Cathode ray tube disassembly and explanation--- make money online

hey guys today I'm taking apart and

electrostatic cathode ray tube this is

what you would find in an old-fashioned

oscilloscope so it was actually a very

small oscilloscope the whole screen

diameter is only about three inches and

I used a diamond cutoff wheel and in

tremble and I cut the back of the tube

open and then pulled out the guts so

this whole electron gun assembly was

inside there and cut the screen off I'm

going to use the screen and another

project that phosphorus screen it's

probably a zinc sulphide screen or

something it's got a nice green dot and

I think I might use the heater out of

this in another project as well so the

glass tube is has a coating on the

inside and I I don't know how to

pronounce this aqua dag a quad egg

whatever it's it's basically a graphite

slurry that was poured inside there to

make the inside of the tube conductive

so the idea is that you don't want

electrons charging up this glass tube so

they've they added that conductive

coating in there so the way the cathode

ray tubes work is by accelerating

electrons and slamming them into a

fluorescent screen so in this case the

electrons start out over here at the

beginning of the electron gun and our

focus accelerated and deflected by this

whole assembly here and make their way

all the way out to the front where they

hit the screen and the fluorescent

phosphor there produces a spot of light

where the electrons are striking so this

technology I guess is kind of going out

of style I mean with plasma and LCD

monitors is not really much need for

this anymore there are a few benefits

though one is that the screen can update

extremely fast so for analog

oscilloscopes you can have the trace

moving extremely fast and be much faster

than you can see with your eye and so

the phosphor has a persistence to it

so it as the trace goes tearing across

the screen the phosphor actually shows

that a line where that electron be

is moving even though the beam itself

was going much too fast to see so let's

take a closer look at the gun here and

see what's going on inside there okay so

here's a close-up of the electron gun

assembly and the electrons start out

inside this metal can here so this uses

a so-called thermionic cathode where

there's a tungsten filament thinner that

gets very hot this is partially why TVs

have so much heat coming out the back

most of the heat that the tube uses is

wasted in this filament here just as

thermal energy that comes off but some

of that energy is used to so-called boil

off electrons off of this cathode and

what happens is there's sort of a cloud

of free electrons inside this can after

you have a cloud of electrons there we

want to shoot them at the screen and the

way to do that is to put a potential

difference in this cloud area so this

can is at a positive voltage and this

one is at a negative voltage and as soon

as the electrons get into that space

between the cans they experience a very

high acceleration force so when I say

potential difference we're talking about

I think for this particular CRT it was

maybe a thousand volts for color TVs

it's quite a bit higher but for

monochrome oscilloscope type things with

just electrostatic deflection it's

really need like a thousand volts maybe

two thousand depending how big the

screen is and then you'll see as the

electrons continue down through here

there's more of these cans electrodes to

deal with and these handle the focusing

operation so as the electrons are beamed

through here they naturally want to

repel from each other because they have

similar charges just sort of like two

pieces of styrofoam that sort of push

against each other the little electrons

try to separate out and the purpose of

these electrodes is to focus the beam

into a pretty narrow path we don't want

the beam to narrow though because if it

strikes the phosphor screen with a tiny

little point

and the dot isn't going to be big enough

to see so really the goal is to get a

dot that's maybe half a millimeter or a

little meter in diameter when the system

is at perfect focus so after we get

through this electrode here the beam is

hopefully focused and will produce a

correctly sized spot on the screen and

the last part of this gun of these

deflection plates so you can see that

there are two plates and there they

angle out at the end and this this is

sort of what spurts say this is the Y

direction if I turn this thing around

you can see that there's plates

orthogonal so it handles both axes there

and by putting a potential difference

across these plates the beam will

deflect and of course in either axis

here's a shot looking down the end of

the gun and what you're seeing are the

two deflection plates for one of the

axes and the hole is the last focusing

electrode there let's take a closer look

at the focusing assembly so what we have

here is one entire metal can electrode

that expands from here all the way to

here then there's a middle one and one

at the end again and as you can see

these two electrodes the first one and

the last one are joins together

electrically and the middle one is at a

different potential there's a different

line that takes that one out outside the

tube and this is known as an ISO lens an

electret when you have electrodes spaced

like this you can call it a lens because

as the beam of electrons fires through

the middle the effect of these potential

differences causes the beam to focus so

it actually is very analogous to light

optics and a convex lens of sorts so the

voltages on these are fairly high -

they're on the order of the accelerating

voltage so if this tubes operating it a

thousand volts maybe there's going to be

800 volts to focus it so in that case

this can and the last can would be at

the accelerating voltage of

in the middle electrode would be at

eight hundred or something like that

I actually didn't use this tube before I

took it apart so I'm not exactly sure

here's another structure that's

interesting

most vacuum tubes have this this is

called a getter and the function of this

is actually just to keep the atmosphere

inside the tube as clean as possible so

it actually doesn't have any effect on

the electronics of the gun its purpose

is to deposit some usually a reactive

metal like magnesium or something like

that on the inside of the glass and so

actually I should have taken a shot of

this first now it just looks like a

bunch of white ash inside here but

originally it was probably a shiny

little patch of metal and so if you look

at most vacuum tubes you'll see that

there is like a a very shiny sort of a

mirrored surface on the inside and the

purpose of that is just to absorb oxygen

and other impurities that might come out

of substances inside the tube so for

example if there are some oxygen atoms

that are stuck on the surface of this

metal can when they put the two together

and suck out all of the air there might

be some oxygen that slowly leeches out

and would degrade the performance of the

tube so it's sort of extra insurance

this little getter here deposits

aluminum on the outside or the on the

inside of the glass and those oxygen

atoms will bond with the magnesium

aluminum or whatever it is and form an

oxide which is stable so it gets the

molecules out of the out of the vacuum

inside there and maintains a good vacuum

okay so here is a little bit more modern

oscilloscopes still fairly old this is a

full analog oscilloscope and it has a

CRT that's very similar to the one that

I just took apart but it has a few small

updates which I'll talk about in a

minute but I wanted to show you this

deflection voltage idea so in the old in

the gun that you saw earlier has these

deflection plates and I said that if you

put a potential difference across these

plates the electron beam as it's coming

through here will get deflected and

that's how

the dot has moved on the screen so let's

see that in action what I've got here is

the X deflection voltage the actual

difference between these plates being

measured on the meter over there and

right now it's just about zero and the

dot is pretty close to the center so if

I turn this so the dot moves to the

right we can see the voltage is climbing

and it's hitting about a hundred volts

as the dot is hitting the right side of

the screen and if we go all the way over

to the left side of the screen we've got

negative 86 volts so pretty close the

the actual center is not quite in the

center of the screen so if I dial this

to zero volts for about half a division

to the left now interestingly if we

monitor the y-axis the voltages that are

not as high and the reason for that is

that the gut the y-axis deflection

plates are closer to the front so that

they have a bigger effect on where the

electron will end it on the screen so

for example if the electron is deflected

in the Y Direction first it has a longer

to go before it hits the screen and

therefore will have a higher deflection

for the same acceleration due to these

plates here so you might wonder if this

is using electrostatic beam deflection

what's going on with this coil of wire

over here this is a trace rotation

equalizer so if there's a part of the

coil or if there's a part of the CRT or

some other piece in the oscilloscope

that gets slightly magnetized the beam

of electrons will rotate about the

center of the CRT axis so this sets up a

magnetic field that you can control with

the trace rotation control on the front

and that will fix the beam and make it

perfectly level with the with the

division markings on the front of the

screen so in this case the magnetic

field is circling around that coil of

wire so the field is basically going in

the direction of the electron beam and

that will cause it to rotate about the

center of that coil this is Sol scope

also has

justing adjustment called astigmatism

and watch what this does now generally

you can't adjust the astigmatism from

the front panel because it's something

that is set at the factory what's

happening here is the voltage between

the deflection plates there's another

electrode hiding between the X and the y

deflection plates that has a different

voltage on it and by changing that

voltage the beam is either flattened out

in the X direction or in the Y direction

so in conjunction with the focus control

the beam can be made into a nice round

point if the astigmatism is off you may

not get a very good focus and if it's

way off you end up with a flat little

squished pancake looking kind of a thing

there instead of a nice dot the tube

that I took apart before that small and

show the electron gun that one did not

have an astigmatism control because it

had used the same focus voltage so there

is no separate adjustment on that one

all right well I hope this is helpful

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