Also, keep in mind that CRTs defocus if the contrast is turned up too high.
The reason for this is that the peak to peak drive of the video signal to the CRT's grid-cathode becomes large enough that it starts to drive the relative grid voltage positive with respect to the cathode, this causes grid current and changes the form of the beam leaving the cathode and the beam focus goes off fairly abruptly at that point.
If a CRT is getting low on emission, the user often rotates the contrast control to a higher level, and the brightness control too ( this sets the DC axis of the relative grid to cathode voltage) in an attempt to gain a better image, and both of these factors can push the CRT gun into grid current and defocus. So a sign of an aged and worn out CRT with low gun emission is when you cannot acquire adequate brightness and contrast before this sudden defocus effect appears.
If the scanning raster is about the correct size, the EHT value is normal because the raster size is inversely proportional to the square root of the EHT value. If the scanning raster increases in size, at higher CRT beam currents (Brightness) the EHT regulation can be poor, and this is called "blooming" , but it was more of a problem in the past in tube sets with tube EHT rectifiers going low on emission and raising the internal resistance of the EHT as a voltage source, than in modern sets with semiconductor EHT rectifiers.
Sometimes it does take some experience examining a VDU or TV that has CRT brightness, focus and contrast issues, to determine if the nature of the problem is in the CRT's support circuitry or the fault of the CRT itself, but if you have worked with CRTs long enough it sort of becomes second nature.
It is not just the gun emission that is important. As CRT's age the phosphor efficiency drops off and even relatively small amounts of gas, which would be well tolerated in tubes for other applications can cause some beam defocus. A TV raster scan is the most demanding application for a CRT, in that the beam's energy has to be high enough to illuminate the entire faceplate, unlike say a scope where the beam can spend much more time delivering energy to the phosphor for a single scanning line. The beam brightness is roughly inversely proportional to its rate of travel. This effect was noted in the early days of TV, and rather than modulating the beam current, the beam velocity was modulated in one TV system, invented by Bedford and Puckle at Cossor in the UK.
In summary, a TV or VDU CRT does have to be in top order to produce a high contrast sharp as a tack image that maintains the beam focus over a wide range of beam currents. When a CRT is brand new the results can be quite good, especially with a well designed video amplifier driving the CRT. Have a look at the monochrome images produced on these Conrac Avionics grade video monitors that I fitted with new old stock 14BAP4 CRT's, un-touched up photos of the CRT's face, for still frame images, are shown on pages 20,21 and 22:
https://www.worldphaco.com/uploads/T...o_monitor..pdf
(PS: this was a video monitor where no schematic or manual was available, so I had to painstakingly copy the whole thing out from the actual set and document it, quite a big job, but it goes to show, you can do it for VDU's, if you are prepared to spend the time that is)
