Anatomy of a CRT Monitor

dezbaz

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In the last few days I found a few TVs with broken necks, So I decided to dissect them and document the results if anyone is interested

CRT is Cathode Ray Tube, AKA: Monitor / TV / Screen / Tube

I firstly was trying to understand how it all works myself, and now I see how amazing it is they ever got these to work

  • So, I got an 8 Pin from a 15" TV. Someone had stolen the yoke for scrap metal, so I kicked the neck off the screen and took it
  • Then I found a 110 degree 25" CRT which was cleanly broken off at the neck, with the yoke intact
  • I would like to do this with a B&W one day, if I ever see one getting trashed

12 Pin, 25", 110 Degree CRT (With a non original neck board on it)


The 3 guns are aimed perfectly through these holes as shown


See the convergence strip held on the Yoke (See pic above - Black Writing)


The funny thing about that little strip, how did it get there? Did they remove the yoke, meaning convergence had to be started again? Maybe it was slipped under, which is why it fell off so easy?? We will never know

So knowing the guns are next to one another may help us understand convergence. Since the beams run almost parallel until they reach the centre of the screen, you can see how a convergence strip (a MU Metal strip) can effect certain colors only, when converging. Also you can see how Green is in the centre and harder to control

There is a lot going on, so I wanted to know what all the pieces were in the neck of the CRT. This pic shows some of the visible levels as viewed from inside the tube.


This pic below shows the (insert correct name of the part here) which would press against the glass (On the opposite side as the adode) and cause the HV potential to be felt here and at the metal it's connected to.


Here are the locations of everything (Heater resistance of this one is 2.2 ohms



8 Pin, 15", 90 Degree CRT

Here is the neck broken off


Here is the glass removed from the neck so I could use an Ohmmeter to bell out the pins


The heater is just a strip of copper (Nichrome Wire?) You can see why a CRT is toast if the heater burns out
 
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So, using the neck connector as a guide we have pinouts for an 8 Pin CRT. It's called 8 pin, but there are more spots (It actually goes up to 9)

8 Pin CRT Pinouts
1: Focus
2: Not Used
3: Blue Gun
4: Heater
5: Heater (3.9 ohms)
6: G1 (GND)
7: Red Gun
8: G2 Brightness
9: Green Gun

Here we go a little further, removing the G2 Grid


There followed by removal of the G1 Control Grid, exposing a clear view of the RG & B guns
 
great dissection...something i can understand since i'm a doctor and took human dissection classes to better learn about the body...your dissection looked and smelled much better than the kind i used to have to do :)
 
I will go further soon, I would like to show EXACTLY where the rings sit on the neck (Like a window or cut-down set of rings)

It would be good to show a precise method for fitting them
 
Nice writeup and pics...

So how does the gun differ between colors? ie, how does the red gun only light the red phosphors on the screen? I've long wondered that and never asked
 
Nice writeup and pics...

So how does the gun differ between colors? ie, how does the red gun only light the red phosphors on the screen? I've long wondered that and never asked

According to this page

There are actually three electron beams and three different kinds of phosphors. Each electron beam is directed by a grid to land on a phosphor of the same color. That way you have three different colors to mix.

So that means the guns are all the same so to speak.

If you refer to a schematic diagram of a color CRT, you see the guns turn on by completing a circuit to GND.

Like any electronic part, you need +ve and -ve, so the +ve is sourced from the anode voltage, and the gun is turned on by adjusting the pots for that color to the optimal range, thus completing the circuit to GND (-ve)
 
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I think that's the "shadow mask" which sits a short distance inside of the inner surface of the face of the screen. I understand they made them of invar... a special alloy with very low coefficient of thermal expansion, so it doesn't change size with temperature (which would cause misalgnment of the beams with the proper phosphor).
 
Makes sense, thanks

I've made some telescope parts out of invar, for a university program, for the same reason... it machines ok but not great
 
Today I went a bit further, trying to understand convergence

The rings were carefully measured and the neck glass was marked to show exactly where they sit

The results are surprising

Here is the untouched yoke, neck and rings


Measuring distance from the plastic shoulder, =29.89mm


Distance recorded



A pic of the orientation of the Yoke, for the record


Some permanent marker lines for the record


A closeup of the neck, with the cloth tape removed and cleaned


Closeup of the rings


The distance from the back of the rings, to the centre of the purity rings: = 23.36mm
 
Add the 2 measurements together
29.89 + 23.36 = 53.25mm

I fitted a strip of tape on the neck and marked the point on the tape where 53.25mm is


Now here is the Wells Gardner Convergence guide for a WG6100


They suggest the centre of the 2 x purity rings be centred over the gap shown above. (60.5mm from plastic shoulder)

My reading (53.25mm from the plastic shoulder)

This is a huge 7.25mm discrepancy to that location

My suggestion, put the rings back exactly where they were before you took them off, NOT where the WG Manual tells you to
 
This pic below shows the (insert correct name of the part here) which would press against the glass (On the opposite side as the adode) and cause the HV potential to be felt here and at the metal it's connected to.

Just seen this thread quoted and I'm answering this...

That is the getter reservoir. The getter is a barium compound that is heated through a RF coil so it evaporates and removes residual gases left inside the tube after evacuation. It can be physically connected either to the electron gun assembly (as in your pic) or to the anode button.
 
Just seen this thread quoted and I'm answering this...

That is the getter reservoir. The getter is a barium compound that is heated through a RF coil so it evaporates and removes residual gases left inside the tube after evacuation. It can be physically connected either to the electron gun assembly (as in your pic) or to the anode button.

Well thank you

So is Barium hazardous, like mercury or to lesser extent lead etc?
 
Well thank you

So is Barium hazardous, like mercury or to lesser extent lead etc?

It is toxic but when a tube breaks the getter instantly reacts with the air rushing in and forms a solid coating on the glass so you can't inhale it and unless you run your tongue on it you can't ingest it either. The phosphor coating is more of a concern as it's a toxic powder that comes off very easily from the glass if touched or rained on.
 
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