Overscan at bottom of screen(?), diagonal lines and green tint

[RyuHayabusa78]

Hi, I just got a MAME arcade machine from a friend and I'm having a few issues with the monitor. I'm a definite newbie at this. I checked out a few FAQs but it's all way over my head because I've never worked on an arcade machine before. My friend told me that it is a Wells Gardner 25 inch monitor, which came with the machine when he bought it. It's an old Neo Geo MVS 4 game cabinet. I'm hoping you guys can help me diagnose what may be going on with this thing.

The first thing is that it displays these very thin diagonal lines across the screen. They are almost unnoticable when playing a game and the screen is full of color. However, when the screen shows a lot of black it is noticable, for instance when playing Pac Man. The black also has a slight green tint to it. Any ideas what may be causing this?

Also, there seems to be some severe overscan along the bottom of the screen that I can't get rid of. I've messed with the vertical screen positioning and resizing but it's not enough. As you can see at the bottom of the pics of Mortal Kombat and the GameEX frontend, there's what looks like the screen rolling at the bottom. Any suggestions? Thanks for any information!

[Kevin Mullins]

On the neckboard there should be knobs for adjusting the three colors, look for one marked Green DRIVE. Turn it down a tad.

They should be right across the top of the nckboard, my be tricky to reach so what your hands and don't bump the neckboard too hard.
Third knob from the right.... fourth knob from the left.

[RyuHayabusa78]

On the neckboard there should be knobs for adjusting the three colors, look for one marked Green DRIVE. Turn it down a tad.

They should be right across the top of the nckboard, my be tricky to reach so what your hands and don't bump the neckboard too hard.
Third knob from the right.... fourth knob from the left.

I tried adjusting the color on the neckboard but it doesn't solve the problem. When I have only the monitor turned on, the screen is almost solid green along with the diagonal lines, as shown in the pic above. Do most arcade monitors look that way when there is no signal being sent to it? This is what it looks like:

[MonMotha]

Try turning down green CUT-OFF (sometimes called BIAS), also on the neckboard.

[RyuHayabusa78]

Try turning down green CUT-OFF (sometimes called BIAS), also on the neckboard.

Not sure where that is. I have a Wells Gardner 25k7191 and here is a pic of the neckboard. Which are you referring to?

I think I may have found the solution to the screen roll along the bottom of the monitor. There's a knob below the neckboard that I think will fix it. Keeping fingers crossed.

[lilshawn]

...

[Kevin Mullins]

[RyuHayabusa78]

Try turning down green CUT-OFF (sometimes called BIAS), also on the neckboard.

I tried that and it didn't help. Thanks for the tip though.

Here's a couple of vids showing what's going on:




It's odd because it shows black and every other color fine. It's as though it only shows the green when there is no color to show, like the quick flash between rounds in Street Fighter 2. Also notice that when Pac Man starts up, it shows black perfectly during the boot-up, but once the game starts the black area of the screen has a green tint. Perhaps it's displaying transparency as green?

[MonMotha]

Looks like the green cut off levels creep up whenever the display is dark.  This indicates to me a possible power supply problem and maybe a leaky transistor.  I'd cap the thing and see if that helps.  You might also try swapping the green drive transistor (one of the big ones on the neckboard) for one of the other 2 (red or blue) and seeing if the problem moves to the color you swapped with (leaving green operating correctly).

[lilshawn]

[Ken Layton]

Try turning the "SCREEN" control on the flyback down a tad bit (counter-clockwise) to eliminate the diagonal 'retrace' lines.

The lower "foldover" problem is adjusted with the 50/60 control as shown here:

http://jstookey.com/arcade/WG_25k7191/

[RyuHayabusa78]

Try turning the "SCREEN" control on the flyback down a tad bit (counter-clockwise) to eliminate the diagonal 'retrace' lines.

The lower "foldover" problem is adjusted with the 50/60 control as shown here:

http://jstookey.com/arcade/WG_25k7191/

Thanks for the info. Adjusting the 50/60 control helped me get rid of the screen foldover. However, adjusting the screen control on the flyback didn't get rid of the lines. I noticed that they disappear from time to time, like when Pac-Man or Galaga are booting up. When they're doing the grid and scrambled graphics, the black is perfect and the lines disappear. After the game loads, the lines appear again. Perhaps the background space on games like Pac-Man and Galaga are not programed to be black but actually empty or trasparent and are black by default?

[MonMotha]

Those lines are definitely a monitor problem, and the monitor has no idea was "transparent" is.  There's no way to tell the monitor to display "transparent", just black.

It is remotely possible that you have a problem with your video amp, if you're using one, but it seems unlikely.

[RyuHayabusa78]

Those lines are definitely a monitor problem, and the monitor has no idea was "transparent" is.  There's no way to tell the monitor to display "transparent", just black.

It is remotely possible that you have a problem with your video amp, if you're using one, but it seems unlikely.

I think I can live with the lines for now. Everything else has been tweaked to my liking now. The green has been minimized, the curl at the bottom of the screen is gone. I'm pretty happy with it now. Thanks to everyone for their help!