View Full Version : I'm looking to fix a computer monitor with VERY poor convergence.


jmetal88
01-16-2016, 02:01 PM
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.

zeno
01-17-2016, 09:35 AM
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:

jmetal88
01-17-2016, 09:53 AM
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?

Chip Chester
01-17-2016, 12:19 PM
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

zeno
01-17-2016, 02:23 PM
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:

jmetal88
01-19-2016, 07:41 PM
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.

jmetal88
01-19-2016, 07:58 PM
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.

andy
01-20-2016, 11:28 AM
Computer monitors often have some electronics for the dynamic convergence. Are there any pots, or adjustable coils on the yoke? The fact that it's far off could mean a failure in that circuit, or just sloppy adjustment.

jmetal88
01-20-2016, 09:11 PM
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.

jmetal88
01-20-2016, 09:14 PM
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.

tom.j.fla
01-20-2016, 09:34 PM
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

jmetal88
01-20-2016, 09:44 PM
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.

tom.j.fla
01-21-2016, 01:20 AM
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

jmetal88
01-22-2016, 09:11 PM
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.

Chip Chester
01-23-2016, 10:46 AM
Here are some options:
http://www.mcmelectronics.com/browse/Trim-Alignment-Tools/0000001399
I'm sure there are more out there, too...


Chip

jmetal88
02-20-2016, 12:15 PM
I finally ordered a kit off eBay today. I guess we'll see toward the end of next week how making these adjustments affects the convergence.

jmetal88
02-20-2016, 06:52 PM
Well, now I have a bigger incentive to get this thing adjusted properly, as my favorite CRT computer monitor, my Samsung Syncmaster 17GL, has just bitten the dust. The image on it went REALLY narrow and unstable today. I unhooked it, took it apart, re-soldered a couple of bad-looking joints on the flyback, and put it back together, but the situation only improved by the instability going away. The picture is much too narrow to be usable on it now. Unfortunately, though I've been able to find the schematics for the Samsung, they are VERY complex, and I'm having a hard time even finding which part of the circuit I think I should be looking at, so I think I'm just going to consider it a loss and focus on making this CompuAdd monitor looking as good as I can instead.

jmetal88
02-24-2016, 05:57 PM
Aw man, I got the alignment kit in, picked the closest fit, and tried to give that inductor at the back of the yoke a turn... It's cemented in place, so it's not going anywhere. In fact, the alignment tool (I had my pick of slightly too small or slightly too big) just started slipping around inside the inductor core after I applied a certain amount of force. I guess my last shot before trying to break the yoke free and move it actually would be re-capping this thing now.

jmetal88
02-28-2016, 12:18 AM
Actually, the more I use this, and the more I look at all the little distortions on the screen, the more I'm suspecting that the yoke has deformed, possibly due to heat or something. It's certainly still glued in place where the three wedges are, but that doesn't mean it hasn't warped in the spaces between them. I might still re-cap it, just to try to give it some extra longevity, but I don't think I can expect an electrical fix for the convergence issues.

jmetal88
02-28-2016, 12:53 PM
Hmm, I've found someone apparently selling a new CRT that matches the one in this monitor. I've e-mailed them to ask if the yoke is attached (they show a picture of a CRT with yoke attached, but have a note saying it may not represent the actual item). If it is, it may potentially solve two problems for me, one being the deformed yoke (if that actually is what's causing my convergence issues) and the other being the small abrasion on the face of the CRT. It's a pity it's as expensive as it is, but considering I've been looking up new-old-stock CRT monitors and most of them sell for about twice as much as this tube, I guess it's not too bad a deal.

old_tv_nut
02-28-2016, 01:34 PM
"may not represent the actual item"

Personally, I would not buy unless the seller provides a picture of the actual item, as well as verifying it's the right one.

MRX37
02-28-2016, 03:14 PM
Another option if all else fails are believe it or not, pieces of refrigerator magnets taped to the corners of the CRT back. It takes some fiddling with size pieces and positioning but it can help.

jmetal88
02-28-2016, 03:51 PM
Another option if all else fails are believe it or not, pieces of refrigerator magnets taped to the corners of the CRT back. It takes some fiddling with size pieces and positioning but it can help.

Yeah, I remember when I took apart the 32" Trinitron TV I had a few years ago I saw that it had magnets glued to the back of the CRT for that purpose. Usually that's only necessary on larger CRTs, though, which is what makes me think something more is wrong with this monitor.

I have a short strip of refrigerator magnet material that I tried positioning on the back of this CRT, but I didn't see any positive effects from it, at least not the way I was trying to use it.

Chip Chester
02-28-2016, 05:09 PM
Have you made the rounds of your local tech buddies, recycling businesses, etc. to see if someone has a decent complete CRT monitor to just give away? It's likely only the better ones were kept around over the years...

Chip

jmetal88
02-28-2016, 05:11 PM
No, but I want this one in particular because it's the same brand as the computer I'm using it with.

wkand
02-29-2016, 01:24 AM
Maybe someone mentioned it, but reading the entire thread, I did not see mention of any setup key or menus. I have never see a CRT monitor that relied entirely on internal adjustments for convergence, brightness, contrast, etc.

jmetal88
02-29-2016, 06:36 AM
Maybe someone mentioned it, but reading the entire thread, I did not see mention of any setup key or menus. I have never see a CRT monitor that relied entirely on internal adjustments for convergence, brightness, contrast, etc.

This is a 1992 monitor, it's too old for that. All the screen size, position, pincushion, etc are all done with individual potentiometers.

kf4rca
03-01-2016, 09:01 AM
I'd try to get your Syncmaster going if at all possible. The Syncmaster is like the Sony Multiscan which is like the NEC Multisync.
Unlike most monitors that are fixed frequency types the Syncmaster is a variable frequency type. It locks to whatever is given it.
If you ever get a Sun Microsystems computer, you will need the Syncmaster.

andy
03-01-2016, 11:25 AM
Have you tried repositioning the yoke? 90% of the dynamic convergence is handled by tilting the front of the yoke. Sometimes the yoke gradually slides back and droops down over the years. The rubber wedges that hold the yoke in position can also move, or fall out. Check out the service manual to any TV with an in-line CRT for some idea of how to adjust the convergence.

Are you sure the CRT is original? One thing that will cause serious convergence problems is trying to use a CRT with an incompatible yoke. You'll end up with the red and blue rasters being either too big, or too small compared to the green.

jmetal88
03-01-2016, 05:58 PM
I'd try to get your Syncmaster going if at all possible. The Syncmaster is like the Sony Multiscan which is like the NEC Multisync.
Unlike most monitors that are fixed frequency types the Syncmaster is a variable frequency type. It locks to whatever is given it.
If you ever get a Sun Microsystems computer, you will need the Syncmaster.

I already gave away the Syncmaster. I was NOT AT ALL interested in trying to troubleshoot the horizontal circuitry on that considering how complex it was.

jmetal88
03-01-2016, 06:03 PM
Have you tried repositioning the yoke? 90% of the dynamic convergence is handled by tilting the front of the yoke. Sometimes the yoke gradually slides back and droops down over the years. The rubber wedges that hold the yoke in position can also move, or fall out. Check out the service manual to any TV with an in-line CRT for some idea of how to adjust the convergence.

Are you sure the CRT is original? One thing that will cause serious convergence problems is trying to use a CRT with an incompatible yoke. You'll end up with the red and blue rasters being either too big, or too small compared to the green.

I have no idea if the CRT is original. Looks original, but I don't know how you tell when you don't have the original bill of materials anywhere.

I understand already that the position of the yoke affects dynamic convergence, but this thing was SOLIDLY glued in place, as were the wedges. I finally managed to break it free a few days ago, after much effort, but I wasn't able to find a position where the convergence was good all over the monitor. The best I could get was the center and top of the monitor having good convergence, but blue was still quite a ways off from red and green at the bottom of the monitor.

kf4rca
03-02-2016, 09:36 AM
Too bad about the Syncmaster. I fixed one once and it was a shorted (melted down) width coil.
I have found that if you have problems getting good purity and /or convergence on a CRT, its usually due to a slipped shadow mask. Usually resulting from a fall.
Sometimes the cabinet cracks and sometimes it doesn't.

andy
03-02-2016, 12:04 PM
It's possible it was just a cheap monitor that never had good convergence. Good convergence is one of the things that you got with the more expensive brands like NEC and Sony.

jmetal88
03-03-2016, 06:22 AM
Too bad about the Syncmaster. I fixed one once and it was a shorted (melted down) width coil.
I have found that if you have problems getting good purity and /or convergence on a CRT, its usually due to a slipped shadow mask. Usually resulting from a fall.
Sometimes the cabinet cracks and sometimes it doesn't.

Yeah, that could be what happened. When I got it, there was a small chip on the base and an abrasion on the face of the CRT, so I wouldn't doubt if it got dropped at some point.

The place with the replacement CRT never e-mailed me back, so I'm not sure what's going on with them.

jmetal88
03-09-2016, 07:08 PM
Got a reply from the CRT place, finally. Turns out they sold off one of their warehouses a while back and didn't successfully remove all the part numbers from their inventory. They don't have the replacement CRT anymore. I guess I'll be stuck with a poorly working monitor now (I don't want to get rid of it because I like that the brand matches my XT clone, it'd just be nice if it wasn't so annoying to use with all those multicolored shadows all around the edges of the screen).

Chip Chester
03-20-2016, 10:08 PM
There's a NEC 17" Multisync 5FG High Resolution Color Monitor on a CL listing near me. (I'm as central as you can get in Ohio.) The listing is not mine, no relation, etc. It says very low hours. You can do a quick search for it, or I'll post the link in $/cents. I had one of these back in the day, and they're quite nice and sharp for a CRT.

Chip

MRX37
03-20-2016, 10:18 PM
This is starting to sound like a Hitachi TV i had years ago. The convergence got worse and worse and it had nothing to do with the yoke.

Again only thing I can recommend are pieces of magnet strips on the back of the CRT.