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CRT repair and troubleshooting

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inside of CRT television with basic components labelled

This page is dedicated to the repair and troubleshooting of CRT (cathode ray tube) televisions, monitors, and other equipment.

Safety Warning:[edit | edit source]

CRT displays contain extremely high voltage (up to 30,000v!) and the acts as a capacitor which can store a lethal charge for weeks after the display has been unplugged. The tube MUST be discharged properly before performing any kind of internal repairs.

Here is a good guide to refer to for discharging CRTs: https://www.youtube.com/watch?v=1CVXzlkOjGg

CRTs also contain mains voltage so general electrical safety precautions should be taken when working on them.

Service Manuals:[edit | edit source]

Due to the facts that most CRTs come from a time when repair was more common and companies didn't claim it was a security risk to have someone replace a failed capacitor, complete service manuals that included full schematics were often given out to independent repair shops, in the cases of older televisions they may have even been included with the television. Many of these manuals and schematics are now available online, simply look up the model of your TV/monitor and look for a service manual.

Descriptions and service manuals for Commodore monitors can be found here: Commodore monitors

Repairs[edit | edit source]

Symptom/Issue Fix
General geometry issues, one side of screen is too disproportionally sized or is generally not scanning properly Although a wide variety of things can cause such issues, one of the most common failures and causes of all sorts of issues in CRTs is capacitor failure. Low quality or aging electrolytic capacitors are often to blame for issues with CRT displays. Here are some general steps for troubleshooting capacitors:
  • Start by doing a quick glance at all the circuit boards, if you see any clearly leaking or bulged capacitors then replace them and clean any potential leakage from the PCB. If you see any capacitors that have blown up or otherwise failed catastrophically then it would be advisable to check the surrounding components that connect to the capacitor as one of them being bad could have caused the cap to fail. (Having the schematic will make this much easier.)
  • Check the capacitors that are near whatever function is not working properly (so if geometry is bad then check the caps that surround the deflection coil plug) If you have a schematic then you can use it to see exactly what capacitors effect the function you are having issues with.
  • If you display has capacitors on the neck board or has any daughterboards then also check those, since the daughterboard can sometimes contain vital components like the jungle chip and contain capacitors.
  • Since no 2 displays are the same, there is no definitive guide as to which capacitors go bad and cause what, the best way to figure out what capacitor to replace is by looking at the board and the schematic.
-Crackling, smoke smell, electrical spark sound that causes an temporary screen blackout.

-High voltage is present but screen is showing no sign of life.

Although this can be an issue with the tube itself, at times these issues can sometimes be caused by issues with the neck board or flyback circuit, sometimes even just dust.
  • The first thing to try would be to remove all dust from the inside of the display, particularly the high voltage areas near the flyback and on the neck board. Sometimes dust build-up can cause a ack of contact between the CRT and the socket.
  • If removing the dust doesn't work then it is advisable to check the neck board for failed components. A bad component on the neck board could be providing a path to ground (which the high voltage will gladly take if presented with the opportunity.) Also the neck board has components that connect direcly into the CRT socket (by extension the CRT itself.) If these have failed then it can cause major issues with the CRT. Check the neck board for failed components and if you find any replace them.
  • Checking for cracked traces or any bad connections on the neck board and flyback circuits. Any cracked trace can mean high voltage is arcing across it, causing a smoke smell and crackling sound, If any are found them repair them with whatever method you choose. Bad connections includes the one with the CRT and the socket, check for any sign of corrosion or other build-up on the CRT pins and in the socket.
  • Check for failed components surrounding the flyback circuit, if something is not working correctly then flyback could be creating too high voltage which can cause arcing and damage to the tube.
Magnet damage, screen is badly distorted in colour and/or geometry.

(See fig 1 for example)

"Magnet damage" in this case refers to when the internal parts of the tube have been affected by a magnet being brought near then which messes up a bunch of things and may make the display seem unusable, however it can sometimes be fixed (depending on severity.) The display needs to be degaussed there are 2 main ways to do this.
  • Using the internal degaussing function: More modern CRT displays will often contain some for on an internal degaussing coil that can be activated from the OSD controls. On monitors it can be found in the user settings or even as a discrete button, the symbol for a degauss button/function is a horseshoe magnet with a cross through it. In TVs it may need to be accessed through a service menu or a service remote, refer to your TV's service manual to see how to access it. Run a single degauss, if it does not fix the problem the wait a few minuets and then degauss again, if it still does not help or does not fully restore the screen then move on to the second method.
  • Using an external degaussing tool: An external degauss coil will be more powerful and effective than the internal function. To do this you will need either a degaussing coil (which can be purchased on eBay) or a strong neodymium magnet. For a degauss coil you simply hold the coil to the screen, turn it on, then slowly pull it away from the screen. To degauss with a neodynium magnet you hold the magnet up the screen then spin the magnet while slowly pulling it away from the screen.
Damaged or failed tube (glass not cracked or broken) In many cases a damaged or failed tube is death knell for a CRT, the tube is the most expensive single part of the display and now that they are no longer in production finding a specific one is harder and more expensive then before. Years ago a damaged tube could be repaired at a component level by carefully cutting open the tube neck then literally rebuilding the electron gun, however shops that used to do this have all closed. The museum of early televisions has CRT rebuild equipment[1] and is looking to one day offer it as a service to consumers. So if your tube has failed and you are dead set on keeping the display alive it may be worth holding out for that service to be available.
Damaged tube (glass cracked or broken) At this point the only way to fix the display is to replace the tube; if you can find a tube for your display for a good price then replacing is fairly straightforward and a service manual can provide additions details.

Picture adjustment[edit | edit source]

NOTE: Most picture adjustments should only be done after it has been confirmed that there are no faulty components on the board and those that are have been replaced.

WARNING: For most of these adjustments to be effective, they must be performed while the CRT is turned on, make sure that you are isolated from ground and are using a nonconductive screwdriver to make adjustments. Stay clear of the neck board, anode cap, and exposed flyback circuitry whenever possible!

Over time, CRTs can go out of focus, misconverge, dim, or develop geometry issues; here is a very basic guide to troubleshooting and fixing such issues:

To start you are going to want a good testing image or program that can display images onto the CRT screen to aid you in adjusting focus or geometry. Here are some ways to do that:

  • NEC test pattern generator is a program that will run on Windows and Linux (using Wine) and will generate test images onto your monitor for convergence, geometry, focus, and more.
  • You can buy or make dedicated hardware that plugs into TVs or monitors and generates test patterns on the screen.
  • You could burn a video calibration disc and play that onto the TV through a game console or DVD player. (Here is an example of one for NTSC TVs: https://archive.org/details/AvCalibrationNtscDvd)
  • Some TVs or monitors may contain built-in test patterns that can be accessed through a service menu, check a service manual to see if they exist for your model and how to access them.
Symptom/Issue Adjustment/Fix
Images on screen appear blurry, particular at high resolutions. Is often caused by an out of focus CRT Firstly, in test pattern generator, go to the "Focus" or similarly named pattern, this will display a bunch of small dots on the screen, the goal of focusing the CRT is to make these dots as small and even as possible. Once the image is prepared it is time to begin making focus adjustments.

Secondly, a magnifying glass may be helpful, particularly for small or high resolution screens in order to be able to see the dot size better.

Some very old CRTs may contain a focus adjustment potentiometer externally, either on the front or the back of the unit, however this is uncommon and this explanation is for CRTs with internal pots on or near the flyback.

  1. Disassemble the monitor/TV, discharge the tube, and identify the flyback transformer and potentiometer locations. You may see 2 or 3 knobs on the side of the flyback transformer. In the case of a 2 knob transformer they will be labelled "Focus" and "Screen" or something similar. In the case of a 3 knob transformer they will be labelled "H Focus" "V Focus" and "Screen" or something similar. If there are not knobs on the side of the transformer then the adjustments may be discrete potentiometers located near the transformer, they will have similar naming on the PCB silkscreen. For this adjustment you will only want to touch the focus potentiometers, do not touch the screen pot.
  2. Turn the CRT on, ensure you are isolated from ground and using a nonconductive object to touch the potentiometers, display the Focus test pattern on screen using the highest quality connection available.
  3. For CRT with single focus pot: Slowly turn the focus pot in a direction while looking closely at the dots displayed on screen by the focus pattern. If the dots are becoming larger, then you need to turn the knob the other way. Once you have figured out the needed direction to turn the pot, look very closely at the dot and zero in on the point where the dot is smallest. For CRT with dual focus pots: When you have 2 focus pots you are looking for not only the smallest dot, but also one that is even in width and height. Begin with either the H or V pot and slowly turn while examining one of the dots displayed on the screen, zero in on the point where the dot is either shortest or thinnest depending on pot you are adjusting. Once you have finished with one of the pots move onto the next one. On the second pot you are looking not only to get the smallest dot, but also a dot that has an even width and height. Therefore after adjusting the second pot, you may need to return the first pot in order to make the 2 even.
  4. Once you have gotten the focus to an optimal point, switch to a screen that is reflective of how you intend to use the CRT (game, desktop, movie, whatever) and ensure that the picture shows up well there and if applicable switch to the resolution that you plan to use the CRT at.
  5. Once you have confirmed that everything is good, add a small amount of glue to the edge of each potentiometer to stop it from drifting in the future. Ensure that this is not extremely strong glue however in case you need to make adjustments again.
Screen is dim, colours not showing up well. While in some cases a dim screen and lack of vibrant colours can be a sign of the phosphors on the screen degrading (which is not really fixable) in some cases it can be fixed by adjusting the brightness. Some televisions and monitors may contain external adjustments for brightness and contrast, however if your CRT does have those or if the external ones do not suffice then here is how to adjust the internal screen pot:
  1. Disassemble the monitor/TV, discharge tube, and identify the flyback transformer. Look for potentiometers on or near the flyback. You are looking for a potentiometer labelled "Screen" or "Brightness". Do not touch any of the other potentiometers.
  2. Prepare an image to display on the CRT that contains section with a lot of vibrant colours AND a large potion of pure black. Turn on the CRT and display this image, once the CRT is turned be careful of high voltage locations and try to only adjust the potentiometers using a nonconductive object.
  3. Find which direction of adjustment increases the brightness of the pot then slowly turn it that way until the image is at a point where the colours are fairly bright and vibrant but not washed out. Pay attention to the black part of the image and ensure that it does not get any brighter, once the black part begins to get noticeably brighter then you have set the brightness too high and it should be turned back down.
  4. Once you have confirmed that everything is good, add a small amount of glue to the edge of the potentiometer to stop it from drifting in the future. Ensure that this is not extremely strong glue however in case you need to make adjustments again.

The "optimal brightness and vibrancy" will be different for different people so it is hard to give an exact measure of where it should be. it is important to note that having the screen too bright can increase the speed in which the phosphor degrades to the point where no amount of adjustment can bring the brightness or colour back. If this adjustment was done when the phosphor was already starting to go then it may work as a temporary fix but it will not last forever.

Misconvergence, one colour sticks out from the rest on edges between light and dark, or in extreme cases the colours have completely separated from each other.

(see fig 2 and fig 3 for examples)

Information coming soon
Tilted Image, the whole image appears to be slightly (or in rare cases drastically) tilted. Issues like this often stem from physical abuse of the display causing the deflection coil assembly to loosen and twist. The deflection coil is responsible for bending and moving the straight beam of the electron gun(s) in a way that creates the picture on the screen, it does this magnetically so if it is physically tilted then the whole image on screen will appear to be tilted.

Most modern CRT monitors and TVs will have some form of tilt correction in the OSD. On monitors it is typically in the user settings while on TVs you may have to access the service menu (Refer to manual on your TV for how to access.) In the case of there only being a small tilt in the display then using these adjustments is a good way to correct it. These adjustments are also a good way to fine-tune after making a physical adjustment.

Not all displays have the tilt correction setting or the tilt correction may not be working/not be enough to correct the tilt on your display. For this you can make a physical adjustment to the deflection coil assembly.

  1. Take note of the direction and severity of the tilt of the screen.
  2. Open the display and discharge the tube.
  3. Loosen the screw that holds the deflection coil assembly, this screw is located at the back of the deflection coil assembly and is normally the very last object around the neck of the tube before it reaches the socket.
  4. Make a rough adjustment to the tilt of the deflection coil based on what you observed from the tilt of the screen. Remember that the deflection coil basically aims the electron gun so if you tilt it to the right, the electrons path will tilt to the right. Since when viewing the screen you are looking at an inverted view of how the electrons hit the screen. Here is a visual to help understand how the gun adjustment works:https://youtu.be/pjTtwBO7UIo
  5. Turn CRT on and display an image and check for tilting. As long as the tilt is very minor, you can begin fine tuning. If you have available OSD controls then you could do the last fine tuning on the OSD, if you do not have OSD controls or if you just want to have the coil as perfectly aligned as possible then you can see the steps for physical fine tuning:
    1. Put an image on screen that has a straight line close to the top or bottom bezel of the display, you can use this as you gauge for tilting.
    2. Ensure the deflection coil screw is tight enough hold the coil place while still being slightly twistable.
    3. Being extremely careful of the neck board and any exposed wiring on the deflection coil assembly. Begin making slight adjustments to the tilt of the deflection coil, after each adjustment check the line parallel with the bezel.
  6. Once the tilt has been corrected to your satisfaction, re-tighten the screw then put the display back together. If your display has tilt correction in the OSD then any further small adjustments can be made in the OSD.

Specific Models[edit | edit source]

Repair guides for specific models of television or monitor.

TVs:[edit | edit source]

Monitors:[edit | edit source]

Other:[edit | edit source]

Images[edit | edit source]

Sources[edit | edit source]