CRT Safety and Basic Guidelines

==Guidelines for working safely on CRT monitors or TVs :==
DISCLAIMER : very high voltage up to 40 KV (40 000V) depending on the model is present inside and the electric charge can still be deadly for a long while after unplugging the device from mains. The electric charge may last months in the worst cases.
If you don't know what you are doing, I strongly advise against tinkering inside a CRT as a first project. They are cool looking, but not safe.
The following information is only here for informational purposes and to reduce risks while working. If you still want to proceed, it's at your own risk and we deny any responsibility should anything bad happen.
Always work inside the CRT with power cord unplugged (duh). While unplugged, turn it on to start discharging some capacitors (it's a start, it's not enough).
Carefully open the device without touching the insides.
Keep one hand behind your back at all times until the tube and capacitors are discharged or while mains power is still present so you avoid current circulating through the heart. Take off any metal jewel, ring or watch, those are entry points into the body for the electricity.
And to avoid releasing the vaccum abruptly or shattering the tube, don't bump into the fragile tip and the board socketed on the end of the tube, called the neck board. Also avoid touching or messing up the yoke and its adjustment rings (the enamled copper wires assembly around the tube), or it'll be a nightmare to make the thing work properly again.
The tube has a positive, the anode, under a rubber cap that you must not touch either as rubber can loose its insulating properties in such old devices. Avoid the high voltage wire as well (usually red) connected to it coming from the board. The other danger zones in a CRT are around the black high voltage flyback transformer on the board (usually covered in black dust from the static electricity, refer to pics below), and the large capacitors on the board (their leads go behind the PCB so you may touch them from under the circuit board, where the solder joints are). These capacitors are usually rated between 100 to 200V, sometimes up to 400V.
Avoid touching the neck board too. And if ever you test the thing open, note the metal heatsinks on the board can be energized or tied to live potential. Older CRTs (1960s to 1980s or so can even have the whole metallic chassis tied to live, especially in arcade cabinets). Also in vacuum tube TVs, some tubes under a metal shield emit X-rays so don't open the shield or stand near those while it's on!
===Discharging procedure===
First, find an insulated screwdriver, a copper cable with insulation (stripped on both extremities) and a pair of thick gloves, preferably heavy duty leather or real electrician gloves.
Strongly attach one end of the wire to your screwdriver shaft, ensuring a good contact. Tie or alligator clip the other end to chassis ground, aka the metal shielding of the case. Sometimes you have a massive metal sheet shield all around the largest side of the tube, sometimes you only have small naked metal braids running along the tube from the corners to the back, usually with one or several springs attached to them (see images at bottom of the page). Locate the proper place and tie your wire to it.
Avoid using the retaining screws in the corners of the tube as contact point, they have a rubber ring and don't make contact with the chassis ground.
Once both ends are properly connected, with one gloved hand, while touching only the screwdriver plastic handle, you can insert the tip of the screwdriver under the rubber cap, reaching for the center. Touch the center part a few times. You may hear a pop or simply nothing.
Remember, you are short circuiting a giant glass capacitor with two plates, so the outside and the inside of the tube. It's like a large Leyden jar. Positive is the anode leading inside, negative is the outside shielding and conductive paint. The charge is held between the two with the glass as an insulator.
Note the tube may even get charged again by sitting alone because of static electricity, so you want to do that again before reconnecting the anode, even if the buildup charge is not as strong.
Next, once you're certain you did the previous step properly, you can separate the anode cap from the tube like this :
Take pictures from all angles before disconnecting any connectors inside, it'll help greatly while putting it back together.
Next come the large capacitors on the board. Reaching them can take a few careful steps disconnecting and removing the board from the device. Be careful you're not touching the solder joints side of the PCB or you could be severely shocked. 
Wear safety glasses, like the ones used for grinding, to avoid melted metal in your eyes in case you short circuit the capacitors accidentally. Start by measuring the 100V or more capacitors between their two corresponding solder joints under the board with a reliable multimeter and probes (real CAT safety rating for such voltages, not a chinese crappy one). Firmly apply the probes on the solder joints without slipping (don't short the cap) in Volts DC, highest caliber of the meter (600VDC usually). If you read below 10V, you may short them with an insulated screwdriver.
If it's higher, they must be discharged with resistors. You have to find a compromise between speed and thermal dissipation. I used a 1 meg resistor because it was 0,5w but it took around 30 minutes for the largest one in my particular monitor. You could also find a large wound resistor of the appropriate rating, let's say around 15-30K ohms and 10 Watts.
Secure the resistor legs to your multimeter probes and monitor the voltage until it goes down to safe levels, around 10V. More can still generate a huge spark, even around 20-30V.
Remove your meter and free the probes from the resistor, then measure again after a minute (it could climb back up). Do that with all large 100V+ capacitors.
Congratulations, your board is now safe to work on. Take off the annoying safety gear and enjoy recapping/RGB modding/dusting with a soft brush :-D
The more dust there is inside, the more the CRT was used and may be worn!
===Common issues, as a starting point (does not replace a full diagnostic with schematic or measurements) :===
Unstable, unreliable, won't stay on, takes an unusually long time to light up or image bouncing : replace electrolytic capacitors, possibly bad flyback, issue with inrush limiting NTC or power regulation
Bad geometry, curved image side changing a lot with brightness : Bad ceramic caps for vsync, Bad flyback
Dim picture brightness or washed out colors : bad voltage regulation, issue with inrush limiting NTC, worn tube
Middle bright spot or vertical straight line on black screen : bad yoke? Major deflection issue
Collapsed image (horizontal straight line in the middle of black screen) : Bad joints on the neck board (micro-cracked) or bad main vertical transistor, convergence circuit issue
Colored patches (mostly green, orange and pink) in large concentric patterns or in a specific spot : degaussing circuit not working because of bad PTC (can you hear it engaging at power on ?) Ensure no unshielded speakers are nearby. Your CRT may have been exposed to unshielded monitors for a long time and degaussing is not enough. Waving a large electromagnet in front of it may help.
Image sideways more than the OSD menu can fix or deformed corners : The yoke has moved, is not stuck properly to the screen. The magnets on plastic tabs glued on the tube have detached, other electronic issue.
Colors overlapping (RGB borders superimposing on image details) : The yoke rings or yoke itself have been moved.
===Useful videos and links===
Another take on the matter at repair.wiki that can be a great complement to this page : https://repair.wiki/w/CRT_repair_and_troubleshooting
Safety and discharge :
https://www.youtube.com/watch?v=a6FMpvs71pc
https://www.youtube.com/watch?v=1CVXzlkOjGg
https://www.youtube.com/watch?v=N39PZB8gYhk
https://www.youtube.com/watch?v=d1mQupQHFkc
https://www.youtube.com/watch?v=DEC75Gb4I-w
https://www.youtube.com/watch?v=JeX5Y7Amk0o
Repair dim image :
https://www.youtube.com/watch?v=hCopsoy0lQI
Choose capacitors for recapping (replaçing all electrolytics) :
https://www.youtube.com/watch?v=zNhjXvzdcLU
https://www.youtube.com/watch?v=uyBjMWzzmf8
Why you should not discharge capacitors with a screwdriver :
https://www.circuitspecialists.com/blog/discharge-capacitors-in-switch-mode-power-supplies/
Useful info about all kind of picture discolouration issues :
http://wiki.diyfaq.org.uk/index.php/CRT_picture_discolouration
https://www.electronicsrepairmadeasy.com/2017/08/solving-crt-television-color-picture.html
===Illustrating images with a Sony monitor from 1999 and an US TV ( TV base pictures courtesy of user RogerB from Discord)===