This section explains the working of colour tubes and will (I hope!) help you to determine tube problems.

The Colour tube basically creates 3 pictures, one of each primary colour: Red, Green and Blue. Most early tubes were of the shadowmask variety. These have phosphor dots on the screen, red phosphors, green phosphors and of course blue phosphors.

There are three guns in the tube neck: one for each colour. In Delta gun tubes (such as the A56-120X used in the BRC3000) these are arranged in a triangle. In PIL (Precision in Line) tubes, they are in a horizontal line.


All the emission from the Red Gun must land only on Red Phosphors, similarly the Green Gun's emission must only hit Green Phosphors and of course the Blue Gun's emission is only intended for Blue Phosphors. At the screen end of the CRT, the shadowmask is employed for this purpose and magnets on the tube neck assembly direct the elecron beams from the three guns accordingly.

Sometimes, this does not happen and the picture becomes IMPURE. The causes of this are misalignment of the magnets above, which are the PURITY magnets, a magnetised Shadowmask or a damaged or bent shadowmask .

The shadowmask is kept free from magnetic charge by being demagnetised every time the set is switched on from cold. There is a black coil wound around the tube, this is the DEGAUSSING coil. It is connected across the mains via the DEGAUSS thermistors. (In more modern sets, these thermistors are replaced by a less reliable little white thing called a Posistor!). This arrangent, which makes a brief buzzing noise when the set is turned on, keeps the tube's shadowmask demagnetised.

This should in most cases cure the problem. If not, repeat the procedure until no further improvement can be made, then proceed to the purity adjustments.

In some cases, the shadowmask becomes excessively magnetised. When this problem occurs, the tube is demagnetised with an external degaussing wand. This is basically a large coil wired across the mains. It should never be left plugged in when not in use. To use first switch on the TV set, display a pure red raster, position it well away from the set, switch on, bring slowly towards the screen, and then draw a figure of eight over the tube face. Don't let the pretty coloured display put you off. Withdraw slowly back from the set, continuing this figure of eight motion. When you're far enough away for there to be no visible effect on the picture, back off a little further and then switch off the degaussing wand.

If this does not completely cure the problem, repeat the procedure above until no further improvement can be made, then move on to purity adjustments on the CRT neck.

Never swtich on or turn off the mains to the degaussing wand when it is near the set. This can MAGNETISE the shadowmask and make matters WORSE!

The Purity magnet adjustments can be found on the tube neck. The procedure for adjusting these will be in the set's service manual. Just as an example, on the BRC3000 series, it involves loosening the two wing nuts on the side of the tube neck assembly and then adjusting them for correct purity in the centre of the screen. They are then pushed forwards together until the whole screen is (should be!) pure. Other sets use purity rings on the tube neck. There is a typical tube neck assembly shown below.

The purity rings are magnetised as shown here.

Finally, if you're very unlucky, the shadowmask could be bent or damaged. In this case, you have two choices: One is to just get it as good as you can, the other is to get a new CRT.

A Pure red raster should look like this. Imupurity will show up as splodges of other colours or paler red.


This is where all those controls on the convergence panel come in. I gave a basic explanation of how to converge a tube under 'The Basics'. The main premise on both STATIC (Centre of screen) and DYNAMIC (The rest) convergence adjustments is to converge Red and Green to create the secondary colour YELLOW, and then to marry the blue up to this.

The first stage is to set up the static convergence, this is done with the convergence magnets and the blue lateral adjustment on the tube neck. Here is the convergence yolk in all its splendour

The aim is to perfectly (sic) superimpose all three primary coloured displays to create a crisp, sharp colour picture. With the colour turned off and the brightness low, there should be no colour fringing. Misconvergence can look rather like a badly printed colour newspaper.

Fortunately, the circuitry on a convergence circuit is relatively easy. If a control seems to be inoperative, the problem is usually the control itself (especially the wirewound pots) or a component associated with it alone. If two controls or more are inoperative or won't set up, look for a common denominator on the circuit.

I have tried to draw a perfect crosshatch with the graphics package. This is what you are aiming for. The centre is aligned first with the static magnets on the tube neck. Then the dynamic controls are used to set up the rest. In practice, it is very hard to achieve perfection.
The Baird 700 Manual specifies that it should look OK from 6 feet away!!!!


The most common problems with an aging CRT are low emission and poor grey scale tracking. I've used a well loved image to illustrate my points in this section. These images have been doctored on good old PaintShop, rather than muck up one of my precious TVs to create a sideshow!! On the right, here is a perfect image of the best ever Daytime TV programme...
It is made up of three images:

...and Blue.
Incidentally, have you noticed that the green image looks brighter than the others, and the blue one the dimmest? This is down to the way the human eye sees colours, it is in fact more sensitive to green.
When the colour control on the TV is turned down or the transmission is a monochrome transmission, only varying shades of GREY should be seen. This is called GREY SCALE and the setting up of this is called setting up of GREY SCALE TRACKING. There should be just the right amounts of all three primary colours to create dark grey, grey, light grey or white. The 'shade' of white is called ILLUMINANT 'C', which is a slightly warmer shade of white than seen on Monochrome Tubes. Like this....
Secondary colours are made up of two of the three primary colours. Therefore, when a particular gun goes down on a CRT, or drive to that gun is low or missing, the overall picture colour will tend towards the secondary colour made up of the two OTHER Colours. For example, if the BLUE Gun was to fail, then the picture would tend towards YELLOW, i.e Red and Green. Here's the three examples...
When the BLUE Gun is weaker, the picture will tend towards YELLOW....
When the GREEN Gun is weaker, the picture will tend towards MAGENTA....
& When the RED Gun is weaker, the picture will tend towards CYAN....
So now we know about colour balancing and how a CRT works. The voltages that you would expect to see on a tube vary from one tube type to another. We'll take one example and look at the voltages.
Each primary colour gun has its own heater and these are normally wiresd in series. The tube also has three cathodes and three grids, one for each colour.
Look at the Diagram below. This is abridged from the BRC2000 manual,and refers to the A53-120X Colour CRT.
These voltages are taken during normal reception. Never try and measure the EHT and focus volts directly, its a good way of ruining your meter. Note also that the voltage on the focus anode only affects focus and does not affect gun emission.
So, what does a bad tube look like? I have tried to simulate a really knackered tube on the right.
The things to look for are lack of definition on bright parts of the picture, flaring (to the right) on bright sections, lack of definition, incorrect colours and grey scale which varies with picture brightness.There can also be a 'metallic' look to the picture. Before condemning the tube, check the voltages on the base and if one primary colour seems to be low or missing, try swapping with another drive. I.E. if there is no red, drive the green gun with the red drive. If there is still plenty green content, the tube is almost certainly faulty.
The simulation to the right is exceptionally bad, and could point to low luminance drive. However it does give some indication of what to look for.


Most tube tester/reactivators will give an indication of the condition of each gun. The best ones (in my opinion) have a scale that reads simply Red, Amber and Green for Bad, Indfferent and Good respectively.
The Reactivator itself acts by increasing the tube heater volts a little and then clears the guns by sparking a voltage across them. This works with some tubes and not others. There is no way of knowing how long the reactivated, or BOOSTED tube will last. Mullard tubes generally reactivate fairly well, but Mazda ones tend to react less favourably. Finally, there is nothing left to reactivate and the tube is now scrap. Gun assemblies are, I am told, still available, so if the tube face is in good condition, the tube can be regunned.
A CRT can be killed by over reactivation, but when there's nothing to lose, there's nothing to lose...

If a tube is slightly weak, the voltage across the heaters can be increased to brighten things up a little. Many sets have a resistor in series with the tube heaters, this can be shorted out and the set re-grey scaled.
The grid voltage can be increased to improve emission from the CRT. This is actually sanctioned in the BRC2000 manual. Remember though, that when a tube is on its way, increasing heater volts can speed up the decay. These are temporary measures.