I'm looking to fix a computer monitor with VERY poor convergence.
 

I know this isn't actually about a TV, but I hope it's close enough to not be off-topic here. I have an early 1990s VGA monitor with VERY poor convergence that I'd like to fix. I've messed with the rings on the CRT neck as much as I dare, and the best picture I get really isn't too far off from the factory positioning (where the white stripe painted down the rings lines up), but that gives me good convergence in the center of the screen and worse convergence the farther I get from there. The absolute worst spots are in the corners of the screen, as white text has a red 'shadow' in the upper left corner and a blue 'shadow' in the lower left corner. The effect is almost like the red part of the picture and the blue part of the picture are 'twisted' in opposite directions from the green part of the picture. I know the first piece of advice when it comes to working on something like this is to replace all the electrolytic capacitors, but before I get into that, I was just wondering if this is a situation where that's likely to help, or if I'm likely to have a more serious problem to fix after that's all done.

If it helps, the monitor is a CompuAdd 51118. On the inside, there are some parts marked JVC, so I'm guessing it's either a re-branded version of a more common monitor or it was manufactured for CompuAdd by JVC.

The rings are only for center convergence & purity.
To adjust sides the yoke must be wedged. Usually
there are 3 rubber wedges between the yoke & CRT
to bend it. Often they fall out over the years.
Start there....

73 Zeno:smoke:

All 3 wedges look like they are in place... Any tips for the manner in which they should be adjusted, or will I just have to try messing with it and see what happens?

You may have addressed this already, but has it been degaussed? If automatic, can you determine if it's actually working?

Along those lines, what it its magnetic environment where it's sitting? Non-shielded speakers nearby, or other significant magnetic toys?

Chip

try bending the yoke edges out & watch the effect. If they need to bend in
the yoke has to be pushed forward a little then the edges bent out, then
wedged. If you do loosen the yoke mark it so you can go back.
If its magnetized it will usually throw off the purity only unless
its real bad. Use an all red, blue or green raster to check purity.
The degausser will make a hum for a second with a COLD start.
A few will do it at turn off but thats rare.

73 Zeno:smoke:

Opened it back up tonight. The yoke and the wedges are all epoxied in place, so I don't think I can move them if that turns out to be the issue. I can't bend the yoke enough to have any kind of effect on the picture because it's on a stiff plastic frame that has very little give to it.

Now that you mention it, though, I'm not sure the degaussing coil is doing anything (if it's made the 'hum', I sure haven't noticed it), so that may be part of the issue. I have a handheld TV degaussing coil I could try on it.

Eh, it looks pretty much the same after using the handheld degaussing coil on it.

I also noticed the picture is narrower at the top of the screen than the bottom, if that's an indication of anything.

...

There is one pot on the yoke that does seem to affect convergence a little, but not enough to make much of a difference. I wasn't sure what it was actually supposed to adjust, though.

Oh! There is also what looks like an adjustable inductor back behind that pot. It could be that that's out of adjustment. I remember wanting to try turning it, but I didn't have a hex key the correct size.

Don't use a metal hex key, that will most likely break the core of the coil and will through off your adjustment. That coil balances the signals going through each Horizonal coil. You need a R F coil alinement set made out of a hard plastic. If you have a Radio Shack still open you can find those tools there or at an electronics supplier. All the best Tom

I do still have a Radio Shack open in town, but I kind of doubt they'd still carry anything like that (I can't find anything on their website that matches that description anyway). But then again, the one near me doubles as a ham radio shop, so it's possible they'd still have stuff that's no longer listed on Radio Shack's website.

I don't know if this catalog number is still valid but my set of "Color TV alignment tool kit" is 64-2220. The five words in quotes is the name of the product. All the best,Tom

I went to my local Radio Shack and asked about it. The guy knew exactly what I was talking about ("It was like a kit of plastic screwdrivers") but said the last one they had was sold over a year ago.

Here are some options:
http://www.mcmelectronics.com/browse...ols/0000001399
I'm sure there are more out there, too...


Chip