K7400 width issue

tnglc79

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I just picked up a Ms. Pac/Galaga today and it was half working half not. The image on the left side of the screen is smaller than the right side. It was also turning to a white screen after a couple hours.
I did a full cap job, double and even triple checked my work and reflowed all the rest f the boards and am sure that it is a solid job. This is definitely not my first rodeo. Now the monitor has been on for six hours still doing fine and not dropping signal but the image is still smaller on the left side. I also cannot move the position any closer to the left edge.
I have tested this chassis on a known good board, known good monitor, and a known good neck board. Same results in any configuration. So the problem must be on the main board.
Tested all test points and getting good numbers. Checked pin 9 of u601 and got 28v.

This is only the second 7400 I have messed with. Am I missing anything or have any of you all fixed this problem? Any help appreciated.
 

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first of all, that's not width. that's actually height. you need to process in your mind that the picture tube is merely rotated. I don't know of any vertical linearity control on these, I'm drawing a total blank on what to even look for, but my advice would be to follow the vertical IC backwards and see where it goes. it probably meets with some part of significance.

the picture turning white on its own is actually what happens on these when the flyback is going bad. since the potentiometers for the screen brightness and focus are sealed inside the HV unit there's no way to clean them that I know of, so you have to replace the whole thing.
 
this is from the wells sight, might be a good place to start checking these parts.

VERTICAL FOLDOVER R618, U601, R303 TOP OR BOTTOM OF SCREEN
ADJUST VR907, VR903, VR902
 
Thanks for the correction on the width vs. height. I was tired last night when I posted. As far as the flyback, it has been on for 12 hours now and still looks great minus the height problem.

One thing that I noticed was the plug for the video sync was wired different than other and looked up a couple schematics and think I have found the problem.

From the schematics of Ms. Pac/Galaga it shows that it has composite sync but it is wired to vertical sync.

current J202
1- red
2-green
3-blue
4-ground
5-sync (vertical)
the rest empty

I believe I should move the sync wire from 5 to either 7 or 10 as they are the composite sync pins. Will give it a try this evening unless someone chimes in something else to help.
 
that could make a difference. you can pop that wire out of the plug with a small screwdriver. far as negative and positive syncs go those things take it either way.
 
I didn't get a chance to mess with it last night, had a long day at the shop. Hopefully tonight I can sit down and figure this happy guy out as I have seen no real info anywhere on this. I am sure this isn't the only monitor with this issue so finding a posting a solution would be helpful to some one else maybe. Not going to lie, I love the challenge, but this machine was bought to put in a shop to make some small change, so having it look perfect is imperative.

that could make a difference. you can pop that wire out of the plug with a small screwdriver. far as negative and positive syncs go those things take it either way.
 
Removed and checked r303, r618 and they were good. VR's also good, scratchy, but good. Guess I'll try switching out U601. I think I have a good one, but not 100% on it. Will be ordering a few parts today anyway so I'll try to buy a few new ones since I have a couple more of these boards. Again, when I find it I will post it. At this point I am going to go to single component switch and test. Not much more left in the circuit, lol.
 
if you've never crimped molex pins before I suggest you learn how. for the monitor plugs they're .156 trifurcon pins, and the plastic housings are called .156 female flat molex. you'll need a molex crimping tool, cause I had a dark age period of trying to do it with pliers and felt like a maroon when I got the real tool. :p

otherwise the screwdriver method of popping pins out will work in cases like these in a pinch.
 
Thanks for that bit of input. I have several styles of crimpers laying around but after googling molex crimper, I have some similar, but not quite the same. I will be picking a pair up since I just made a deal with a local guy to repair around 30-40 monitors for route machines and am working on a deal with another guy to help fix and maintain his entire arcade. I am sure they will come in handy at some point as I am sure I will eventually break one and have to replace it. Plus it never hurts to have more tools.

if you've never crimped molex pins before I suggest you learn how. for the monitor plugs they're .156 trifurcon pins, and the plastic housings are called .156 female flat molex. you'll need a molex crimping tool, cause I had a dark age period of trying to do it with pliers and felt like a maroon when I got the real tool. :p

otherwise the screwdriver method of popping pins out will work in cases like these in a pinch.
 
Thanks for that bit of input. I have several styles of crimpers laying around but after googling molex crimper, I have some similar, but not quite the same. I will be picking a pair up since I just made a deal with a local guy to repair around 30-40 monitors for route machines and am working on a deal with another guy to help fix and maintain his entire arcade. I am sure they will come in handy at some point as I am sure I will eventually break one and have to replace it. Plus it never hurts to have more tools.


i have the best crimpers i think on my site for sale.
https://www.arcadepartsandrepair.co...ckets-pins/open-barrel-contact-crimping-tool/
these will crimp everything that takes two different waldon crimpers and mine crimp the wire and the jacket with one crimp and you can't over crimp and break the back of the pin.
 
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