XEGS S-Video


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I recently found some good articles on converting Atari 8bits to s-video, but none of the circuits seemed match the ones in my XEGS system. However, I was able to use them as general guidelines, and came up with step-by-step instructions of my own:

1.  The ascii schematic below was the normal production circuit
in my XEGS, don't know if there are different models like with the 
7800.

2.  To convert to s-video do the following:
    a. Replace R79 with 10 ohms (8.2 ohms also good)  
    b. Add a 10uf (20v or more) electrolytic cap from R79
       (transistor side) to ground, negative side of cap goes
       to ground of course            
    c. Cut Q3 emitter pin and bend up, coat with a little solder
    d. Wrap two 75 ohm resistors together at one end and solder,
       (82 ohms should be OK if you can't get 75)
    e. Solder wrapped end of resistors to Q3 emitter pin
    f. For the other end of the resistors: one goes to ground,
       the other to the luma wire of your cable
    g. Add a 100uf (20v or more) electrolytic cap between 5v
       and ground, somewhere near all this
    h. Repeat c-f for Q5, except this goes to the chroma
       wire of your cable 
    i. Solder the other conductor in both cables to ground

3.  I don't know if it's neccessary to cut the emitters, or if this
circuit will work with less components, all I know is that this worked
for me.

4.  Can't make up my mind if the color are now too strong.  If they are
too strong for you, try increasing R55. 

oversize 190kb image of connections (still not big enough)
                    
                                   
         5v--------+-------------------,
                   |                   |
             1.6K  |    51             |
           ,-/\/\/-+--/\/\/--,         |
           |  R81      R79   |         |
           |                 |         |
luma plus  |        ___/-----'         |               To Modulator
sync lines |       /|/c\ 3904          |                   Pin 2 --------,
       ----+------(b|   ) Q3        ___/                                 |
           |       \|\e/           /|/c\ 3904                            |
           |           \-----+----(b|   ) Q4                             |
           |                 |     \|\e/    33      18                   |
           |                 |         \--/\/\/-+-/\/\/--+------|(-------+
           |  2K       100   |             R53  |  R52   |      C55      |
           `-/\/\/-,--/\/\/--`                  |        |               |
              R80  |   R78                      |        |  1k     1.6k  |
                   |                            |        `-/\/\/-+-/\/\/-'
               GND \/                           |           R51  |  R93
                                                |                |
                                                |            GND \/
                      5v---------,              |
                                 |              |
                              ___/              |
GTIA    5.6k    _______      /|/c\ 3904         |
P21 ---/\/\/---|_______|----(b|   ) Q5          |
        R55        L5        \|\e/              |
                                 \---+----|(----'    
                                     |    C44
                           180       |
                    ,-----/\/\/------'
                    |      R54
                GND \/
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