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Author Topic: NeoGeo mvs-4-25 (WG K7000 compatible chassis?) w/ color separation, size issue  (Read 968 times)

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Sememmon

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Hello!

I've got a Neo-Geo mvs-4-25 with a Zenith tube and, I think a WG K7000 (or compatible) chassis based on reading the description here http://www.homearcade.org/BBBB/monitor78.html . The board on my chassis has no discernable identification, but the ICs (7823 & 1397) do match the description and the board looks very similar to the one in the photo. There's also a K7000 built by Zenith there as well http://www.homearcade.org/BBBB/7000z.html that could be it, hard to tell from the picture.

Here are some (less than stellar) photos of my monitor / chassis: https://photos.app.goo.gl/aXAJ5Q2G2T5b85L97

Anyway, the original issue that started me down this monitor rabbit hole is that at the top-left corner of my monitor, I have color separation. It is visible in the cross hatch test in my photo album above. I know nothing about CRTs. What would cause that? How can I fix it?

In addition, the horizontal size of the image is too wide for the monitor. To avoid having a wrapped-around-like bar on the right side of the screen, I have to move the position as far away as I can. However, this causes things on the left edge (and still somewhat on the right) to be off screen. Is there a way to shrink the image width a bit?

Lastly, I'm always reading/hearing about cap kits being a fantastic thing. Given that these are the only issues my monitor is experiencing, would a cap kit still help? Is there anything else I should be doing to maintain the life of my monitor? I'm thinking of at least pulling out the chassis and cleaning it, so any other service I can do would be great. I've popped/discharged it before with a grounded flathead .. and I know my way around a soldering iron, so not too worried about any of that.

Any help / advice / info would be greatly appreciated! Thank you.

-Sem

lilshawn

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this does not look like a WG chassis to me. this looks like one of the many sharp image (or it's knockoffs). I can't be certain, as I've only actually had to fix 2 or 3 sharp image monitors in the last 10 some odd years.

your separation in the corner as you call it (called convergence) is somewhat normal for an analog monitor like this one.... it's fairly typical to have some convergence issues in the corners. sometimes little magnets are placed around on the tube to help direct the beams properly and perhaps one has fallen off. check the surface of the tube for evidence of tape residue or glue that may have held something to it.

it's tough to get a perfect image, especially with these old analog monitors. You really need a microprocessor controlled deflection system to really get a perfect image. trust me, once you get a game on there and you are playing it, you will never notice the convergence issue, and in all seriousness, it's not that bad.

as for your width issue... if you have the ability, change out those capacitors. it could be the cause. the capacitor used for tuning the width are very small value and if they are old and starting to drift off, it can make it too wide, or completely unadjustable.

Sememmon

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Thank you for the response! I've actually just lived with the separation for quite a long time. As you said, it's just a minor annoyance. However, I recently decided to learn more about my cabinet and clean it up. I believe that the tube has a coil all the way around it? Maybe that has something to do with it? I will take a look for missing magnets as you said.

Sounds like the cap kit would be a good thing. So, would it be the WG K7000 kit? That Sharp Image link I shared seemed very similar (including the ICs). I was figuring that'd be the kit I needed.

Thanks for the help!

lilshawn

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the coil all the way around it is called a DEGAUSS coil. it temporarily passes mains voltage through a coil of wire looped many times around the tube.

When the monitor is first powered on, the power flowing through the coil is great, and a huge alternating magnetic field is produced... then after a few seconds (using a circuit) reduces the power flowing through to coil down to nearly zero essentially turning it off. what this does is demagnetize the screen. magnetization can happen if the monitor is moved or turned while powered on... or a magnetized piece of metal goes near it. typically low magnetizations can be removed this way, while larger color distortions (like from magnets being brought to the screen) require a larger, more powerful degausser. I, for instance  have a handheld plastic hoop about 12 inches in diameter with a power switch that I can manually activate and blast a screen to eliminate stray magnetic fields.

Sememmon

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Thanks for the info about the degauss coil. I had a hunch about some of that, but good to know how it all works.

Which cap kit should I get?

lilshawn

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the more I look at this, the more it reminds me of a Kortek monitor. maybe a Kortek KTW-N-26? It's hard to piece together with all the extreme closeups and parts here and there.



if so... this is the kit for it...

https://www.arcadepartsandrepair.com/store/monitors/monitor-cap-kits/kortek-ktw-n-26-monitor-cap-kit-105c-nichicon/

Sememmon

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Apologies for the poor quality pictures, but I was trying to be careful about what I was touching and haven't yet removed it from the cabinet. That said, that does look a lot like the chassis I have! Strange that the ICs would be the same as in that Sharp Image one. It looks pretty similar, too.

Thanks for the identification help. I appreciate it!

On a side note... the cooling fan... Any thoughts where I could find a replacement? Mine is in need of retirement. It looks pretty standard. Wondering if I could find a nice Noctua that I could replace it with.

Hm, and another question. Would a cap kit help the high pitched whine my cabinet emits?

lilshawn

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On a side note... the cooling fan... Any thoughts where I could find a replacement? Mine is in need of retirement. It looks pretty standard. Wondering if I could find a nice Noctua that I could replace it with.

Pull off the fan and take a look. as long as it says it's 12 volts, you can replace it with any standard computer fan. cut it off, take it with you and match it up with a new one. it looks about an 80mm. if you measure between the screw holes...


32mm between screw holes   40mm fan size
40mm between screw holes   50mm fan size
50mm between screw holes   60mm fan size
60mm between screw holes   70mm fan size
72mm between screw holes   80mm fan size
83mm between screw holes   92mm fan size
105mm between screw holes   120mm fan size


Hm, and another question. Would a cap kit help the high pitched whine my cabinet emits?

nope not in the slightest. that whine you hear is because it's refresh frequency (15 khz) is within the range of human hearing. (typically 20hz to 20 khz) once you get to medium resolution (25 khz) and VGA (31 khz), you fall outside of hearing range and you don't hear it.

once you have it in a cabinet and have to all buttoned up, you will hardly notice it.

Sememmon

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Awesome, thank you so much for the info!

Will get that cap kit in and report back on my results. Thanks! :cheers:

lilshawn

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no problem. Once you start doing the cap kit i recommend taking one capacitor out and switching it with it's replacement... one at at time... taking careful note of it's orientation... noting which way the polarity the capacitor is installed. that way you don't miss any and get them in the right way. all the circuit boards are marked for polarity printed right in the area where it installs... but it's known that  a couple chassis have some markings for the capacitors polarity erroneously backwards... the capacitor is installed currently correct (otherwise it would blow up.) it's best to note how it's installed and replace it the same direction, ignoring the markings just in case of manufacturer error.

Sememmon

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Thanks for the advice about the caps! I will definitely heed it. =]

One other question, while I await my cap kit... something I've wondered since I bought my cabinet. On the .. "neck" on the back of the tube, there are some adjustable rings. What're they for? I should probably just try changing them and see, but haven't wanted to .. make things worse than they are. :o

Here's a pic of what I'm talking about: https://photos.app.goo.gl/9vbuX6d3ews9m6if7

lilshawn

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These adjust the color convergence...they are called purity rings. If it needs adjusting, head over to the wellsgardner website and download the service manual for the K7500. In it, is a set of directions on what adjusting each ring does what to the screen.

Usually they are set at the factory and only need to be changed if they get knocked or broken.

Sememmon

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Installed my cap kit this evening. Everything seemed to go well, except that I appear to have lost my blue gun. What was white is now yellow and the right-most pair of knobs on the neck board now do nothing. I'm guessing I've got a bad solder joint. Going to let it sit overnight and then discharge and pull it back out to closely check my soldering job again, but thought I'd ask here to see if I'm looking for the right thing. Any ideas?

Edit: That service manual was an interesting read. Now I know what the extra rubber wedge I have is for. =]
« Last Edit: June 14, 2019, 03:38:35 am by Sememmon »

lilshawn

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could just be a broken solder joint. check the transistor solder points on the neck board.

Sememmon

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I seem to be making things worse instead of better. :laugh2: Now I've got a solid horizontal line at the center of the screen. Some quick googling resulted in some ideas, so looks like I'm pulling it back out of the cabinet again. At least I'm getting pretty comfortable fiddling with this. Kinda regretting the cap kit at this point, though. Hopefully this journey has a happy ending.

lilshawn

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yer fallin' apart mate!

loss of vertical deflection. issue with the vertical driver IC or power supply to it.

probably flat one attached to the straight heatsink all by itself. check for broken traces especially if you've grabbed that heatsink to pick it up or move it around...or propped it up on it (especially while doing the capacitor swaps)

Sememmon

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So, a couple questions about this. I've reheated the solder on a few things including the LA7831. With the old solder on this board.. is it better to remove/resolder? Or is just remelting the original solder ok? Also, when I took a look at the LA7831, 3 of the 10 pins are not soldered (2 on one end, 1 on the other). It looks like they're not actually used, but I can't be certain. Should I solder them? Or leave it as is? This isn't something I've touched before and it was working previously so I'm figuring it was intentional... just kinda odd as it looks like the only thing on the board that wasn't soldered.

Thanks for the help btw. I realize this thread got a bit off track from the original post, but I'm really appreciating the support.

Edit: As far as I can tell, the LA7831 looks ok. I don't see any damaged traces. I'm not sure how to test that it is getting the appropriate power...
« Last Edit: June 15, 2019, 09:00:23 pm by Sememmon »

lilshawn

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 it does have a few pins that don't do anything (N/C) Vin is on pin 8. the datasheet is all in japanese, but the best i can muster is it's supposed to be anything 18 to 27 volts.

also check pin 5 and make sure there is something there. this is the voltage input used to drive the vertical deflection. probably going to be like 60 volts or something like that...as long as it's not zero or near zero, it should be okay.

best way to check these voltages is to carefully solder a small wire onto the pin you need to test (or a component pad directly attached to it.) and attach your meter to this wire. that way you can keep your mitts out of the high voltages and safely power it up.

only power it on long enough to get a reading, blasting your tube with a solid line of  electrons meant to be spread out over a whole screen is not great for it.


Sememmon

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I attached a wire to pin 5 and pin 8 and got a reading from my multimeter of 28V on both of them.

So... guessing that means power to the IC is ok? But signal from the IC is not? Not sure.

What's next? Replace the IC?

Thanks again for the help! This is proving to be quite the trial for my limited electronics skill.

lilshawn

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if it worked and you farted around with it...and it now no longer works... generally something you've done has broken it. usually it's something snapped like a connection or solder joint is all. something simple. search around... wiggle anything that sticks out and see if something on the solderside is flapping in the breeze.

you seem to have voltage...so that's a good start. maybe the input is not receiving a signal. possibly another chip like the sync separator probably the dip package towards the video input connector...the LC7823. it takes the sync signal and separates the horizontal and vertical and sends them on their way....if the vertical pulse isn't extracted and sent to the driver IC, the 7831 won't drive.

Sememmon

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I've gone all over the the components.. nothing seems loose. I'm figuring either I there's a bad solder joint.. or I broke a trace I'm not seeing.. or I damage a component.

I just don't know how to tell which it is. =( I've got a replacement IC coming for the LA7831. Maybe that'll help. =|

lilshawn

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check R91 (3.9 ohms)

D13 on diode check mode should read somewhere abouts 0.4 to 0.5something probably have to lift one end out of circuit to get a good reading.

C48 double check polarity. if it's backwards it could be dragging down the voltage causing it to act dead.

if you are changing out IC3, you may as well order and change IC2 as well. they are pretty tightly integrated to each other and a detrimental failure on IC3 can cause failure in the other by back feeding into the IC by voltage going "in" through the output.

Sememmon

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Thanks for the suggestions. I will see about swapping IC2. I had an arcade tech visit today, but we were unable to make any headway. It was good to have someone with more knowledge check my work, but he admitted to not being a monitor expert.

Regarding the voltages you suggested checking, I'm not sure how I can go about doing that with the monitor in the cabinet. It'd be pretty awkward. =| Would be a lot easier if I had an external power supply so I could power it up on my desk. I suppose I could solder in a bunch of test wires like with IC3... hm.

Sememmon

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Finally got this resolved. Chad at arcadecup.com did a great job fixing me up. He replaced multiple ICs as well as fixed a trace and solder pad point. Everything is good to go now and looks better than it did when we started.

I also spent a little time trying to fix the top-left corner convergence. I've made some headway there, but it seems to be very much a black art with a lot of trial and error. Hopefully I'll figure it out eventually, but am pretty happy with how well it is performing now.

lilshawn, thank you so much for the assistance throughout this issue. I really appreciate it! :cheers:

lilshawn

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cool cool. if you want a little insight into setting up convergence, the Wells gardner K7500 service manual has a really good write up (starting on page 5)... it's basically universal since most tubes are designed in similar fashions.

https://www.wellsgardner.com/wp-content/uploads/2016/08/K7500.pdf