CRT picture discolouration
CRTs, the bulky picture tubes used in old TVs, monitors and projection sets, sometimes deteriorate before the end of life of the appliance. When CRT emission falls, picture discolouration occurs.
The treatment of such defects can also result in severe smear on the picture.
The good news is that both such faults are usually repairable, and that such repairs are fairly straightforward and well within the skill set of some DIYers.
CRTs are no longer popular, but are still in use in projection sets, historic tvs & computers, some military testgear, and special applications.
Cause of the problem
Over time the emissive coating on the CRT cathodes becomes contaminated, and loses emission.
Different tube designs deteriorate at different rates. Some may have poor emission after a few years, while some carry on in regular use for decades without any loss of performance.
Its fair to say that many other faults and misadjustments are also blamed on failing tube emission, since the problem is now fairly well known, whereas a full undertanding of CRT colour issues is not.
How colour channels work
Colour CRTs have 3 colour channels, Red Green and Blue. All on screen colours are made by combining 1, 2 or 3 of these 3 colours. White contains all 3. If you look very closely at a picture on a CRT you'll see the 3 individual colour dots that make up each pixel.
If what should be white areas have become:
- aqua: they lack red
- purple: they lack green
- yellow: they lack blue
- blue: they lack red & green
- green: they lack red & blue
- red: they lack green & blue
and the lacking colour is the problem channel.
When picture discolouration occurs, observation of picture colours in various areas of the picture can help pinpoint the cause.
A healthy white patch anywhere on the picture means there is no emission fault with the tube. Healthy white means all 3 cathodes are emitting well, the problem lies elsewhere. If its impossible to get clean bright white on screen, there's either a tube emission problem or a video driver fault...
Discolouration in dark areas only is caused by misadjustment of the video drive circuitry. This is not a tube fault, and can normally be corrected by adjusting the video background (and sometimes drive) controls.
Discolouration in light or white areas indicates a problem with either tube emission, or misadjustment of the 3 video drive channels.
Discolouration in all areas (eg light, mid and maybe dark picture areas are yellowed, caused by loss of blue) usually indicates a problem with tube emission. A video driver failure is also possible. Green failure causes dark red/blue/purple pictures. Red failure can look almost normal, except that all reds are missing.
Any discoloration that suddenly changes between healthy colour and wrong colour is caused by a circuit or wiring fault, normally a bad connection somewhere. This is not a tube problem.
A screen displaying a colour where it should be dark:
- if the single colour is dim, its a video drive misadjustment
- if the single colour is bright, its a video circuit fault
Trippy or rainbow discolouration - the whole picture is wildly and seemingly randomly strongly discoloured - caused by dented or detached shadownmask. Caused by a blow to the tube, and sometimes unsafely fixed the same way.
Severe horizontal smearing or flaring indicates insufficient emission on a tube that has been rejuvenated. Colours are sometimes normal, sometimes not, but the cause is still inadequate emission plus rejuvenation damage.
Dim picture, despite user controls being set to max(with or without discolouration), is usually caused by poor tube emission. Thre's no limit to how dim it can become.
A silvery appearance on a black & white set's picture is due to poor tube emission. This silveriness goes away if contrast is turned down, but of course the picture is then too dim.
Pictures that show poor colour when first switched on, improving to reach healthy colour after a while, have a mild tube emission problem.
There are 4 possible approaches to poor tube emission, with varying degrees of success.
The simplest option is to increase the drive of the affected colour channel. This resolves mild cases, but typically discolouration reappears after a while as emission deteriorates further.
See also How to adjust video drive
Rejuvenation is popular with repair shops. Its a quick operation, and works with many tubes, though far from all.
However its downsides are sizeable. First the boost only lasts short term, with reappearance of the fault in typically 1-3 months. Second it affects the tube in such a way that when the fault reappears, severe smearing of the picture also occurs, which is worse than the original problem.
Heater boosting is a bit more work than the other options, but produces a long term fix lasting years, even with Sony CRTs. (See The repair trade re the unsatisfactory results of low levels of boost)
The repair consists of increasing the voltage applied to the tube heater. This increase emission in 2 ways:
- low emission areas of cathode run hotter, improving their emission
- the area of cathode hot enough to emit electrons increases, bringing some new emissive area into service.
How much boost?
- I've routinely used 33% voltage boost for lasting repairs. Severe cases can require more.
- 5 or 10% sometimes gets a good picture, sometimes not, but emission tends to worsen over time.
10% EHT boost certainly improves picture quality when the tube deteriorates, but it can increase x-ray output to some extent, and the voltage increase can sometimes cause arcing or total failure.
EHT boost should not be employed. The improvements it makes can be obtained in safer ways.
The picture quality of a boosted tube is in nearly all cases 100% good.
However there are exceptions, as it depends on tube condition to some extent. Severe emission problems tend to result in poor colour tracking after boost. If one colour channel is almost entirely missing from the picture before boost, then its probable that colour tracking after boost will be poor. This means that on a really bad tube, although white and black may be set correctly, intermediate colours may still retain a bit of colour distortion.
Repair or replace
Only the end user can decide what an existing set is worth to them once repaired, and which is the best option. CRTs repaired by heater boosting normally last many years.
Boosting also performs much better than tube replacement in projection sets, as replacement of one of the 3 tubes tends to cause colour mistracking, as well as being unnecessarily expensive.
Heater boosting does increase the power consumption of a set, but only to a trivial degree. Adding 3 watts to consumption costs about 22p per year for a set used 2 hours every day. The delivery or collection of any replacement set would use far more energy, let alone its manufacture and shipping halfway aross the world.
How to boost
This article will concentrate primarily on voltage boosting, since its both effective and DIYable.
There are 3 possible ways to boost heater voltage
Adding an extra turn to the LOPTF (line output transformer) is probably the simplest boosting method. This works for TVs with heaters driven from a winding on the LOPTF, ie most TVs from the 1950s to early 90s.
The method is to add an extra turn round the outside of the LOPTF, and connect this in series with the wires to the tube heater. Connected one way round it will much reduce emission, and connected the other way it will boost. Since TVs usually use a 3 turn winding to supply the tube heater, 1 extra turn gives 33% boost and 2 extra turns 66% boost.
This wire must be well insulated, as the outer winding of the LOPTF carries EHT, typically 19-24kV.
Note that multimeters produce very inaccurate readings of the voltage produced by LOPTF windings.
Some TVs feed the tube heater from a regulated dc power supply line. The simplest way to boost this is to disconnect the tube heater from the 12v line and connect to the unregulated side, where voltage is typically in the region of 17v.
TVs seldom have no suitable power takeoff point, and LOPTFs that aren't accessible enough to add an external winding. If such is ever encountered, a standalone transformer may be used to supply the tube heater. This is fitted inside the cabinet, but keep it away from the tube as much as possible.
This option is mainly useful for people that don't want to delve into the circuitry to find a suitable power take off point. Transformers are readily available in standard voltages of 6v, 9v, 12v, 15v and 18v from any electronic component supplier. 6v is good for 4v tubes, 9v for 6v tubes and 18v for 12v tubes.
How much boost
33% voltage boost is good for minor emission problems. The discolouration may be substantial, but the maximum brightness of each colour channel is still fairly high.
50% or 66% boost is suitable for heavier problems, where one or more colour channel shows substantial loss of brightness.
Tubes that smear have been rejuvenated, and are more likely to need higher levels of boosting such as 50% to prevent any recurrence of smearing.
How to adjust video drive
Video drive adjustment may be required after boosting. Drive adjustment also solves many of the problems in the Diagnosis section that are not caused by tube problems.
Use of a testcard makes adjustment much easier, but is not absolutely essential. These are readily available online. One should be used when setting up a computer monitor, where colour accuracy is more important than on a TV.
The 3 colour channels each have 2 controls: drive and background.
- Drive is adjusted to correct white
- Background is adjusted to correct black level
Both controls affect both black and white to some degree, so precise setting may require more than one tweak of each control.
1960s-80s sets normally have RG and B adjustment pots on the main PCB. A few have only 2 adjustable colour channels, with often another preset that needs to be set first. A small number of low end sets have no colour background controls, and are unlikely to ever give good performance in the darkest areas of the picture.
Old sets tend to accumulate a layer of dust on the pot track, causing an unreliable connection when adjusted. The solution to this is to twiddle the pot back and forth a few times over the desired adjustment point.
More recent sets tend to be software controlled, and are adjusted via the on screen menu.
How to rejuvenate
Rejuvenation involves discharging a capacitor from grid to cathode with the tube heater on, and is explained on various websites. But its not really recommended. It may enable repair techs to charge for a quick repair to used sets, but a rejuved tube usually has too short a life to be a good deal for the set owner.
Sometimes sets have been reported to last over a year after a rejuve, but this only happens in a minority of cases.
How to maximise performance with user controls
This is not a repair, but can improve performance on borderline sets a bit to to give a few more months to decide what action to take, or find a replacement etc.
Due to the relatively long time constant of the electron cloud, peak white level can be improved a little by adjusting the brightness control to ensure some of the screen area is totally black, rather than near black. Correct black level adjustment gives a better picture than inaccurate adjustment, and deliberate misadjustment to get a bit more black area may improve the picture slightly more.
Note the naming of brightness and contrast controls is historic and somewhat misleading; the brightness control primarily adjusts black level. The controls interact on most sets, so contrast may also need a tweak.
To see how effective this approach could be, an experiment was conducted to push this method to its limits and observe the results.
A 1970s 13" sony trinitron tv tube was selected for experiment. Tube emission was so low that no picture was visible on screen at all. These Sony trinitron tubes were famous for failing emission, and failing to respond usefully to rejuvenation.
Approx 70% voltage boost was applied to the tube heater. 10% voltage boost was also applied to final anode EHT.
The resulting picture was bright and clear, and a full colour palette was restored.
- Colour tracking (channel matching) was poor.
- The CRT heaters glowed yellow rather than red/orange.
- The EHT boost caused very occasional arcover (not due to the heater boost)
- The set was used like this for years and no deterioration of picture quality was experienced.
The so-so quality of the end result was due to the severe tube condition, the fact that it worked at all was impressive. For comparison, rejuvenation did more or less nothing for these tubes.
CRT TVs and microwaves are the 2 consumer appliances with the most risks under the hood. CRT TV risks include:
- live chassis
- stored mains power in reservoir capacitors
- EHT typically 19-24kV
- stored EHT in the tube
- focus voltage sometimes a few thousand volts
- fragile rear of picture tube, & the picture tube often explodes if the unprotected rear is broken
- nasty voltages in many areas of the circuitry
- instability, some CRT TVs are very close to tipping over when the back is removed, and not a lot more stable with the back on.
This is inevitably an incomplete list. If you don't know the risks and how to mitigate them, you should not work on CRT TVs.
The repair trade
Heater boosting is not normally used today in the repair trade, for a few reasons
- rejuvenation is quicker, and ensures the customer will come back later for another set
- The widely known results of boosting come from the historic practice of 10% boost, which does not produce a good level of boost or a long lasting result.
- The knowledge & skill set is not there
- There are few techs now willing to undertake even a minor modification to restore equipment to good service.
- There's more profit to be made from a replacement tube than a smaller repair
- And there's more profit to be made selling new sets than repairs
Rarely a set can have a problem with heater boosting. Any repaired set should always be soak tested before release. If it produces a plasticcy smell after being on for a while, this can be caused by plastic on the tube neck running hot. This is rare, but if it occurs the boost percentage will need to be reduced, or a tiny fan fitted (eg a laptop fan) to reduce tube neck temperature.
If you're just fixing your own set its unlikely you'd encounter this issue.