Understanding How CRTs Work And How They Are Numbered

Each of the CR70 “BEAM BUILDER"™Universal CRT Analyzer and Restorer testsclosely duplicates the operation of the CRTin the circuit. This Tech Tip reviews howCRTs work and how they are numbered, soyou can relate CRT testing to the functionsof the operating CRT.

How CRTs Work

The concept behind a CRT is very simple:an electron beam from the cathode strikesthe screen which gives off light. Circuitsexternal to the CRT deflect this beam so itfills the whole viewing screen of the CRT.Some CRTs, like those used in scopes,contain special internal deflection plates.

The CRT can be divided into three majorgroups of elements: the cathode, thecontrol grids, and the phosphor screen.The cathode is the source of the electronbeam. The cathode is coated with amaterial (such as barium or thorium) whichgives off large numbers of electrons whenheated by the filament. The hot cathodeemits electrons which form a cloud aroundthe cathode until something pulls themaway.

The next group of elements, directly infront of the cathode, consists of severalgrids. Each grid has a specific function.The grid closest to the cathode, G1 is thecontrol grid. A bias voltage (negativevoltage compared to the cathode) is placed

on this grid which repels the electronsback towards the cathode. The amount ofnegat ive b ias de te rmines how manyelectrons pass towards the screen. Thecontrol grid is cylindrical in shape, havinga hole in the center. The electrons arepulled through this hole by the next grid(G2), called the screen grid, forming a thinstream of electrons.

The screen grid has a positive voltage on itto pull the electrons through the controlg r id . On ly the e lec t rons wh ich passthrough the hole in the control grid formthe true beam current or the current thateventually strikes the face of the CRT. Thescreen grid is shaped like the control gridand also has a hole through which theelectron beam passes.

Fig. 1: The electrons produced by the cathode are regulated and accelerated until they collide with the phosphor to produce light. (Courtesy GE)

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Fig. 2: A CRT gun has five basic elements.

One or more accelerating grids follow thescreen grid (each of which has increasinglevels of positive bias) to increase thespeed of the electron beam. In addition,one or more focus grids form the electronbeam into a fine hair-like thread which hitsthe phosphor screen, resulting in a verytiny spot of light.

The f ina l g roup o f CRT e lements i sresponsible for producing the visual image.These elements include the phosphorscreen and a structure called the secondanode. The second anode is quite similar inshape to the other grids but it has a veryhigh positive potential on it. An aquadagcoating lines the inside of the CRT betweenthe second anode and phosphor screen.This is at the same potential as the secondanode and keeps the beam from beingdistracted off course.

The second anode speeds the electronbeam to an extremely high rate. The beamthen suddenly collides with the phosphorscreen, causing light to be emitted by thephosphor. A color CRT has a phosphorscreen with three colors of phosphor (red,blue, and green) which are very close toeach other. Each one is struck by electronsfrom the corresponding beam from one ofthree identical electron gun assemblies.

In order for an image to be produced on thescreen, the electron beam must be made tovary in intensity by changing the biasbetween the cathode and control grid.

The CRT beam is cut off with the normal DCbias which is applied to the control grid.This results in a black level beingdisplayed on the screen, since no beamc u r r e n t r e a c h e s t h e s c r e e n . V i d e oinformation is applied to the cathode,control grid or to both the cathode andcontrol grid, which reduces the amount ofG1 bias, enabling more of the electronbeam to str ike the phosphor screen.Reducing the bias to zero allows maximumbeam current and maximum CRTbrightness. Varying the bias producesbrightness levels between cutoff and fullbrightness.

Understanding CRT Numbers

Most CRTs are registered according tosome kind of industry standard. Thesestandards define certain characteristics ofthe tube. Since Apri l 1, 1982, thesestandards have been combined into oneworldwide standard. Prior to that date,several non-universal standards wereused.

Old Standards

The CRTs which were introduced prior toApril 1, 1982 were registered differently inthe United States, Japan, and Europe. Ingeneral, the registration number brokedown into three parts. This is the

Fig. 3: The old standard can be broken intothree groups.

numbering system with which everyonehas become at least somewhat familiar.

The first part of the CRT number is a seriesof d ig i ts wh ich s ign i fy the min imumdiagonal viewing measure of the CRT. ForAmerican tubes, which are listed in thefirst half of the CR70 Setup Book, this sizeis in inches. Thus, a 19VACP22 wouldhave a viewing diagonal measure of 19inches. Japanese tubes, which appearlater in the book, have this distancemeasured in millimeters.

The next part of the CRT number consistsof one to four letters which designate aparticular CRT within a group of CRTs

Fig. 4: The WTDS system uses six groups of symbols to identify CRTs.

having the same screen size. The final partof the CRT designation indicates the type ofphosphor used. Black and white videoCRTs use a P4 designation for Americanlistings or a B4 listing for Japanese listingswhile color CRTs use a P22 (American) orB22 (Japanese) listing. Computer CRTs orscope CRTs may use some other type ofphosphor, and will have a different numberfol lowing the “P” or “B”. But, as yousee, the "P" or "B" is not part of the tubedesignation.

Some CRTs have listings that do not have a"P" or "B" ending. The most commonnon-standard ending is ' "TC01 " or“TC02”. These CRTs always have bondedyokes (or some other component) per-manently attached to the CRT neck. The“TC” ending simply indicates the type ofyoke plug the CRT uses to connect to thechassis. The CRT is identical to one with a"P22" or "B22" ending. For example, a15VAETC01 is identical to a 15VAEP22.Only the standard (P22 or 822) listingsappear in the CR70 Setup Book.

WTDS Standard

Since April 1, 1982, a new system forcategorizing and numbering CRTs hasbeen in use. This system is officially calledWorldwide Type Designation System

(WTDS) for TV picture tubes and monitortubes. Until the adoption of this system,American, Japanese, and European tubemanufacturers had al l numbered theirtubes differently. This has led to confusionand incomplete or inaccurate information inthe past. The new WTDS numbering is ane f f o r t t o s i m p l i f y a n d u n i f y C R Tdesignations.

The WTDS number consists of six groupsof symbols. The first symbol defines theapplication of the tube. This symbol isalways a single letter; either an A forpicture tubes or an M for monitor tubes.The second group of symbols is a two digitnumber wh ich de f ines the min imumdiagonal view. This measurement is alwayslisted in centimeters (1 inch = 2.54 cm.).

The next group of symbols consists ofthree letters. These letters designate afamily code for the CRT. Tubes within aparticular family have specific mechanicaland electrical characteristics. These lettersare assigned alphabetically beginning withAAA, followed by AAB, AAC, etc. Tubeswhich contain the same sequence of lettersare identical as far as their setup for theCR70 is concerned.

One or two digits follow the family code.These digits indicate a specific member

within a part icular family. A differentmember number would be assigned totubes within the same family that havedifferent neck diameters, for example. Asingle digit member symbol indicates amonochrome tube while a two digit numberindicates a color tube.

Following the one or two digit membersymbol is the phosphor type designation.Color picture tubes are designated by thesingle letter X, while color monitor tubesm a y h a v e s o m e o t h e r s i n g l e l e t t e rdesignation. Monochrome picture tubesare designed by the two letters, WW. Othermonochrome tubes, such as monitors,have a different, specific two letter code todesignate the phosphor type.

S o m e t u b e s c o n t a i n i n t e g r a l n e c kcomponents, such as bonded yokes. Thesetubes have a sixth group of symbolsassigned to them. A two digit number isused to define the characteristics of theseintegral neck components.

Tubes which follow the WTDS numberformat are listed in the CR70 Setup Book.The whole WTDS number is included in thesetup book, (even though the three letterfamily code is the only part of thedesignation which is required for propersetup), to avoid the possibility of confusionwith some other number that may appearon the CRT, such as a manufacturer’s partnumber. Tubes having the sixth symbol(designating an integral neck component)are listed without this sixth symbol just as"TC01" endings are deleted from theconventional CRT numbers.

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