Having Trouble With Monitor Losing Sync on Flashes

fezmonkey

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So, we're having this weird issue with a QNIC CGM-2500 monitor in a Neo Geo. Whenever a game has a flash in it, like during duper moves in fighters or big explosions in shmups or what have you, the monitor will desync for a split second and then recover.

Here's a video I took of the KoF 2001 intro, you can see it do this about six times:
https://youtu.be/J2j0QlXB8ME (audio is kind of loud, but unnecessary so you can turn it off)

The monitor has had a cap kit not too long ago. This seemed like it was potentially a flyback problem, so the flyback has been replaced. I kind of remember it being worse before the flyback replacement, so that may have helped a little, but it's been a while and I've had to work on so much stuff since then I can't be 100% sure on this.

I found the manual for the monitor and it has a flowchart that says that IC701 controls horizontal and vertical osc, so I replaced that. No effect there either. The Sync interface is IC401 and I believe I replaced that as well to no effect. I can double check on that tomorrow.

We had this issue with a K7000 too and I couldn't pin it down there either. Has anybody encountered this problem before and had any luck fixing it with something not related to the parts I've already gone through?
 
i don't know about your chassis, never worked on one.

the K7000 suffers from this if the brightness and contrast are adjusted improperly.

what it does in the K7000 is hog the B+ and cause it to drop so much so quickly that it affects the chassis operation in a whole.

when a screen goes from black to wight or vise versa, any chassis that has a weak or slow PS in it will do the same thing.

it is real common on CAPCOM, NAOMI and many different NEO GEO games when they flash.

try color and wight balancing your chassis again, DO NOT adjust the brightness past it's mechanical center ever, and ONLY adjust the contrast past it's mechanical center after you adjust the screen brightness on the fly and the colors.

this way you don't over drive the brightness and contrast circuit and make them fight each other.

you may need to dial back the brightness and just barley adjust the contrast if you can not find a happy medium.

you can see what i do in my K7000 video.
start at the 18:00 Minuit mark.

lets hope it works for your chassis.



i have a K7000 in my NAOMI Guilty Gear XX and it took me over 45 minuits to find the proper balance so it would not dip out like yours.

i eventually had to let the game play in actual game mode by letting one of the computers beat my dummy pick, to find the proper brightness and contrast level.

after that it all looked great.

Peace
Buffett
 
I had the same thing happen with my G07 in a Dragon's Lair. Buffett hit the nail on the head, it was the contrast and brightness adjustments.
 
I'll have another go at it again based on your suggestion, but that was the first thing I did before I started replacing parts. I should've mentioned that in the first post. The only way to get it to stop was to adjust it so low that the games looked really horrid. But I'll fiddle with it some more today.
 
Got this fixed a little while ago and forgot to supply an update. It was in fact the brightness and contrast adjustments (this time) but in the opposite way. The brightness was actually fairly low and I had to turn it up more and the contrast down to get it to stop.

As I stated previously though, I spent a lot of time doing adjustments before replacing the flyback and replacing the flyback did make it better before these final adjustments got rid of the issue entirely.

So if anybody else comes here because they have this problem, you should start by spending a lot of time fiddling with balancing the brightness and contrast, but if it never seems to get it right, I'd say consider a new flyback though I worked on this over such a long period of time and did a bunch of other stuff during then, I can't state with absolute certainty it will help you.

Thanks again for the suggestions.
 
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