Monitor Repair

General information about repairing CRT monitors found in most older arcade cabinets. Anyone attempting to repair an arcade CRT should be quite sure that they understand what parts are dangerous and how to discharge a CRT. You can view a video on YouTube as to how to do this, but be sure you know what you are doing!

Common vocabulary that is necessary to understand the following tips:

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🔗Repair Tips

🔗 Make Your Own Convergence Strips

Convergence strips are small plastic strips with a permalloy tip which can be used to adjust the convergence of a CRT around the corners. They can be easily manufactured at home on the cheap. You will need the following supplies:

  • Styrene Plastic sheet, of any thickness
  • Kapton Tape, of any width
  • Ultraperm 80 metal sheet, of any size

The styrene sheet should be cut up into strips 1 cm wide and around 8-10 cm long. The ultraperm sheet should be cut into 1cm x 1cm squares. Optionally you can use contact adhesive to affix the ultraperm to the styrene. Use the kapton tape to secure the ultraperm square to the end of the styrene strip as well as insulate it from the coils. Do not use other tape as the permalloy will get hot and kapton tape is specifically designed for heat insulation. Once made, you can secure these strips to the tube of a CRT using double-sided tape on the back or regular tape across the front.

🔗 Adjusting The Yoke

Under most circumstances, you should never need to touch the yoke of your CRT. Once tweaked it can be very difficult to align the yoke properly so that you get good geometry, convergence and purity. That said, there are two situations in which you might need to adjust the yoke on your CRT. Note that you must do these adjustments with the CRT on and displaying a picture so that you can see the effects of your adjustment directly. Invest in a good pair of lineman gloves so that you can safely grasp the yoke to make adjustments. Note also that some yokes are held in place by two set screws instead of one.

The recommended picture to display on your CRT while making these adjustments is a cross hatch grid pattern. This will allow you to clearly see the rotation, geometry and convergence while you are making the adjustments. Also, it is recommended to use a cross hatch that colors the outside edges with red and the center with pure white. This is so that you can see if you have messed with the purity while adjusting the yoke. If you have an all white cross hatch pattern it can be difficult to see if you've screwed up the purity because the shifted colors will still sum up to white. Using a pattern with red borders will allow you to quickly see purity issues while still giving you a white pattern in most of the screen to judge convergence. You can find such a test pattern in the 240p test suite or in the naomidiag test suite.

The first problem that can be fixed with a yoke adjustment is a rotated picture. If your picture has good geometry and convergence but appears rotated a few degrees clockwise or counterclockwise, then your yoke needs to be rotated. Loosen the set screw holding your yoke enough to be able to twist it back and forth, but not enough for it to slide backwards down the neck. Then once you've gotten the picture level, tighten the set screw. Be careful not to tweak the yoke left, right, up or down as this could affect convergence and geometry. Note that the wedges between your tube and yoke might be glued to the yoke itself so this can make adjustments a bit fussy.

The second problem that can be fixed with a yoke adjustment is geometry, convergence and purity issues. If your monitor appears to sag on the left and right, has weird curves on one side of the picture, has incredibly poor convergence or even has purity issues, a yoke reseat can help. Loosen the set screw holding your yoke enough to slide it forward and back and snug it up against the wedges of your tube. With any luck, this should resolve any geometry or purity issues you are experiencing, or at least make them better. Under very dire circumstances you may want to pull out the wedges, reposition the yoke for better purity/convergence/geometry and then reinsert them once you have tightened the yoke set screw down again. Be warned, repositioning a yoke without the factory wedges is very time consuming and difficult. It may take hours of fine tweaking to get it just right and you may very well make things worse.

🔗 Adjusting the Convergence Rings

The convergence rings are a set of six rings (sometimes 8 on high-end CRTs) that control fine tune purity adjustments as well as static convergence. The rings operate as pairs. For each pair, the degree difference between the two rings controls the strength of the adjustment and the position of the pair relative to the neck controls the adjustment location. Note that these work on polar coordinates, so rotating a pair of rings together will adjust the desired setting in a circular fashion. The adjustment itself can be a bit finicky for the inexperienced so it is recommended to use a sharpie or white-out marker to draw a line through all six of them before making any adjustments. That way, when you inevitably screw it up, you can "reset" back to how it was and try again.

The first pair of rings (closest to the tube itself moving down the neck) controls the purity. This is a fine tune adjustment for the instances where the yoke is seated correctly but you have a small splotch of color purity issues. Normally you should ignore these rings, but if you have a discolored spot on your monitor on solid color screens and degaussing it has not fixed the problem, you can play with these rings to attempt a fix. The recommended test pattern to display when adjusting these rings is a solid full-screen red, green or blue screen. That way you can check the purity across the whole tube. These screens can be found in the 240p test suite or naomidiag. If you have CraftyMech's Test Pattern Generator, you can display the checkerboard pattern and then turn off two of the three guns at a time.

The second pair of rings controls the red-blue static convergence. This allows you to rotate the red and blue guns so that they line up just right. If it appears that your red or blue gun is misaligned compared to the rest of the picture then these rings should be adjusted. Not again that these are adjusted by spreading the two rings out to control the magnitude of difference between the red and blue guns, and then rotated into position by rotating both rings simultaneously. The best test pattern to display here is a magenta cross hatch pattern so that you can see the red and blue convergence without green getting in the way. This can be found in naomidiag, or you can use CraftyMech's TPG on the checkerboard pattern with the green gun turned off.

The third pair of rings controls the green-magenta static convergence. This set of rings assumes that you have already adjusted your red-blue convergence, hence why it is magenta (red + blue) versus green. This allows you to rotate the green gun with respect to the other two guns so that you get a crisp image. Much like the other two, spreading the rings controls the magnitude of the adjustment, and rotating the rings as a pair allows you to rotate the green into position. The best test pattern to display here is a white cross hatch pattern so that you can see the green line up with the red/blue guns. This can be found in the 240p test suite or naomidiag. If you are using a CraftyMech TPG you can use the checkerboard pattern with all guns turned on.