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Old 03-28-2011, 03:26 PM
leonk leonk is offline
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General color tube convergence question .. what gives?

This might not be the right forum, but this sure is the right place to ask this question, due to the caliber of individuals that frequent this forum. So here goes:

One of my hobbies is arcade video games. And I've had a chance to work on a few arcade machines, restoring them, recapping the chassis, etc. Most of the tubes I work on are designed for VGA input (mostly 15khz signal, some go up to 25khz). But it seems that no matter what vintage of tube I work on (late 80's to late 90's, 19" all the way up to 29") they all have the same problem. Terrible convergence!

When connected to a game PCB or PC, I'm able to put up a grid pattern to check convergence. In the center, the image always looks perfect (nice straight white lines both horizontally and vertically). Yet when you look at the pattern at the edges, 99% of the time, convergence is off (blue/red always a bit off .. and you end up seeing not a solid white line, but rather 3 thin different color lines).

What gives? Why are CRT arcade monitors always showing convergence issues? Is this common also for CRT tube TV's, except one never notices this because we don't look at grids on the screen? Is the quality of CRT's shipped to arcade monitor suppliers different than TV suppliers? (My 29" arcade monitor has a CRT tube made by Toshiba .. my 25" arcade monitor has a CRT made by Zenith)
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Old 03-28-2011, 03:36 PM
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miniman82 miniman82 is offline
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Those have inline guns, right?

Stray magnetic field, maybe?
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Old 03-28-2011, 06:57 PM
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compu_85 compu_85 is offline
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This is something I've wondered about myself, how do you adjust the dynamic convergence on a semi modern set?

-J
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Old 03-28-2011, 09:39 PM
leonk leonk is offline
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the tubes are standard TV tubes. You adjust convergence using the standard convergence rings on the yoke. Many also have permanent magnets glued to the CRT around the yoke to assist with convergence in special cases where the rings can't fix all convergence.
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Old 03-29-2011, 01:18 AM
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colorfixer colorfixer is offline
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Some tubes used in games are monitor grade tubes: M48AA... 19VLU...

Convergence issues in these monitors come from a few different issues:
-poor installation of replacement tubes (many of the wrong type) and servicing by their techs.
-warped yokes and convergence/purity magnets from the years (and years and years) of use and heat inside the cabinets.
-manufacturers choosing to cut corners in their monitors to maximize profits.
-CRT's improved over the years such that we expect better performance.


Remember that these games were designed to get as much $ for their owners in a short period and were never designed to last as long as some of them have.

I opened a NIB game a couple of years ago, and the setup on the monitor wasn't that bad considering that the game was shipped over 25 years earlier, and the setup was done in a factory 1/2 way across the globe without knowing where they were going to be used.

Some monitors need to have more than a few stick on magnetic strips to dial the convergence in spot on:
http://www.youtube.com/watch?v=QYB1L...eature=related
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Old 03-29-2011, 01:28 PM
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compu_85 compu_85 is offline
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Where does one buy those lill self stick strips? I assume they can help correct geometry issues on B&W CRTs as well?

-J
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Old 03-30-2011, 12:23 AM
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ChrisW6ATV ChrisW6ATV is offline
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I started my electronics career repairing arcade video monitors in 1981. I could get the convergence darn-near perfect on most of them, and I replaced a lot or CRTs when they arrived damaged (we were a factory service shop for several game manufacturers). These were all 19-inch CRTs, which were standard in the Berzerk/Defender era. (Maybe larger CRTs are not as well converged in general; I have not worked on those.) Yes, these were inline-gun CRTs, with modified JVC chassis and 15 kHz input. Static/center convergence was set with the rings on the neck (after doing purity and positioning the yoke), then center-of-edge convergence was done with judicious tilting of the yoke's front diameter, which was not up against the CRT even though the back of the yoke was already tightened to the neck with its band. With the proper position, the yoke was held in place with three rubber wedges about 120 degrees apart on the cone of the CRT. Lastly, corner convergence and some edge fine-tuning was done as needed with the small magnets on the cardboard strips. The strip has a thin small magnet on one end and some adhesive tape on the other end; the whole thing is about 2.5-3 inches long. You slide the magnet between the yoke and the CRT bell and watch the raster with a crosshatch pattern and move it until the error was gone, or reduced. A good one needed only three strips (one in each of three corners); some of the worst needed up to ten strips to get them into really good convergence.

I used similar strips when I have replaced Sony Trinitron CRTs on 25-27" monitors in more recent years.
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Old 03-30-2011, 08:31 PM
julianburke julianburke is offline
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The little yellow strips with the tiny magnet on the end was called a "spoiler". We also used "chevrons" to help make purity. If they didn't have the convergence ring with the adjustable tabs, there was a magnetic ring that was tie strapped to the neck that was set with a computer and could not be adjusted. Most all large rectangular tubes will have pincushion or convergence issues on the outer edge esp when they age.

Watch these videos:
http://www.youtube.com/watch?v=421_3KY8oaU
http://www.youtube.com/watch?v=QYB1L...eature=related
http://www.youtube.com/watch?v=5NwMP...eature=related
http://www.youtube.com/watch?v=0iEl-...e=more_related
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Last edited by julianburke; 03-30-2011 at 09:03 PM.
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