynamic convergence right and stuff the wedges back under the yoke to hold it precisely in place and glue them back down. The recheck purity. There you have it. Easy as pie. Some folk would have you believe no-one in their right minds adjusts these things. Well, balls. Someone did it at the factory, and they did it the way I just described. All you need is the right tools (pattern generator), patience, and time. Tony's Notes on Setting Convergence on Older Delta Gun CRTs (From: ard12@eng.cam.ac.uk (A.R. Duell)) The older delta-gun tubes (3 guns in a triangle, not in a line) can give **excellent** pictures, with very good convergence, provided: You've set those 20-or-so presets correctly - a right pain as they interact to some extent. The CRT is set up in the final position - this type of tube is more sensitive to external fields than the PIL type. Both my delta-gun sets (a B&O 3200 chassis and a Barco CDCT2/51) have very clearly set out and labeled convergence panels, and you don't need a service manual to do them. The instructions in the Barco manual are something like: "Apply crosshatch, and adjust the controls on the convergence board in the numbered order to converge the picture. The diagrams by each control show the effect". Here's a very quick guide to delta gun convergence where the settings are done using various adjustments on the neck of the CRT (if you don't have a service manual but do know what each control does, and where they all are - otherwise, follow the instructions in the service manual --- sam): Apply a white crosshatch or dot pattern to the set. Don't try and converge on anything else - you'll go insane. It's useful to be able to switch between those 2 patterns. Before you start, set the height, width, linearity, pincushion, etc. They will interact with the convergence. Also check PSU voltages, and the EHT voltage if it's adjustable. That's where you do need a service manual, I guess. Turn off the blue gun using the A1 switch, and use the red and green static radial controls to get a yellow croshatch in the middle of the screen. These controls may be electrical presets, or may be movable magnets on the radial convergence yoke (the Y-shaped think behind the deflection yoke). Turn on the blue gun and use the 2 blue static controls (radial and lateral) to align the blue and yellow crosshatches at the center of the screen. Some manufacturers recommend turning off the green gun when doing this, and aligning red with blue (using *only* the blue controls, of course), but I prefer to align blue with yellow, as it gives a check on the overall convergence of the tube. Turn off the blue gun again. Now the fun starts - dynamic convergence. The first adjustments align the red and green crosshatches near the edges - I normally do the top and bottom first. There will be 2 controls for this, either a top and a bottom, or a shift and a linearity. The second type is a *pain* to do, as it's not uncommon for it to affect the static convergence. Getting the red and green verticals aligned near the edges is a similar process. You now have (hopefully) a yellow crosshatch over the entire screen. Now to align the blue. This is a lot worse, although the principle is the same. Turn on the blue gun again, and check the static (center) convergence To align the blue lines with the yellow ones, you'll find not only shift controls, but also slope controls. Use the shift controls to align the centers of the lines and the slope controls to get the endpoints right. These interact to some extent. You'll need to fiddle with the controls for a bit to work out what they do, even if you have the manual. The convergence over the entire screen should now be good.... A word of warning here... The purity is set by ring magnets on almost all colour CRTs, but on PIL tubes, there are other ring magnets as well - like static convergence. Make sure you know what you are adjusting. Jerry's Comments on Convergence and Other Advanced Adjustments (From: Jerry G. (jerryg@total.net).) Convergence alignment is not something you can do yourself unless you have the proper calibration instruments and skills. It takes lots of experience and time. There are published specs for most of the good monitors. Most of the time they are as follows: There is the 'A area', 'B area', and 'C area'. On a 15 inch monitor the A area would be a diameter of about 4 inches. The B area would be about 7.5 inches. The C area would be the outside areas including the corners. These numbers are approximate. There are actually standard specs for these areas. They are expressed in percentage of screen viewing area. Therefore the inches would vary with the CRT size. The higher the price (quality) of the monitor CRT, yoke, and scanning control circuits, the tighter the convergence can be aligned by the technician. For the A area on a good monitor, the maximum error should not exceed 0.1 mm. For the B area it should not exceed more than about 0.25 mm. And for the C area, it can be allowed up to about 0.3 mm. Most of the monitors that I have repaired, seen, and used did not meet these specs unless they were rather expensive. With these specs there would not be any real visible misconvergence unless you put your nose very close to the screen... A lot of the ones in the medium price range they were about 0.15 mm error in the A area, about 0.4 in the B and greater than in the C area. This also annoys me because I am very critical. If one has the skills and test gear he or she can do a better job on most monitors. It is a question of the time involved. To see the convergence errors a grating or crosshatch pattern is used. A full raster color generator is required for the purity adjustments as well. This is necessary to align the landing points of the CRT guns. The exact center reference and purity adjustments are done with the ring magnets on the CRT neck. The yoke position angle adjustments are also done for the side and top-bottom skewing as well. Everything interacts! The corners are done with various sorts of slip or edge magnets. As for corner convergence skewing, button magnets are used. The color purity will be effected as you go, and must be also corrected. These adjustments interact on one another, and the processes continues until the convergence and purity are good at the same time...! I don't recommend the amateur or hobbiest, or even the do-it-yourselfer to attempt this alignment procedure. The test gear would exceed the cost of a really good monitor anyways...!!! And without the proper skills required, he or she would only make it worse anyways... As for purity specs, the color change from any corner to any corner must not exceed an error of more than 200 degrees Kelvin. The error in the B area should not exceed 300 degrees kelvin. This applies to a white raster. Most of the monitors I see don't get better than about 300 degrees Kelvin. And some are even 1000 out! The purity errors are best checked with a full Red raster using 100 % saturation. Then the other color vector angles are checked with cyan, and then magenta. The color temperature stability should be the same in all aspects. A color spectrometer should be used to judge this error factor. As far as the eye is concerned, it will see a purity error of more than about 500 degrees Kelvin if the person knows what to look for... When changing the CRT, this alignment must be done completely. Most shops do not even employ people who are skilled to a proper alignment, or don't even own the instruments to do it right, and the poor customer get back a monitor that is not in specs...! CRTs with No Purity or Static Convergence Rings I have a late model TV with a 13 inch tube with no static purity or convergence rings. I don't get to see that many modern tubes so this was a bit of a surprise or maybe I just hadn't noticed before on small CRTs if they didn't have purity/convergence problems. I do see it has wrapping of a rubber-ferrite-permalloy type material where the ring assembly would go. I assume that this is magnetized selectively at the factory to adjust purity/convergence? The yoke has the usual position and tilt adjustments. This one was an RCA/GE CRT. What this means is that if you were to accidentally bring a strong permanent magnet near the base of the CRT or a strong degaussing coil, there is a slight possibility of totally messing up this compensation requiring replacement of the CRT. I don't know how possible this is without really working at it! (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).) Since eternity, Philips CRTs have not had external multipole magnet rings around the neck. There is an iron ring inside the neck, at the end of the electron gun assembly. This ring is permanently magnetized in the factory by a strong outside magnetic field at a later stage of the production. Further responsibility for purity, convergence and geometry is in the design of the coil windings distribution and some metal parts. Final purity adjustment is achieved by matching a tube with a coil and then shifting and tilting the coil slightly. This explains why Philips CRTs are always sold as a matched combination of tube and coil, contrary to some other brands. Projection Set Convergence Adjustment Principles (From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).) CRT projection displays require much convergence correction, especially since the 3 tubes aim at the screen under different angles. Generally the green Horizontal convergence coil is not driven because that is a geometry correction which is taken care of by the horizontal deflection circuit. The 3 vertical convergence coils usually also take care of vertical geometry correction (N-S corrrection) because the vertical deflection circuit is generally a standard direct-view type. Add to that a severe keystone correction for the Red and Blue tubes. The convergence waveforms used to be generated from an analog polynomial generator. The components are then weighted and summed to form a Taylor polynomial. Consider the adjustment of horizontal convergence, then typical polynomial components are: 1 (shift),

x (amplitude),

x^2 (linearity),

y (rotation or tilt),

y^2 (bow),

x*y (keystone),

x^2*y (dunno if it's used).

x*y^2 (pin-cushion),

x^3 (side linearity).

x*y^4 (corner pin-cushion)

Adjusting convergence is a highly iterative (read: costly) process because each potentiometer tends to influence the whole screen. Also, this method is not easily extendible to higher order adjustments for more accuracy. That's why better waveform generators have been designed, like digital look-up tables with D/A converters (which are quite expensive) and spline-like waveform generators (which are cheap and easy to adjust, the Philips design is called Convergence Spline Processor, it's digital too). Monitor Tune-Up? (The following from: Bob Myers (myers@fc.hp.com).) Most manufacturers will quote an MTBF (Mean Time Before Failure) of somewhere in the 30,000 to 60,000 hour range, EXCLUSIVE OF the CRT. The typical CRT, without an extended-life cathode, is usually good for 10,000 to 15,000 hours before it reaches half of its initial brightness. Note that, if you leave your monitor on all the time, a year is just about 8,000 hours. The only "tuneup" that a monitor should need, exclusive of adjustments needed following replacement of a failed component, would be video amplifier and/or CRT biasing adjustments to compensate for the aging of the tube. These are usually done only if you're using the thing in an application where exact color/brightness matching is important. Regular degaussing of the unit may be needed, of course, but I'm not considering that a "tuneup" or adjustment. A Discussion on Correction Magnets (From Ludwig (eastcomp@gmx.de).) When repairing and recalibrating color monitors of different brands, one experiences those "dirty little tricks" called correction magnets, which have different forms, sizes and magnet strength, and which are attached at different locations somewhere near the electronic beams at the neck of the tube. These magnets are used to correct bad edge geometry/convergence and problems with color convergence at various locations on the screen. Depending on the quality (i.e., magnetic geometry) of the tube and the deflection coils/fields there are monitors, which have only few (or even none) of these correction magnet, while others (some brands are "famous" for this) are really clustered with these magnets. The magnets can have different forms and sizes: Most often there are used small and thin, weak magnets, which are glued to the end of a plastic stripe. These stripes are inserted into the small gap between the tube and the deflection coils (ferrite coil) and the fixed by glue, silicon or plaster. This magnets are weak and therefore have to be positioned very near to the electron beam at the neck of the tube. They usually are intended to correct bad convergence at the corners and edges of the picture. Plastisized magnets (e.g., 4x4x1 mm, 3x10x1 mm, or 10x10x0.2mm), which have a much bigger magnetic strength, are either glued to the the edges of the plastic case of the deflection coils or - if the magnet is not so strong - to the tube itself. These types of magnets often are used to correct larger deficiencies in geometry - and to a lesser extent - in convergence. Those are my observations, but what I'd like to know is this: Why aren't such magnets demagnetized during the power-on degaussing?

Aren't such magnets demagnetized, if one uses an extra demagnetizing coil for removing undesired magnetic fields at the tube? Are those demagnetizing coils harmful to the different correction magnets on/at the neck of the coil? What type of magnets are used for those correction magnets ? (barium titanate, other types of ferrites?). (From: Sam.) The answers to both (1) and (2) is that if using the internal degauss coil and/or properly positioned (front of CRT only) external coil, the strength of the field is (hopefully) insufficient to affect the correction magnets. That is why one should NEVER attempt to degauss in the rear of the TV or monitor or inside the case! I don't believe the magnets are made of anything special - they appear to be similar to your typical refrigerator (note holding) magnets in composition and strength. (From: Ludwig.) By the way: Almost any monitor, which is older than 1-2 years has developed deficiencies in convergence, geometry and sharpness, and has to be recalibrated, if you'd prefer an optimal picture (and being careful with one's eyes). It's not quite easy to do fine recalibration of convergence and geometry (even modern monitors allow only to correct coarse via OSD menus), because during recalibration the monitor has to be at power-on state, i.e. high voltages are at every edge of the monitor. I successfully used household rubber/plastics gloves to do the recalibration by repositioning the above mentioned magnets while the monitor is powered on. Using household rubber/plastics gloves is also a valuable means to prevent beginners from electric shock, and therefore should be recommended for every job to do with the monitor case open and power on (even just for monitoring electronic signals with an oscilloscope).



Back to CRT FAQ Table of Contents. CRT and CRT Related Maintenance and Repair Preventive Maintenance - Care and Cleaning Preventive maintenance for a TV or monitor is pretty simple - just keep the case clean and free of obstructions. Clean the CRT screen with a soft cloth just dampened with water and mild detergent or isopropyl alcohol. This will avoid damage to normal as well as antireflection coated glass. DO NOT use anything so wet that liquid may seep inside of the monitor around the edge of the CRT. You could end up with a very expensive repair bill when the liquid decides to short out the main circuit board lurking just below. Then dry thoroughly. Use the CRT sprays sold in computer stores if you like but again, make sure none can seep inside. If you have not cleaned the screen for quite a while, you will be amazed at the amount of black grime that collects due to the static buildup from the CRT high voltage supply. There is some dispute as to what cleaners are safe for CRTs with antireflective coatings (not the etched or frosted variety). Water, mild detergent, and isopropyl alcohol should be safe. Definitely avoid the use of anything with abrasives for any type of monitor screen. And some warn against products with ammonia (which may include Windex, Top-Job, and other popular cleaners, as this may damage/remove some types of antireflective coatings. To be doubly sure, test a small spot in corner of the screen. In really dusty situations, periodically vacuuming inside the case and the use of contact cleaner for the controls might be a good idea but realistically, you will not do this so don't worry about it. (From: Bob Myers (myers@fc.hp.com).) Windex is perfectly fine for the OCLI HEA coating or equivalents; OCLI's coating is pretty tough and chemical-resistant stuff. There may be alternative (er..cheaper) coatings in use which could be damaged by various commercial cleaners, (For what it's worth, OCLI also sells their own brand of glass cleaner under the name "TFC", for "Thin Film Cleaner".) I have cleaned monitors of various brands with both Windex and the OCLI-brand cleaner, with no ill results. But then, I'm usually pretty sure what sort of coating I'm dealing with...:-) Monitor coatings are always changing; besides the basic "OCLI type" quarter-wave coatings and their conductive versions developed to address E-field issues, just about every tube manufacturer has their own brew or three of antiglare/antistatic coatings. There are also still SOME tubes that aren't really coated at all, but instead are using mechanically or chemically etched faceplates as a cheap "anti-glare" (actually, glare-diffusing) treatment. In general, look in the user guide/owner's manual and see what your monitor's manufacturer recommends in the way of cleaning supplies. Shorts in a CRT Occasionally, small conductive flakes or whiskers present since the day of manufacture manage to make their way into a location where they short out adjacent elements in the CRT electron guns. Symptoms may be intermittent or only show up when the TV or monitor is cold or warm or in-between. Some possible locations are listed below: Heater to cathode (H-K). The cathode for the affected gun will be pulled to the heater (filament) bias voltage - most often 0 V (signal ground). In this case, one color will be full on with retrace lines. Where the heater is biased at some other voltage, other symptoms are possible like reduced brightness and/or contrast for that color. This is probably the most common location for a short to occur.

Cathode to control grid (K-G1). Since the G1 electrodes for all the guns are connected together, this will affect not only the color of the guilty cathode but the others as well. The result may be a very bright overloaded *negative* picture with little, none, or messed up colors. The one exception is where the G1 is connected directly to ground. Then, it's possible for a short from one cathode to G1 to only affect that color.

Control grid to screen (G1-G2). Depending on circuitry can result in any degree of washed out or dark picture.

Screen to focus (G2-F). Screen (G2) and focus voltage will be the same and the controls on the flyback will interact. Result will be a fuzzy white raster with retrace lines and little or very low contrast picture. Symptoms will be similar to those of a flyback with breakdown in the focus/screen divider network.

Focus to high voltage (F-HV). High voltage will be pulled down - probably arcing at the focus spark gaps/other protective devices. Line fuse and/or HOT may blow.

Other locations between electron gun elements as feed wires. Replacing the CRT may be required but there are a variety of 'techniques' that can often be used to salvage a TV that would otherwise end up in the dump since replacing a CRT is rarely cost effective: Isolation - this will usually work for H-K shorts as long as only one gun is involved. Blowing out the short with a capacitor - depending on what is causing the short, this may be successful but will require some experimentation. Placing the CRT (TV or monitor) face down on a soft blanket and gently tapping the neck to dislodge the contamination. Depending on the location of the short, one side or the other might be better as well. A combination of (2) and (3) may be required for intermittent shorts which don't appear until under power. See the sections below for additional details. However, for shorts involving the focus and high voltage elements, even a sharp edge can result in arcing even if there is no actual short. There is no remedy for these types of faults. Providing Isolation for a CRT H-K Short This procedure will substitute a winding of your own for the one that is built in to the flyback to isolate the shorted filament from the ground or voltage reference. Note that if you have a schematic and can determine where to disconnect the ground or voltage reference connection to the filament winding, try this instead. The flyback is the thing with the fat red wire coming out of it (and perhaps a couple of others going to the CRT board or it is near this component if your set has a separate tripler) and may have a couple of controls for focus and screen. It should have some exposed parts with a ferrite core about 1/2-3/4" diameter. The filament of the CRT is the internal heater for each gun - it is what glows orange when the set is on. What has happened is that a part of the fine wire of the bad color's filament (assuming this is indeed your problem) has shorted to the cathode - the part that actually emits the electrons. Normally, the heater circuit is grounded or tied to a reference voltage so when it shorts to the cathode, the cathode voltage level is pulled to ground or this reference. You will need some well insulated wire, fairly thick (say #18-22). Find a spot on the flyback where you can stick this around the core. Wrap two turns around the core and solder to the CRT filament pins after cutting the connections to the original filament source (scribe the traces on the board to break them). Make sure you do not accidentally disconnect anything else. This winding should cause the filaments to glow about the same brightness as before but now isolated from ground. If they are too dim, put another turn on the flyback to boost the voltage. (Don't go overboard as you may blow the filament totally if you put too many turns on the core - you then toss the TV or monitor.) Route the wires so that there is no chance of them getting near the high voltage or any sharp metal edges etc. Your picture quality may be a tad lower than it was before because of the added stray capacitance of the filament wiring being attached to the (formerly bad) video signal, but hey, something is better than nothing. Rescuing a Shorted CRT If the short is filament-cathode (H-K), you don't want to use the following approach since you may blow out the filament in the process. If this is the case, you may be able to float the filament and live with the short (see the document: Notes on the Troubleshooting and Repair of Television Sets. Shorts in the CRT that are between directly accessible electrodes can be dealt with in a more direct way than for H-K shorts. At this point you have nothing to loose. A shorted CRT is not terribly useful. If the short is between two directly accessible electrodes like cathode-grid, then as a last resort, you might try zapping it with a charged capacitor (see below for K-G1 short). Unplug the CRT socket! Start with a relatively small capacitor - say a few uF at a couple hundred volts. Check to see if the short is blown after each zap - few may be needed. Increase the capacitance if you feel lucky but have had little success with the small capacitor. If the fault is intermittent, you will, of course, need to catch the CRT with the socket disconnected and the short still present. Try some gentle tapping if necessary. If you do this with the charged capacitor across the suspect electrode, you **will** know when the short occurs! See Sencore Blows Away CRT Failures With CR7000 by Randy Fromm for what a commerical CRT restorer can do as well as more on CRT failures in general. (Local backup at Sam's Copy of Sencore Blows Away CRT Failures With CR7000 by Randy Fromm.) (From: Terry DeWick (dewickt@esper.com).) I have seen this problem many times, shorted CRT red cathode, tap neck of CRT (not hard enough to brake, but close) or hit with a Tesla coil, we use one in shop, remove CRT board, run coil around pins for about 10 seconds, would you believe there is a service bulletin from Philips on this and focus shorts - I do not have a copy - I just helped write it - demonstrated use of coil to the service engineer and fixed 2 bad tubes in process. (From: JURB6006 (jurb6006@aol.com).) If it takes more than about 400 mA to clear the short the cathode is usually toast. The big cap makes it a big thing, but only for a certain time, this is based on thermal mass. Recently after I had tried everything I decided to use external power to remove a G1 short. This was the most solid short I've ever seen, I used an old AT PC power supply and all I got was to make the internal "wires" to the elements RED HOT!!! The best way to remove a G1 short is (I don't do it this way all the time it is too much trouble). Completely disconnect the cathode of the offending gun. Connect the cathode to a clip lead. Connect the G1 to ground, not DAG on an NAP and if it's an NAP you must isolate it and connect it to ground, if not an NAP 99% of the time you don't have to do it. You still need to tie G1 to ground, but in most sets you can just do it. As the short starts to manifest itself, touch the clip lead from the cathode to the video output 220 VDC source. (From: Arfa Daily.) My B&K CRT analyser / restorer removed shorts by discharging a cap across the affected electrodes. I seem to recall that g1-k shorts were actually more common, due to 'burnt out' emissive material falling off the cathode and lodging in the g1-k gap. The location of the short was indicated by neon lamps, the intensity of the glow giving an indication of the resistance of the path. When the zapping to