Need S-Video out help

[Viso]

I have tried to add S-Video output to my 7800, but there are some problems that required me to make odd changes to the circut to get any usable output.

 

The problems I'm having are darkened vertical lines and a different brightness when the output is given to a different device. The vertical lines don't really look like lines because of the space between each horizontal line, but there are regularly spaced columns where the image is darker. So along a horizontal line that should be the same brightness, it alternates between brigher and darker. The effect is not present when the color is white (or any grey). It can be seen very promenately in Ballblazer - in fact, it is so bad with Ballblazer that I haven't decided if RF looks worse or better yet.

 

When I connect the output to my VCR (I've got an S-VHS one, so it takes the S-Video input), I get an image that, save for the above problem, is wonderful. When I connect it to my receiver's front panel video input, the result looks darker and the colors more muted. If I adjust the brighness output, I can get the video through the receiver to look almost bright enough to be right, but then the VCR shows it to be too bright and washed out.

 

The circut I used is very similar to the CD4050 A/V mod sans CD4050 that has been mentioned on Atari age. You can find a 2600 schematic here: http://www.atariage.com/forums/viewtopic.php?t=17748

The exact circut I used started out several months ago as another mod listed here: http://www.geocities.com/atari7800mod/7800...nstruction.html

I modified the circut to include the transistor and two 75ohm resistors on the chroma output as present in the CD4050 mod.

 

When I connected the circut as described, I did not get an image. I found that until I connected the output to my VCR (or receiver), there where chroma and luma signals at the output end of the circut. With the S-Video cable connected to the VCR (or receiver), the signals at the output end of the circut where grounded. I checked this with an osciliscope.

 

To get an image, I stuck a couple wirse onto the emitter ends of the two transistors, and connected the other end of each wire to either a chroma or luma line on the S-Video out. This eliminates the 75ohm resistor between the transistors and the circut's output.

 

What have I done wrong with the circut and how do I fix it?

[jsoper]

I didn't follow all that, but this mod has always worked great for me (it doesn't use any chroma transistors):

http://home.earthlink.net/~resqsoft/7800_mod.htm